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INCUBATION

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Originally appearing in Volume V14, Page 364 of the 1911 Encyclopedia Britannica.
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INCUBATION and INCUBATORS. The subject of " incubation " (Lat. incubare, to brood; in-cumbere, to lie on), a term which, while strictly signifying the action of a hen in sitting on her eggs to hatch them, is also used in pathology for the development within the body of the germs of disease, is especially associated with the artificial means, or " incubators," devised for hatching eggs, or for analogous purposes of an artificial foster-mother nature, or for use in bacteriological laboratories. Life is dependent, alike for its awakening and its maintenance, upon the influence of certain physical and chemical factors, among which heat and moisture may be regarded as the chief. It is therefore obvious that any method of incubation must provide for a due degree of temperature and moisture. And this degree must be one within limits, for while all organisms are plastic and can attune themselves to a greater cr less range of variation in their physical environment, there is a given degree at which the processes of life in each species proceed most favourably. It is this particular degree, which differs for different species, which must be attained, if artificial incubation is to be successfully conducted. In other words, the degree of temperature and moisture within the incubation drawer must remain uniform throughout the period of incubation if the best results are to be reached. It is not easy to attain these conditions, for there are many disturbing factors. We may therefore next consider the more important of them. The chief causes which operate to make the temperature within the incubator drawer variable are the changes of 'the temperature of the outer air, fluctuations in the pressure of the gas when that is used as the source of heat, or the gradual diminution of the oxidizing power of the flame and wick when an oil lamp is substituted for gas. Also, the necessary opening of the incubator drawer, either for airing or for sprinkling the eggs with water when that is necessary, tends to reduce the temperature. But there is another equally important though less obvious source of disturbance, and this resides within the organism undergoing incubation. In the case of the chick, at about the ninth or tenth days of incubation important Changes are occurring. Between this period and the fourteenth day the chick becomes relatively large and bulky, and the temporary respiratory organ, the allantois, together with its veins, increases greatly in size and extent. As a consequence, the respiratory processes are enabled to proceed with greater activity, and the chemical processes of oxidation thus enhanced necessarily largely increase the amount of heat which the chick itself produces. Thus an incubator, to be successful, must be capable of automatically adjusting itself to this heightened temperature. The drawer of an incubator is a confined space and is usually packed as closely as possible with the contained eggs. The eggs are living structures and consequently need air. This necessitates some method of direct ventilation, and this in its turn necessarily increases the evaporation of water vapour from the surface of the egg. Unless, therefore, this evaporation is checked, the eggs will be too dry at the period-from the tenth day onwards— when moisture is more than ever an important factor. There is, according to some poultry authorities, reason to believe that the sitting hen secretes some oily substance which, becoming diffused over the surface of the egg, prevents or retards evaporation from within; presumably, this oil is permeable to oxygen. In nature, with the sitting hen, and in the " Mamal " artificial incubating establishments of the Egyptians, direct air currents do not exist, owing to the large size of the chambers, and consequently incubation can be successfully achieved without any special provision for the supply of moisture. Artificial incubation has been known to the Egyptians and the Chinese from almost time immemorial. In Egypt, at Berme on the Delta, the trade of artificial hatching is traditionally transmitted from father to son, and is consequently confined to particular families. The secrets of the process are guarded with a religious zeal, and the individuals who practise it are held under plighted word not to divulge them. It is highly probable that the process of artificial incubation as practised by the Egyptians is not so simple as it is believed to be. But as far as the structures and processes involved have been ascertained by travellers, it appears that the " Mamal " is a brick building, consisting of four large ovens, each of such a size that several men could be contained within it. These ovens are in pairs, in each pair one oven being above the other, on each side of a long passage, into which they open by a circular aperture, just large enough for a man to obtain access to each. The eggs are placed in the middle of the floor of the oven, and in the gutters round the sides the fire is lighted. The material for this latter, according to one account, consists of camels' dung and chopped hay, and according to another of horses' dung. The attainment of the right degree of heat is apparently reached wholly by the skill of the persons employed. • When this has been attained, they plug the entrance hole with coarse tow. On the tenth to twelfth days they cease to light the fires. Each " Mamal " may contain from 40,000 to 8o,000 eggs. There are 386 " Mamals " in the country, which are only worked for six months of the year, and produce in that time eight broods. Many more than two-thirds of the eggs put in are successfully hatched. It is estimated that 90,000,000 eggs are annually hatched by the Bermeans. A method of incubating that appears to have been altogether overlooked in England—or at least never to have been practised —is that carried on by the Couveurs or professional hatchers in France. They make use of hen-turkeys for the purpose, and each bird can be made to sit continuously for from three to six months. The modus operandi is as follows: a dark room which is kept at a constant temperature throughout the year contains a number of boxes, just large enough to accommodate a turkey. The bottom of the box is filled with some vegetable material, bracken, hay, heather, straw or cocoa-fibres. Each box is covered in with lattice-work wire, so arranged that the freedom of the sitting bird is limited and its escape prevented. Dummy eggs, made by emptying addled ones and filling with plaster of Paris, are then placed in the nest and a bird put in. At first it endeavours to escape, but after an interval of a few days it becomes quiet, and the dummy eggs being then removed, fresh ones are inserted. As soon as the chickens are hatched, they are withdrawn and fresh eggs substituted. The hen turkeys are also used successfully as foster-mothers. Each bird can adequately cover about two dozen eggs. Incubation as an industry in Europe and America is of recent development. The growing scarcity of game birds of all kinds, coincident with the increase of population, and the introduction of the breech-loading gun, together with the marked revival of interest in fancy poultry about the year 1870, led, however, to the production of a great variety of appliances designed to render artificial incubation successful. Previously to this, several interesting attempts had been made. As long ago as 1824, Walthew constructed an incubator designed to be used by farmers' wives with the aid of no more than ordinary household conditions. It consisted of a double-walled metal box, with several pipes opening into the walled space round thesides, bottom and top of the incubator. These pipes were connected with an ordinary kitchen boiler. Walthew, however, constructed a fire grate, with a special boiler adapted to the requirements of the incubator. Into the walled space of the incubator, steam from the kitchen boiler passed; the excess steam escaped from an aperture in the roof, and the condensed steam through one in the floor. Ventilating holes and also plugs, into which thermometers were placed, pierced the door of the incubator. . In 1827, J. H. Barlow successfully reared hens and other birds by means of steam at Drayton Green, Ealing. He constructed very large rooms and rearing houses, expending many thousands of pounds upon the work. He reared some 64,000 game birds annually. The celebrated physician Harvey, and the famous anatomist Hunter were much interested in his results. To John Champion, Berwick-on-Tweed, in 187o, belongs, how-ever, the credit of instituting a system which, when extended, may become the system of the future, and will rival the ancient " Mamals " in the success of the incubation and in the largeness of the numbers of eggs incubated. He used a large room through which passed two heated flues, the eggs being placed upon a table in the centre. The flues opened out into an adjoining space. The temperature of the room was adjusted by personal supervision of the fire. This system, more elaborated and refined, is now in use in some parts of America. Bird Incubators. Owing to the great variety in the details of construction, it is difficult to arrange a classification of incubators which- shall include them all. They may, however, be classified in one of two ways. We may either consider the method by which they are heated or the method by which their temperature is regulated. In the former case we may divide them into " hot-air " incubators and into " hot-water " or " tank " incubators. In the latter case we may classify them according as their thermostat or temperature-regulator is actuated by a liquid expanding with rising temperature, or by solids, usually metals. In America incubators of the hot-air type with solid and metallic thermostats are most used, while in Europe the " tank " type, with a thermostat of expansible liquid, prevails. For the purpose of more adequately considering the various forms which have been in use, or are still used, we shall here divide them into the " hot-air " and " hot-water " (or " tank ") classes. In the hot-air types the incubator chamber is heated by columns of hot air, while in the tank system this chamber is heated by a tank of warmed water. (a) Hot-Water Incubators.—In 1866 Colonel Stuart Wortley de-scribed in The Field an incubator constructed upon a novel principle, but which appears never to have been adopted by breeders. The descriptive article is illustrated with a sketch. Essentially the incubator consists of four pipes which extend across the egg chamber some little distance above the eggs. The pipes pass through holes in the side of the incubator, which are furnished with pads, so as to render their passage air-tight. Externally they are connected with a boiler. This is provided with a dome through which steam escapes, and also with a glass gauge to show the height of the water within the boiler. The water in the boiler is kept at the-boiling point, and the temperature of the incubator is regulated by adjustment of the length of the hot-water pipes within the egg chamber. To raise the temperature, a greater length of the pipes is pushed into the chamber, and to reduce it, more of their length is pulled outwards. It is claimed for this instrument that since the temperature of boiling water at any particular locality remains practically constant, the disadvantages due to fluctuations in the activity of a lamp flame or the size of a gas flame are obviated. But it has the serious &S.-advantage that there is no automatic adjustment to compensate for fluctuations of atmospheric temperature. And experiments by C. Hearson have shown that even if the temperature of the tank or source of heat be constant, that of the incubator drawer will nevertheless vary with fluctuations of external temperature. Probably if the mechanical difficulties of providing a self-regulator were over-come, it would prove an efficient and reliable incubator. The difficulties do not seem to be insuperable, and it appears possible that a thermostatic bar could be so arranged as to automatically increase or decrease the length of hot-water pipes within the incubator, and therefore the incubator temperature. Another early form of incubator is Brindley's, which was first in use about 1845, and in his hands it appeared occasionally to act successfully, but it never became generally used. The egg chamber was lined with felt, and was placed beneath a heated air chamber, the floor and roof of which were composed of glass. The air chamber was heated by a number of hot-water pipes which were connected with a copper boiler. This latter was heated by means of a lamp so constructed as to burn steadily. The temperature of the air chamber was regulated within certain limits by means of a balanced valve, which could be so adjusted that it would open at any desired temperature. In Colonel Stuart Wortley's incubator the hot-water tubes passed directly into the egg chamber, and in Brindley's into a chamber above it. But in other forms of incubators in which the principle of an external boiler connected with water tubes is adopted, the latter pass not into the egg chamber nor into an air chamber, but open into and from a tank of water. The floor of this tank forms the roof of the egg chamber, so that the eggs are heated from above. This device of warming the eggs from above was adopted in imitation of the processes that presumably occur with the sitting hen; for it is generally assumed that the surface of the eggs in contact with the hen is warmer than that in contact with the damp soil or with the material of the nest. One of the earliest of this form of incubator is that invented by F. Schroder, manager of the now extinct British National Poultry Company. In this incubator the form is circular, and there are four egg drawers, so that each one occupied the quadrant of a circle, and the inner corner of each drawer meets in the middle of the incubator. Front -the centre of the incubator a vertical chimney passes upwards and opens out from the inner corners of the four egg drawers. This chimney acts as a ventilator to the incubating chambers. These latter are open above, but their floors are made of perforated zinc, and when in use they are partially filled with chaff or similar material. Under them is a tank containing cold water and common to all four drawers; the slight vapour rising from the surface of the water diffuses through the egg drawers and thus insures a sufficient degree of humidity to the air within. Above the egg drawers is a circular tank containing warm water. The floor of this tank constitutes the roof of the egg drawers, while the roof forms the floor of a circular chamber above it, the side wall of which is composed of perforated zinc. This upper chamber is used to dry the chicks when they are just hatched and to rear them until they are strong enough for removal. It is partially filled with sand, which serves the double purpose of retaining the heat in the warm-water tank beneath and of forming a bed for the chicks. The water in the warm-water tank is heated by means of a boiler which is external to the incubator, and in communication with the tank by means of an inlet and an outlet pipe. There is no valve to regulate the temperature, and the latter is measured by means of a thermometer, the bulb of which is situated not in the incubator drawers, but in the warm-water tank. This is a wrong position for the thermometer, since it is now known that the temperature of the water tank may be different by several degrees to that of the egg drawer; for with a fall of external temperature that of the latter necessarily tends to fall more rapidly than the former. But, none the less, in skilful hands this incubator gave good results. T. Christy's incubator, which we shall describe next, has passed through several forms. We shall consider the most recent one (11894). The incubator (fig. 1) is double walled, and the space between the two walls is packed with a non-conducting material. In the upper T C p H 0 part of the incubator there is a water tank (T) divided by a horizontal partition into two chambers, communicating with each other at the left-hand side. Below the tank is the incubation drawer (E), which contains the eggs and also a temperature regulator or thermostat (R). The tank is traversed by a ventilating shaft (V), and inserted into this is a smaller sliding tube passing up to it from a hole in the bottom of the incubator drawer. The floor of the incubator drawer is perforated, and beneath it is an enclosed air space which opens into the sliding air shaft just described. Fresh air is let into the incubator drawer from a few apertures (I) at its top. The ventilating shaft (V) is closed externally by a cap (C), which can be raised from or lowered down upon its orifice by the horizontal arm (H) working upon pivot joints at (P). This arm is operated by the thermostat (R), through the agency of a vertical rod. The water in the tank is heated by an external boiler (B) through two pipes, one of which (T) serves as an inlet, and the other (L) as an outlet channel from the tank. These two pipes do not open directly into the tank, but into an outer vessel (0) communicating with it. Communication between this vessel and the tank may be made or broken by means of a sliding valve (S), which is pierced by an aperture that corresponds in position with the upper of the two in the wall of the tank when the valve is up. When this valve is in its upper position, the tank (T) communicates with the outer vessel (0) by two apertures (A and A'), the top one being the inlet and the lower one the outlet. These coincide in position with the tubes from the boiler. This latter (B) is a conical vessel containing two spaces. The heated water is cons tained in the outer of these spaces, while the central space is an air shaft heated by a lamp flame. This particular form of the boiler results in the water at its top part being more heated than that in its lower. As a consequence of this, a continual circulation of water through the tank ensues. The more heated water, being specifically lighter, passes into the outer vessel, where it remains among the higher strata, and therefore enters the tank through the upper aperture. In passing along the upper division of the tank it becomes slightly cooled and sinks therefore into the lower compartment, passes along it, and out through the aperture A'. Hence it passes into the lower portion of the boiler, where it becomes warmed and specifically lighter; in consequence it becomes pushed upwards in the boiler by the cooler and heavier water coming in behind and below it. Should the temperature in the incubator drawer rise, the bimetallic thermostat (R) opens out its coil and pulls down the vertical rod. This simultaneously effects two things: it raises the cap (C) over the ventilating shaft and allows of a more rapid flow of fresh air through the incubator drawer, and it also lowers the slide-valve (S) so that the tank becomes cut off from communication with the outer vessel (0) and therefore with the boiler. The temperature thereupon begins to fall and the thermostat, coiling closer, raises the vertical rod, closes the ventilating shaft, and once more places the tank in communication with the boiler. The structure of the thermostat is liven below. The Chantry Incubator (Sheffield) is also an incubator with a hot-water tank, the circulation of which is maintained by an outside boiler. Its temperature is regulated by a metal regulator. In Schroder's and Christy's incubators the hot-water pipes from the boiler simply entered the warm-water tank but did not traverse it. In the two incubators to be next described the hot-water pipes are made to pass through the water in the tank, and are so arranged as to minimize the possibility that the outside of the tank may become colder than the centre. Both of them are also fitted with an ingenious though slightly complex valve for maintaining an approximately constant temperature. Halsted's incubator was the earliest of this type. Since his original form was constructed he has designed an improved one, and it is this latter which will be described. The egg drawer (E, fig. 2) lies beneath the warm-water tank (T), and above this is a nursery (N). The egg drawer is ventilated by two tubular shafts (V), of which only one is represented in the illustration; the tubesare about 21 in. in diameter, and each one is fitted at its upper end, where it opens into the nursery, with a swing-valve (V') which turns upon a horizontal axis (A), in its turn connected, by means of cranks (C) and shafts (S), with the heat regulating apparatus (R). A space of about 2 in. between the top of the incubating drawer and the warm-water tank is necessary for the insertion of this :-apparatus. The water in the tank (T) is heated by means of the boiler (B) ; the tank and boiler are connected by the two pipes (I) and (0), of which one is the inlet and the other the outlet channel. The boiler consists of an FIG 2.-Halsted's Incubator. inner (I') and an outer (0) division in communication with each other below. The latter is cylindrical in form, while the outer wall of the former is cylindrical and its inner wall conical. The conical wall of the inner boiler is the surface which is heated by the lamp (L). The arrangement of the inlet and outlet tubes is important. In the illustration, for the sake of clearness, they are represented as one above the other. In reality they lie in the same plane, and the fork (F) of the inlet pipe similarly lies in the horizontal plane and not vertically as represented. The inlet pipe not only differs from the outlet pipe in the 1111111111111 V' possession of a forked end, but it is carried to the farther end (not closely, according as the flame is to be smaller or larger respectively. shown in the diagram) of the water tank, while the outlet pipe opens from about the middle of the tank. The inlet pipe is connected with the inner portion of the boiler and the outlet one with the outer portion. The result of this adjustment of the parts is that the warmer water of the inner boiler, being specifically lighter than the cooler water of the outer boiler, rises up and passes through the inlet pipe (I) and is discharged into the tank through the two divergent orifices of the fork (F). Here the water strikes the side wall of the farther end of the tank and is reflected back along the back and front walls towards the nearer side. Hence it is again reflected, but in the opposite direction, and now forms a central current, which is directed towards.the centrally situated orifice of the outlet tube (0). Through this it passes to the outer boiler, and sinking towards the bottom, reaches the base of the inner boiler. Here it becomes heated and lighter and consequently rises to the top, and once more passes through the inlet pipe to the water tank. The warm water thus travels round the outer walls of the tank and the cooled water is conducted away along the middle portion. A more equable distribution of temperature over the roof of the incubating chamber is thus ensured than would be the case if the heated water were discharged either into the centre or at any other single point only of the tank. To a very large extent, the efficiency of this apparatus depends l serted over the orifice of the upon the approximately perfect performance of the lamp. A good, I ventilating shaft (A) which steadily burning one should be employed, and only the best oil used; for, should the wick become fouled the flame cannot freely burn. For this reason it is better to use gas, whenever obtainable. The maintenance of an approximately uniform temperature is obtained by allowing the heated air of the egg-drawer to escape through the two ventilating shafts (V). The swing-valves of these are opened or closed by means of the regulator (R). This latter consists of a glass bowl prolonged into a tube, about 8 in. long and three-eighths of an inch in diameter. The glass tube swings upon an axis (A) which is situated as near as possible to the bowl of the regulator. The axis is connected with a crank (C') which is disposed so as to act as a lever upon the vertical shaft (S), which in its turn is connected with the upper crank (C) ; this works the axis (A') of the swing-valves, and so can open or close the apertures of the ventilating pipes. The bowl of the regulator is filled with mercury to such an extent that at the temperature of zoo° F., and when the tube is slightly inclined upwards from the horizontal it just flows slightly into the tube from the bowl. On the lever-crank (C') a weight is slung by a sliding adjustment, and is so placed that when the temperature of the egg-drawer is Io3° it just balances the tube of the regulator when it is slightly inclined upwards. Should the temperature of the drawer now rise higher the mercury flows towards the distant end of the tube and, causing it to fall down, brings about a rotation of the regulator axis and as a consequence the opening of the ventilat- ing valves. A transverse stay prevents the limb of the regulator that of Charles Hearson. This differs from any of those described from quite reaching the horizontal when it falls. As the temperature cools down the mercury contracts and retraces to the nearer end of the tube and to the bowl, and consequently results in the upward inclination of the limb; the valves are thus closed again. The egg-drawer (E) is specially constructed so as to imitate as nearly as possible the natural conditions that exist under a sitting hen. The drawer is of wood and contains a zinc tray (Z) into which cold water is placed. Fitting into the zinc tray is another zinc compartment, the floor of which is made of a number of zinc strips (X) transversely arranged and placed in relation to each other like the limbs of an inverted A. The limbs are so disposed that those, of one series do not touch the adjacent ones, and in fact a space is left between them. Thus a number of parallel troughs are formed, each of which opens below into the moist air chamber of the cold water tray beneath. In practice these troughs are covered with flannel which is allowed to dip into the water of the tray. Thus the eggs lie in a series of damp troughs and their lower surfaces are therefore damper and colder than their upper ones. This incubator, if carefully worked and the necessary practical details observed, has the reputation of being an efficient machine. Somewhat similar to the Halsted incubator, but differing from it in the nature of the boiler and in the temperature regulator, is the Graves incubator, made in Boston, U.S.A. The incubator itself (fig. 3) consists of an incubating or egg-drawer (E) heated from above by a warm-water tank (T). Below the egg-drawer is a tank containing cold water, the vapour of which passes through the perforated floor of the former and keeps the air of the egg-chamber slightly humid. Above the warm-water tank is an air chamber (AC) to serve as a non-conducting medium and to prevent therefore undue loss of heat. Above this is a nursery or drying chamber (N), closed in, with a movable lid. The warm-water tank is heated by means of a simple boiler (B) from which an inlet tube (I) carries heated water to the tank; the tube traverses the length of the tank and discharges at its farther end (not shown in the diagram). From the nearer end of the tank an outlet tube (0) passes out and opens into the boiler at a slightly higher level than the inlet one. The boiler is heated by an evenly burning lamp below, of special construction. The rectangular tube through which the wick passes is bevelled at its outer end, and upon this bevelled edge a metal flap (F) is allowed to rest more or less The wick is, of course, bevelled to correspond to the form of its tube. The metal flap is raised or depressed by means of levers connected with the heat-regulator. When it is depressed upon the wick the flame is lessened; and it becomes proportionately bigger as the flap is raised more and more. The heat-regulator consists of a glass tube (T) which runs the whole width of the incubation chamber and lies in contact with the floor of the warm-water tank ; it is filled with alcohol. Externally to the incubator this tube is connected with a U-shaped one containing mercury. The free limb of the U-tube contains a piston (P) which rests upon the surface of the mercury in that limb. From the piston a piston rod (PR) passes vertically upwards and is connected with a lever (1.) which operates, through the agency of a second lever (L') the movements of the ventilating valve (V) in F opens from the roof of the incubator drawer. The lever (L) is further connected with a spiral spring (S) which works the metal flap of the lamp already described. The height of the piston in the U tube can be so adjusted, by varying the quantity of mercury in the tube, that when the temperature of the incubation drawer is 103° F., the ventilating valves are closed and the wick is burning to its full extent. Should the temperature rise, the alcohol in the glass tube (T) expands and causes the mercury in the free limb of the U tube to rise. This carries with it the piston, and this movement brings about the opening of the ventilating valves, and at the same time, through the agency of the lever (L) and the spiral spring (S) the metal flap is brought down upon the wick, cutting off more or less of the flame. Should the temperature then fall to Io3° or lower, the contraction of the alcohol reverses these movements, the valve closes, and the wick once more burns to its full extent. In practice, the boiler and the temperature regulator are duplicated, there being a set on both sides of the incubator. Any slight irregularity on the one side may be thus compensated for by the other side, Graves's incubator has the reputation of being a good machine. Among the most recent type of incubators made in England is in the simplicity and ingenuity of the heat regulator, and in that the tubes which traverse the water tank are hot-air flues, carrying the air heated by the flame and not warm water. Consequently a further simplification is introduced inasmuch as no boiler is required. The essential features of this incubator are shown in fig. 4. The internal parts of the incubator are insulated by a double wall, the interspace being packed by a non-conducting material, which is not shown in the figure. The incubation or egg-drawer (E) is heated by the warm-water tank (T). Beneath the egg-drawer is a zinc tray (Z), so constructed that in the central part the floor is raised up into a short cylinder. Around the raised cylinder is a wide trough containing water and into this dips a canvas cloth which is stretched out over a perforated zinc support (F). By this means an extended moistened surface is produced which allows of a rapid evaporation. F E S r...F_ z v ~l The floor of the incubator, which is raised by short feet from the table on which it stands, is perforated in the central portion by a number of holes, and which are so situated that they lie beneath the raised cylinder of the cold-water tray (Z). The incubation-drawer is thus supplied continuously by a slow current of moistened air because the air in the upper part of the drawer, i.e. in contact with the floor of the warm-water tank, is the warmest and lightest. It therefore tends to diffuse or pass through the narrow slits between the drawer and the walls of the incubator, and also through the aperture in the front wall of the egg-drawer, through which a thermometer is placed. To replace the air thus lost, fresh air passes in through the holes in the bottom of the incubator, and on its way must pass through the pores of the damp canvas which dips into the water in the zinc tray (Z). The warm-water tank is heated by an inlet (I) and outlet (0) flue which are, however, continuous. The inlet flue opens out from a vertical chimney (C), the air in which is heated either by a gas flame or that of an oil lamp. The outlet or return flue passes back through the width of the tank and opens independently to the exterior. The vertical chimney (C) is capped by a lid (L) capable of being raised or lowered upon its orifice by the lever (L'). When the cap is resting upon the chimney all the heated air from within the latter passes through the flues and heats the water in the tank. If the cap is widely raised, practically all the heated air passes directly upwards through the chimney and none goes through the flues. If the cap be but slightly raised, part of the heated air goes through the flues and part directly escapes through the aperture of the chimney. The movement of the lever (L') which raises the cap (L) is deter-mined by the thermostatic capsule (S), situated within the egg-drawer. The principle upon which this capsule is designed is that the boiling point of a liquid depends not only upon temperature but also upon pressure. A given liquid at ordinary atmospheric pressure will boil at a certain degree of temperature, which varies for different substances. But if the pressure be increased the boiling point of the liquid is raised to a higher degree of temperature. A liquid when it boils passes into a gaseous condition and in this state will occupy a very much larger volume some two or three hundred times—than in the liquid condition. If, therefore, a hermetically sealed capsule: with flexible sides be filled with some liquid which boils at a given temperature, the sides of the capsule will distend when the temperature of the air round the capsule has been raised to the boiling point of the liquid within it. The distension of this capsule can be used to raise the lever (L'). The thermostatic capsule is placed on a fixed cradle (F) and is filled with a mixture of ether and alcohol, the proportions being such that the boiling point of the mixed liquid is too° F. Between the capsule and the lever (L') is a vertical rod (V), articulating with the lever as close as possible to its fulcrum (M). The articulation with the lever is by means of a screw, so that the necessary nice adjustment between the height of the rod (V), the thickness of the capsule and the position of rest of the damper (L) upon the chimney, can be accurately made. The temperature at which it is desired that the liquid in the capsule shall boil can be determined by sliding the weight (W) nearer or farther to the fulcrum of the lever (L'). The farther it is moved outwards, the greater is the pressure upon the thermostatic capsule and consequently the higher will be the boiling point of its contained liquid. By means of the milled-head screw (A), the height of the lever at its outer end can be so adjusted that when the liquid of the capsule is not boiling the damper (L) closes the chimney, but that when it does boil the damper will be raised sufficiently high from it. If the weight is pushed as far as it will go towards the fulcrum end of the lever, the temperature of the egg-drawer will never rise more than toe F. because at this temperature and under the pressure to which it is then subjected, the liquid in the capsule boils, and consequently brings about the raising of the damper. It matters not, therefore, how high the flame of the gas or lamp be turned, the temperature of the egg-drawer will not increase, because the extra heat of the enlarged flame is passing directly outwards through the chimney, and is not going through the flues in the tank. In order to raise the temperature within the incubation chamber to 102° or 103°, or any other desired degree, the weight (W) must be moved outwards along the lever (L'), about t in. for every degree of temperature increase desired. This thermostatic capsule works admirably, and the incubator will work for months at a time and requires no adjustment, however much, within the limits of our climate, the external temperature may vary. The capsule, like all other thermostats in which the expansible substance is a liquid, is, however, dependent upon external pressure for the point at which its contained liquid boils and therefore, for the degree of temperature prevailing within the incubator drawer. It is therefore responsive to variations in atmospheric pressure, and as the barometer may fall t or 2 in., this may possibly make a difference of two or three degrees in the fluctuation of temperature within the egg-drawer. It is not, of course, often that such large oscillations of the barometer occur, and as a matter of practical experience, under ordinary conditions, this incubator will work for months together without attention with only half a degree variation round the point at which it was set. ' Greenwood's incubator (fig. 5), named the Bedford, resembles Hearson's in that hot-air flues (F and F') and not hot-water pipes, 363 traverse the water tank (T). And the method of regulation of the temperature is much the same, i.e. a thermostat (V) operating upon a lever which raises a cap (C) from off the aperture of the main flue (F) and thus allows all the heat of the flame to pass directly outwards, without passing through the series of flues (F) which horizontally traverse the water- tank. Fresh air enters through a wide circu- W lar aperture (A) which surrounds the main flue, and it thus becomes partially warmed before entering the egg-chamber. The eggs are placed upon a perforated floor (E) lying over water baths (B). The water tank (T) lies in the centre of the incubation chamber and is traversed through its central axis by the main hot-air flue (F). From this, four horizontal flues pass outwards through the water and open into small vertical flues, which in their turn communicate with the exterior. The thermostat (V) consists of a glass tube of peculiar form. This is closed at the end of its short limb and open at its other extremity on the long limb. The bent portion of the tube is filled with mercury and between the mercury column and the closed end is a small quantity of ether. The thermostat is lodged in a box (G), which forms part of the lever (L). At one end this lever is pivoted to a fixed arm, and at the other to the vertical rod .which operates the ventilating cap (C). If the temperature should rise, the ether in the thermostat expands and pushes the mercury column up along the inclined long limb. This disturbs the equilibrium of the lever (L), and it descends downwards, pulling with it the vertical rod, and thus raising the cap over the main flue. If the temperature falls the reverse series of changes occur. The temperature at which the cap will be raised can be adjusted within limits by the position of the weight (W) and by the adjustment of the degree of inclination of the thermostat. The Proctor incubator, made at Otley, is apparently, in its main features, similar to the Greenwood. Somewhat similar, in certain features, to the Greenwood is the Winchcombe. Its improved form, in which metal replaces the wood casing, is named the Gladstone. In it there is a combination of the hot-air and the water-tank systems of warming the incubation chamber. The wall of the incubator is double, and the space between the outer and inner wall is packed with a non-conducting material. The incubation chamber is heated above by a water-tank (fig. 6 T) A which is traversed by a main vertical flue (F) and four subsidiary horizontal ones which discharge externally. The main flue, how-ever, in passing up to enter the water tank traverses the egg-chamber, and therefore serves to warm it, as in the hot-air type of incubator, by the heat of the flue itself. Around the lower half of the flue is a water vessel consisting of two concentric containers (C), holding water. In the space between these concentric containers, fresh air passes in through the aperture (A), and before it reaches the egg-chamber it passes through coarse canvas which dips into the water in the containers, and is therefore kept permanently moist. The containers are filled from a water tank (S) outside the incubator. Air passes out from the egg-chamber through the aperture (0). The temperature is regulated by a bimetallic thermostat (see below), which operates two levers, that by their arrangement can raise or depress the cap (D) over the main flue (F). The temperature at which this occurs will be determined, within limits, by the position of the adjustable weight (W). Tomlinson's incubator, designed in 188o, is novel in principle.
End of Article: INCUBATION
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Additional information and Comments

I think that the article about incubation is quite correct and exhaustive. I also have a similar writing in my site and I did it in part from a National Geographic article (1927) in part from the specific treatise of Reaumur "Art de faire éclorre et d'élever en toute saison des oiseaux domestiques" (1751). Some days ago I wrote to the Egyptian Academy in Rome in order to identify Bermé in Nile's Delta quoted by Reaumur and by you in your article but they replied that they are not able in indentifying this little town. With National Geographic it is impossible to enter in touch and the German Geographic Society doesn't work with strange and curious persons like me. Are you able in identifying Bermé quoted by Reaumur? My site is : www.summagallicana.it. Many thanks for your attention. Kind regards.
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