See also:

• HELIOSTAT (from Gr. rtXtor, the sun, vrares, fixed, set up)

HELIOSTAT (from Gr. rtXtor, the See also:

• SUN
• SUN (0. Eng. sonne, Ger. sonne. Fr. soleil, Lat. sol, Gr. ijXior, from which comes helio- in various English compounds)

sun, vrares, fixed, set up)  , an See also:

• INSTRUMENT (Lat. instrumentum, from insincere, to build up, furnish, arrange, prepare)

instrument which will reflect the rays of the See also:

• SUN
• SUN (0. Eng. sonne, Ger. sonne. Fr. soleil, Lat. sol, Gr. ijXior, from which comes helio- in various English compounds)

sun in a fixed direction notwithstanding the See also:

• MOTION (Lat. motio, from movere, to move)
• MOTION, LAWS OF

motion of the sun . The See also:

• OPTICAL

optical ' apparatus generally consists of a See also:

• MIRROR (through O. Fr. mirour, mod. miroir, from a sup-posed Late Lat. miratorium, from mirari, to admire)

mirror mounted on an See also:

• AXIS (Lat. for " axle ")

axis parallel to the axis of the See also:

• EARTH
• EARTH (a word common to Teutonic languages, cf. Ger. Erde, Dutch aarde, Swed. and Dan. jord; outside Teutonic it appears only in the Gr. 'pq'e, on the ground; it has been connected by some etymologists with the Aryan root ar-, to plough, which is seen in
• EARTH, FIGURE OF
• EARTH, FIGURE OF THE

earth, and rotated with the same angular velocity as the sun . This construction assumes that the sun describes daily a small circle about the See also:

• POLE (FAMILY)
• POLE, REGINALD (1500-1558)
• POLE, RICHARD DE LA (d. 1525)
• POLE, WILLIAM (1814—1900)

pole of the See also:

• CELESTIAL, OR STELLAR

celestial See also:

• SPHERE (Gr. vclsa?pa, a ball or globe)

sphere, and ignores any diurnal variation in the See also:

• DECLINATION (from Lat. declinare, to decline)

declination . This variation is, however, so small that it can be neglected for most purposes . ' See also:

• DOLLOND, JOHN (1706—1761)

Dollond provides for changing the See also:

• POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power by sliding the See also:

• LENS
• LENS (from Lat. lens, lentil, on account of the similarity of the form of a lens to that of a lentil seed)

lens d nearer to or farther from a . Many forms of heliostats have been devised, the earliest having I been described by Wilhelm See also:

• JACOB
• JACOB, JOHN (1812-1858)

Jacob s'Gravesande in the 3rd edition of his Physices elementa (1742) . One of the simplest consists of a I See also:

• PLANE

plane mirror rigidly connected with a revolving axis so that the See also:

• ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle between the normal to the mirror and the axis of the instrument equals See also:

• HALF

half the sun's polar distance, the mirror being adjusted so that the normal has the same right See also:

• ASCENSION
• ASCENSION, FEAST OF THE

ascension as the sun . It is easily seen that if the mirror be rotated at the same angular velocity as the sun the right ascensions will re-See also:

• MAIN (from the Aryan root which appears in " may " and " might," and Lat. magnus, great)
• MAIN (Lat. Moenus)

main equal throughout the See also:

• DAY (O. Eng. dreg, Ger. Tag; according to the New English Dictionary, " in no way related to the Lat. dies")
• DAY, JOHN (1574-1640?)
• DAY, THOMAS (1748-1789)

day, and therefore this See also:

• DEVICE

device reflects the rays in the direction of the earth's axis; a second fixed mirror reflects them in any other fixed direction . See also:

• FOUCAULT, JEAN BERNARD LEON (1819—1868)

Foucault's See also:

• HELIOSTAT (from Gr. rtXtor, the sun, vrares, fixed, set up)

heliostat reflects the rays horizontally Fin. t. in any required direction . The principle of the apparatus may be explained by reference to fig . 1 . The axis of -rotation AB bears a rigidly attached See also:

• ROD (O.E. rodd, probably related to Norw. rudda, stick, rodda, stake)
• ROD, EDOUARD (1857-1910)

rod DBC inclined to it at an angle equal to the sun's polar distance .

By adjusting the right ascension of the plane See also:

• ABC

ABC and rotating the axis with the angular velocity of the sun, it follows that BC will be the direction of the See also:

• SOLAR, SOLLER (Lat. solarium, Fr.• galetas, Ital. solaio)

solar rays throughout the day . X is the mirror rotating about the point E, and placed so that (if EB is the See also:

• HORIZONTAL

horizontal direction in which the rays are to be reflected) (t) the normal CE to the mirror is jointed to BC at C and is equal in length to BE, (2) the rod DBC passes through a slot in a rod ED fixed to, and in the plane of, the mirror . Since CE equals BE these directions are equally inclined to, and coplanar with, the normal to the mirror . Hence See also:

• LIGHT

light incident along the direction BC will be reflected along CE . Silbermann's heliostat reflects the rays in any direction . The principle may be explained by means of fig . 2 . AB a. is the axis of rotation, BC an adjustable rod as in Foucault's construction, and BD is another rod which can be set to the direction in which the rays are to be reflected . The rods BC and DB carry two small rods EF, GF jointed at F; at this See also:

• JOINT (through Fr. from Lat. junctum, jungere, to join)

joint there is a See also:

• PIN (a doublet with " pen " from Lat. pinna, feather, pinnacle, which is said to contain the same root as irirvs, pine tree, and properly to mean a sharp point or end)

pin which slides in a slot on the rod BH, which is normal to the mirror X . Theto the framework pate, the members of which are parallel to the incident and reflected rays SO, OR, and the See also:

• DIAGONAL (Gr. &a, through, ywvia, a corner)

diagonal pf is perpendicular to the mirror . The framework is attached to two See also:

• INDEPENDENT

independent circular arcs Cs and rr' having their centres at 0 and provided with clamps D and A on the axis F of the instrument . The arc Cs is graduated, and is set so that the angle See also:

• COD

COD equals the See also:

• COMPLEMENT (Lat. complementum, from complere, to fill up)

complement of the sun's declination .

This can be effected (after setting the axis) by rotating Cs until a See also:

• NEEDLE (O. Eng. ncedl; the word appears in various forms in Teutonic languages, Ger. Nadel, Dutch naal, the root being ne-, to sew, cf. Ger. nahen, and probably Lat. nere, to spin, Gr. vi)ats, spinning)

needle indicates true See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

time on the See also:

• HOUR

hour See also:

• DIAL

dial B . The arc rr is set so as to reflect the rays in the required direction . The axis F of the instrument is set at an angle equal to the See also:

• LATITUDE (Lat. latitudo, latus, broad)

latitude of the See also:

• PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place of observation and in the See also:

• MERIDIAN
• MERIDIAN (from the Lat. meridianus, pertaining to the south or mid-day)

meridian by means of the See also:

• SCREW (O.E. scrue, from O. Fr. escroue, mod. ecrou; ultimate origin uncertain; the word, or a similar one, appears in Teutonic languages, cf. Ger. Schraube, Dan. skrue, but Skeat, following Diaz, finds the origin in Lat. scrobs, a ditch, hole, particularl

screw K, and rotated by clockwork contained in the See also:

• BARREL (a word of uncertain origin common to Romance languages; the Celtic forms, as in the Gaelic baraill, are derived from the English)

barrel H . The setting in the meridian is effected by turning the instrument after setting for latitude until a pin-hole See also:

• APERTURE (from Lat. aperire, to open)

aperture s and a small See also:

• SCREEN (usually, but very doubtfully, connected with Lat. scrinium, a box for holding books, from scribere, to write; a connexion with Ger. Schranke, barrier, has been suggested)

screen P, placed so that Ps is parallel to CO, are in a See also:

• LINE

line with the sun . Many other forms of heliostats have been designed, the See also:

• CHIEF (from Fr. chef, head, Lat. caput)

chief difference consisting in the See also:

• MECHANICAL

mechanical devices for maintaining the See also:

• CONSTANT, BENJAMIN (1845-1902)

constant direction of the reflecting See also:

• RAY (Lat. raia)
• RAY (or WRAY, as he wrote his name till 1670), JOHN (1628-1705)

ray . One of the most important applications of the heliostat is as an See also:

• ADJUNCT (from Lat. ad, to, and jungere, to join)

adjunct to the newer forms of horizontal telescopes (q.v.) and in See also:

• CONJUNCTION (from Lat. conjungere, to join together)

conjunction with spectroscopic telescopes in observations of eclipses .