Pliofof/rdphic 0/i(nfii)(/ of fh" Hedven^, aS 



adjnstuients, and with a slmtter to cover the o'ject glass, 

 wliicli can be worked easih' from the eye end. The guider 

 telescope has an object glass of ten inches opening, and 

 thirteen feet focal length, and is fitted with a micrometer, 

 with long range slides and an electric lamp apparatus for 

 illuminating the spider webs in the eye piece, and for 

 illuminating the various setting circles, scales, ka. Every 

 means for setting, clamping and moving are found within 

 convenient reach of the hands, when the eye is at the guider 

 telesco])e. The whole of the moving parts, which amount 

 to nearly two tons, are so balanced and counterpoised as to 

 be operated with the greatest ease, and kept in rotation by 

 the clock work with wonderful smoothness and precision. 

 Nearly everything depends on the precision with which the 

 clock moves the telescope, so as to keep the stars apparently 

 stationary in the field of view. For this pvn-})ose, Sii- H. 

 (irubb has devised a very beautiful arrangement, which, 

 however, is very difficult to clesciibe without models. The 

 mechanism consists first of a powerful clock geai", driven by 

 a heavy weight and controlled by a peculiar kind of governor. 

 This clock work alone drives the telescope, so as to follow 

 diurnal rotation very closely, and will keep a star for an 

 hour together in the field of view of the telescope, but does 

 not control it so accui-atel}- as to keeyj a star bisected on a 

 single spider line in the eye piece of the guider. To secure 

 this, the maker has made a special electric controlling 

 apparatus, which may be tlms described: — The driving 

 clock being adjusted to go as accurately as possible, one of 

 tlie astronomical clocks in the Observatory is made to send 

 a momentary galvanic current every second to an apparatus 

 attached to the driving clock, called the detector. This is a 

 wheel driven by the clock rotating in forty seconds, having 

 three series of contact teeth on its periphery ; pressing on 

 the.se teeth are throe springs electrically connected with 

 another apparatus called a "distributor," which consists of 

 three pairs of electro magnets operating a lever capal»le of 

 moving right oi left and making contact with platinum 

 points, or of being held in a central ]wsition by the central 

 electro magnet. The action is this : — The driving clock is 

 set going, and the astronomical cl(x;k made to send its 

 momentary currents every second through the detector to 

 one or other of its three springs; if through No. 1, the dis- 

 tributor instantaneously moves to the left platinum point; 

 if through No. 3, to the right, and if through No. 2 to the 



