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age a (From a diagram in Proc . Inst . Ciro . Eng.) into the chimney from the fan has a line parallel to that of the fan-shaft and of the fan-blades, and, as a consequence, as each blade passes this shutter, the stoppage of the discharge of the air is instantaneous, and the sudden change of the pressure of the air on the face of the blade whilst discharging and the reversal of the pressure, due to the vacuum inside the fan-casing, cause the vibration hitherto inseparable from this type of ventilator . As an illustration of the effect of the pulsatory action of the Guibal shutters the following figures may be given: a fan having ten arms and running, say, sixty revolutions per minute, and working twenty-four hours per day, gives (10 X 6o X 6o X 24=) 864,000 blows per day transmitted from the tip of the fan-vanes to the fan-shaft; the shaft is thus in a constant state of tremor, and sooner or later reaches its elastic limit, and the consequent injury to the general structure of the fan is obvious . This difficulty is avoided by cutting a A-shaped opening in the shutter, thus gradually decreasing the aperture and allowing the air to pass into the chimney in a continuous stream instead of intermittently . The action of this regulating shutter increases the durability and efficiency of the fans in an important degree . In towns like Liverpool and Birkenhead any pulsatory action would he readily felt by the inhabitants, but with the above: arrangement it is difficult to detect any sound whatever, even when standing close to the buildings containing the fans . The admission of the air on both sides is found in practice to conduce to smooth running and to the reduction of the side-thrust which occurs when the air is admitted on one side only . The fans are five in number: two are 4o ft. in diameter by 12 ft. wide, and two 3o ft. in diameter by loft. wide, one of each size being erected at Liverpool and at Birkenhead respectively . In addition, there is a high-speed fan 16 ft. in diameter in Liverpool which throws 300,000 cub. ft . The following table gives the result of experiments made with the ventilating fans of the Mersey railway: Fan at z n ti A Y,t - C c o ff; c s ~~ c E o c« c4.a oy ~[ a, `w 'OC o g C ~... a .

ie h a y ~ 3 ~ ~ E a ES E c o o lfamilton

Street, 30 10 47 113 I.30 1895 214,135 Birkenhead Shore Road, 40 12 45 41 2.50 32881 134,685 Birkenhead 1 James Street, 40 12 45 72 2.45 2465 178,880 Liverpool . James Street, 30 10 6o 6o 2.30 2062 123,720 Liverpool . Bold Street, 162 — — — — — 300,000 Liverpool . Total 951,420 • i The central point of the Severn tunnel (fig . 15) lies toward the Monmouthshire bank of the river, and ventilation is effected from that point by means of one fan placed on the surface at Sudbrooke, Monmouth, at the top of a shaft which is connected with a horizontal Ventilating Fan 4oxret--__ Monmouthshire ®2Jy` heSn otSevern ( too 7 in 3 4 7 miles '*----Total:englh of Tunnel 4 miles 624 yards . heading leading to the centre . This fan, which is 40 ft. in diameter by 12 ft. in width, removes from the tunnel some 400,000 cub. ft. per minute, and draws in an equivalent volume of fresh air from the two ends . About 1896 an excellent system was introduced by Signor Saccardo, the well-known Italian engineer, which to a great extent has minimized the difficulty of ventilating long tunnels under mountain-ranges where shafts are not available . This system, which is not applicable to tunnels in which underground stations exist, is illustrated in fig . 16, which represents its application to the single-line tunnel through the Apennines at Pracchia . This tunnel is one of fifty-two single-line tunnels, with a gradient of I in 40, on the main line between Florence and Bologna, built by Thomas Brassey . There is a grey deal of traffic which has to be worked by heavy locomotives .

Before the

installation of a ventilating system under any condition of wind the state of this tunnel, about 3000 yds, in length, was bad ; i In the case of this circular drift-way a velocity of 4000 ft. per minute was subsequently attained . 2 Quick-running fan.but when the wind was blowing in at the lower end at the same time that a heavy goods or passenger train was ascending the gradient the condition of affairs became almost insupportable . The engines, working with the regulators full open, often emitted. large quantities of both smoke and steam, which travelled concurrently with the train . The goods trains had two engines, one in front and another at the rear, and when, from the humidity in the tunnel, due to the Fan (From the Proc . Inst . Cis . Eng.) steam, the wheels slipped and possibly the train stopped, the state of the air was indescribable . A heavy train with two engines, conveying a royal party and their suite, arrived on one occasion at the upper exit of the tunnel with both enginemen and both fire-men insensible; and on another occasion, when a heavy passenger train came to a stop in the tunnel, all the occupants were seriously affected . In applying the Saccardo system, the' tunnel was extended for 15 or 20 ft. by a structure either of timber or brickwork, the inside line of which represented the line of maximum construction, and this was allowed to project for about 3 ft. into the tunnel . The space between this line and the exterior constituted the chamber into which air was blown by means of a fan . Considering the length of tunnel it might at first be thought there would be some tendency for the air to return through the open mouth, but nothing of the kind happened . The whole of the air blown by the fan, 164,000 cub. ft. per minute, was augmented by the induced current yielding 46,000 cub. ft. per minute, making a total of 210,000 cub. ft.; and this volume was blown down the gradient against the ascending train, so as to free the driver and men in charge of the train from the products of combustion at the earliest possible moment .

Prior to the installation of this system the drivers and firemen had to be clothed in thick woollen garments, pulled on over their ordinary clothes, and wrapped round and round the neck and over the head; but in spite of all these precautions they sometimes arrived at the upper end of the tunnel in a state of insensibility . The fan, however, immensely improved the condition of the air, which is now pure and fresh . In the case of the St Gotthard tunnel, which is 91 M. in length and 26 ft. wide with a sectional area of 603 sq. ft., the Saccardo system was installed in 1899 with most beneficial results . The railway is double-tracked and worked by steam locomotives, the cars being lighted by gas . The ventilating plant is situated at Goschenen at the north end of the tunnel and consists of two large fans operated by water power . The quantity of air passed into the narrow mouth of the tunnel is 413,000 cub. ft. per minute at a velocity of 686 ft., this velocity being much reduced as the full section of the tunnel is reached . A sample of the air taken from a carriage contained 10.19 parts of carbonic acid gas per 10,000 volumes . In the Simplon tunnel, where electricity is the motive power, mechanical ventilation is installed . A steel sliding door is arranged at each entrance to be raised and lowered by electric power . After the entrance of a train the door is lowered and fresh air forced into the tunnel at considerable pressure from the same end by fans . The introduction of electric traction has simplified the problem of ventilating intra-urban railways laid in tunnels at a greater or less distance below the surface, since the absence of smoke and products of combustion from coal and coke renders necessary only such a quantity of air as is required by the passengers and staff . For supplying air to the shallow tunnels which form the under-ground portions of the Metropolitan and District railways in London, oren staircases. blow-holes and sections of uncovered track ate relied on .

When the lines were worked by steam locomotives they afforded notorious examples of bad ventilation, the proportion of g 7 SFr Chamber ~I II IY111ii ~l4 Air Chamber

Gloucestershire carbonic acid amounting to from 15 or 20 to 60, 70 and even 89 parts in Io,000 . But since the adoption of electricity as the motive power the atmosphere of the tunnels has much improved, and two samples taken from the cars in 1905 gave 11.27 and 14.07 parts of carbonic acid in Io,000 . When deep level " tube " railways were first constructed in London, it was supposed that adequate ventilation would be obtained through the lift-shafts and staircases at the stations, with the aid of the scouring action of the trains which, being of nearly the same cross-section as the tunnel, would, it was supposed, drive the air in front of them out by the openings at the stations they were approaching, while drawing fresh air in behind them at the stations they had left . This expectation, however, was disappointed, and it was found necessary to employ mechanical means . On the Central London railway, which runs from the Bank of England to Shepherd's Bush,adistance of 6 m., the ventilating plant installed in 1902 consists of a 300 h.p. electrically driven fan, which is placed at Shepherd's Bush and draws in fresh air from the Bank end of the line and at other intermediate points . The fan is 5 ft. wide and 20 ft. in diameter, and makes 145 revolutions a minute, its capacity being Ioo,000 cub. ft. a minute . It is operated from I to 4 a.m., and the openings at all the intermediate stations being closed it draws fresh air in at the Bank station . The tunnel is thus cleared out about 2t times each night and the air is left in the same condition as it is outside . The fan is also worked during the day from II a.m. to 5 p.m., the intermediate doors being open; in this way the atmosphere is improved for about half the length of the line and the cars are cleared out as they arrive at Shepherd's Bush . Samples of the air in the tunnel taken when the fan was not running contained 7.07 parts of carbonic acid in 10,000, while the air of a full car contained IO'7 parts . The outside air at the same time contained 4.4 parts . A series of tests made for the London County Council in 1902 showed that the air of the cars contained a minimum of 9.6o parts and a maximum of 14.7 parts .

In some of the later tube railways in London—such as the

Baker Street and Waterloo, and the Charing Cross and Hampstead lines—electrically driven exhaust fans are provided at about half-mile intervals; these each extract 18,500 cub. ft. of air per minute from the tunnels, and discharge it from the tops of the station roofs, fresh air being conveyed to the points of suction in the tunnels . The Boston system of electrically operated subways and tunnels is ventilated by electric fans capable of completely changing the air in each section about every fifteen minutes . Air admitted at portals and stations is withdrawn midway between stations . In the case of the East Boston tunnel, the air leaving the tunnel under the middle of the harbour is carried to the shore through longitudinal ducts (fig . 3) and is there expelled through fan-chambers . In the southerly 5 M. of the New York Rapid Transit railway, which runs in a four-track tunnel of rectangular section, having an area of 65o sq. ft., and built as close as possible to the surface of the streets, ventilation by natural means through the open stair-cases at the stations is mainly relied upon, with satisfactory results as regards the proportions of carbonic acid found in the air . But when intensely hot weather prevails in New York the tunnel air is sometimes 5° hotter still, due to the conversion of electrical energy into heat . This condition is aggravated by the fine diffusion through the air of oil from the motors, dust from the ballast and particles of metal ground off by the brake shoes, &c . Volume of Air Required for Ventilation.—The consumption of coal by a locomotive during the passage through a tunnel having been ascertained, and 29 cub. ft. of poisonous gas being allowed for each pound of coal consumed, the volume of fresh air required to maintain the atmosphere of the tunnel at a standard of purity of 20 parts of carbon dioxide in 10,000 parts of air is ascertained as follows: The number of pounds of fuel consumed per mile, multiplied by 29, multiplied by 500, and divided by the interval in minutes between the trains, will give the volume of air in cubic feet which must be introduced into the tunnel per minute . As an illustration, assume that the tunnel is a mile in length, that the consumption of fuel is 32 lb per mile, and that one train passes through the tunnel every five minutes in each direction; then the volume of air required per minute will be 32 lb X 29 cub. ft . X 5o0 =185,600 cub. ft . 21 minutes .

Corrosion of Rails in Tunnels.—Careful tests made in the

Box and Severn tunnels of the Great Western railway, to ascertain if possible the loss that takes place in the weight of rails owing to the presence of corrosive gases, gave the following results: Box TUNNEL (1 m . 66 chains in length) . Percentage of Wear per annum . lb per yard Down line, gradient falling I in too— % per annum . At east mouth 0'439 = 0'377 28 chains from east mouth 1 Boo =1.540 48 chains from east mouth 2.110=1.810 1 m . 8 chains from east mouth 2.88o =2.48o At west mouth o.64as= =c2Up line, gradient rising 1 in too At east mouth 0.620=0.575 t m . 8 chains from east mouth 1.500=1.380 t m . 28 chains from east mouth ..