228 Seientific Proceedings, Royal Dublin Society. 



the end of its run ; consequently its differential circle has to be re-set every 

 time tlie sector is wound back, or at any time the clock is stopped. 



There is a second objection also, that the vernier by wliich this circle is 

 read varies its place from time to time, and may be anywhere round the circle ; 

 consequently it is less convenient to read than a fixed vernier, more especially 

 in the case of large instruments, where it is necessary to read these circles 

 through a microscope from a distance. 



In the large equatorials which are now being built for Johannesburg and 

 Santiago, an arrangement has been made at the suggestion of Sir David Gill, 

 F.K.S., by which this differential circle is kept continually moving in the proper 

 direction— not by the main clock of the equatorial itself, but by a series of 

 electrical contacts from the sidereal clock of the observatory. So long as 

 these contacts act perfectly, this completely obviates the first objection above 

 mentioned; but there is still the objection remaining that the vernier is to 

 be found in various positions round the circle under different circumstances. 

 I have therefore devised another form in whicli this objection is avoided. 



In this last form (see fig. 3, Plate XVII.) tlie differential circle is caused 

 to travel backwards as regards the polar axis, by a piece of clockwork which is 

 carried upon the axis itself, and in this way it is possible to read tlie actual A.R. 

 of a star from a fixed vernier without any reference to the sidereal clock, and 

 this small subsidiary clock which is carried upon the polar axis is always 

 acting, whether the telescope is in use or not — or whether the main clock is 

 working or not — so, as long as this is kept wound up and going, the instru- 

 ment can be set to actual A.R. without reference to the sidereal clock, and 

 thus avoids both the objections mentioned above to the existing forms. 



Desckiption of Fig. 3, Plate XVII. 



AA. A.R. circle read by vernier Ffor actual right ascension, and also by vernier 

 V (which is carried on an arm fastened to the polar axis) for sidereal time. 



The A.R. circle AA. is strung loosely on the boss of the disc B.D., so that it can 

 be set to correct reading at any time, after which it is kept revolving backwards on 

 the polar axis by the independent piece of clockwork and escapement C, the driving 

 force of which is a weight attached to a cord passing round the F-grooved circle//. 



