GrKUBB — Improvements in Equatorial Telescope Mountings. 227 



Improved Arrangement of Differential Hour-Circles for 

 Equatorial Telescopes. 



The position in the lieavens of auy celestial object at any given moment 

 is determined by : — 



(«) Its declination (that is the number of degrees, etc., north or south 

 of the Equator) ; 



[b) Its B.A. (that is, its angular distance from a certain fixed point in 



the heavens measured in a direction parallel to the Equator) ; 

 and 



(c) The time as shown by the sidereal clock. 



Setting a telescope in declination is comparatively simple, because the 

 instrument has only to be turned until the correct declination is read upon the 

 declination circle ; but setting in the other direction is not so simple, because 

 it depends upon two quantities, that of its right ascension and the time as 

 given by the sidereal clock at that particular moment. As this time is con- 

 tinually varying, the setting in this direction is more complicated than in the 

 direction of declination. If a single circle be used, the reading of that circle 

 from a fixed vernier should be the difference between the time as given 

 by the sidereal clock and the A.R. of the star. This involves a reading 

 of the clock and an arithmetical operation for every setting, which is 

 troublesome in an observatory ; and to avoid this trouble a device 

 was introduced (I think by the late Sir George Airy) of a second circle, 

 which was strung loosely upon the axis, and could be set from a fixed vernier 

 to read the time as given by the sidereal clock. Differential readings 

 between this and the circle fixed to the axis would give true A.R.s, without 

 the troublesome arithmetical computation. As, however, the setting of this 

 differential circle (which should always read sidereal time) varies from 

 minute to minute, the device was adopted of connecting it to the main driving 

 clock of the equatorial, so that when once set for sidereal time it would still 

 read, at any other time of the night, true sidereal time, as it was carried round 

 by the clock-work at the corresponding rate. So long as that circle was kept 

 moving round by the equatorial clock differential readings between the two 

 circles would give the correct A.R. of any star that the telescope was pointed 

 upon without the arithmetical operation of subtraction. In other words, it 

 formed a mechanical subtraction machine. ■ This is the form that is usually 

 adopted in modern equatorial telescopes. 



There are objections, however, to this arrangement. Most telescopes are 

 driven by a sector or portion of a circle only ; and this has to be wound back at 



