OVKMBEK 1, 1900.] 



KNOWLEDGE. 



247 



In order to surmount these vai-ious obstacles it was 

 wisely decided that the mounting be that of the sidtrostat, 

 such as perfected by Lt?ou Foueault. a man of remark- 

 able mechanical ingenuity. The siderostat is an old 

 invention. During the eighteenth century, Parrault had 

 already construct.ed a mechanism based on the same 

 principle, while " in 1790, ' says Lalande,* " an able 

 London optician, named Brown, made a telescope whose 

 tube is always horizontal, and in which a plane mirror 

 reflects the image of the object into the eye-piece." 



The siderostat thus consists of a plane mirror, M 

 (Fig. 1), moved by a clockwork motion communicated 

 to the axis PQ, parallel to the axis of the earth, and 



two axes, a horizontal axis, at right angles to the [)a[)i'i- 

 at O, and a vertical axis OO'. At the back and centre 

 of the mirror's cell is fixed a rod, ON, on which glides 

 a muff, N, hold by a fork, PN, attached to the polar 

 axis. The length of the fork, PN, being equal to the 

 distance OP, the triangle NOP is, in all positions of the 

 mirror, isosceles, and / " — / ''■ ''''it / <' = / n = / m, 

 the angles of refleition and iiuiiii'iice. Henci^ / in ■=. /h, 

 and thus the direction of the fork PN is always parallel 

 to the incident ray. 



Now, in virtue of the problem of revolving mirrors, 

 the angular velocitv of tlie mirror is one-half that of 

 the celestial sphere. This is obtained by imparting to 



Fig. 2. — The Great Siderostat of Paris, 1900. .\, Riglit ascension axis; ]{, Riyht ascension circle; C, Ueclinafcion axis; 

 D, Declination semi-circle; E, Fork attached to the declination axis; F, Muff held by the fork ; G-, l!rass rod fixed nonniill^ to tlie 

 mirror's cell; H, Cell of the mirror; 1,. Silver on glass mirror of the siderostat ; K, Screw allowing; of tlie mirror being laken out of the cell ; 

 L, Counterpoises equilibrating the mirror ; M, Great forked support of the mirror ; N, Cylinder containing mercury, enabling the floating 

 of the mass M; O, Telescope for reading the divisions of the riglit ascension circle; U', Telescope for the <lecli nation circle: ]',, Dandle 

 for slow horary movements ; P„, Handle for rapid movements in right ascension ; P,, Handle for motion in declination ; 1*,, Handle tor 

 winding the clock ; Q,, Strings for clamping and unclamping in right ascension ; R, Clockwork motion ; .S, Weight of tlie clock. 



sending the reflected beam along the optical axis of a 

 fixed telescope, AB.f The mirror is moveable round 



* Quoted by Delaunay, Court EUmentaire cCAstronomie, 7e cd., 

 p. 174. 



t For clearness's sake, the refractor has been monstrously shortened 

 on the above diagram. 



the axis PQ a double velocity, so as to make it rotate 

 once in 24h. Inasmuch as the centre of rotation of the 

 mirror is at O, and not on the middle of NO, its rotation 

 is twice slower than that of the axis, being effected once 

 in 48h., since an angle at the circumference of a circle 

 is one-half the angle at the centre. 



