VOL. LXXXV.] PHILOSOPHICAL TKANSACTIOKS. 603 



time, he may order the whole frame to be moved by the great round motion, 

 which ought to be in readiness ; and may even keep his object in view, as I have 

 often done, by screwing the telescope backwards as fast as the round motion 

 advances it. Then screwing himself forward again, he may repeat these succes- 

 sive motions as long as he pleases. 



In those observations, which I have called sweeps (from the method of oscil- 

 lating or sweeping over an arch, which at first I had adopted in the way of right 

 ascension, but which in the year 1783, I reduced to a systematical method of 

 sweeping over zones of polar distance), several conveniences are required : the 

 principal of them are as follow. An assistant, provided with an apparatus for 

 writing down observations; with catalogues of stars, atlasses, and other re- 

 sources of that kind. A small apartment, as near to the observer as possible, 

 in which this apparatus, with candles and other conveniences, may be inclosed. 

 A sidereal time-piece. A right ascension apparatus. A polar distance apparatus. 

 A polar distance clock. A zoned catalogue of the stars. And a ready commu- 

 nication between the observer and assistant, both ways. There is also wanting, 

 a person to give the required motions for sweeping the zones of the heavens. 

 A micrometer-motion to perform the sweeps. A zone-piece, to point out the 

 required limits of the intended zones. A small apartment to inclose these mo- 

 tions, and the candle which is required for the workman. And a ready com- 

 munication, for the observer to direct the workman in the required motions. 

 The distance between the observatory and the end of the telescope, is evidently 

 too far for a conversation in the open air, between the observer and assistant ; 

 especially as the latter, on account of his candles, must be inclosed ; and ought 

 not to leave his post at the time-piece and writing-desk. Add to this, that when 

 the observer is elevated 30 or 40 feet above the assistant, a moderate breeze will 

 carry away the sound of his voice very forcibly. A speaking-pipe was therefore 

 necessary, to convey the communications of the observer to their destination. 

 At the opening of the telescope, near the place of the eye-glass, is the end of a 

 tin pipe, into which at the time of observation a mouth- piece, may be put, which 

 can be adapted, by drawing out, or turning side-ways, so as conveniently to 

 come to the mouth of the observer, while his eye is at the glass. This pipe is 

 14^ inch in diameter, and runs down to the bottom of the telescope, to which it 

 is held by proper hooks, that go into the tube, and are screwed fast at the inside. 

 When it is arrived as near to the axle as convenient, it goes into a turning joint; 

 thence into a drawing tube, and out of this into another turning joint; from 

 which it proceeds by a set of sliding tubes towards the front of the foundation 

 timber. 



The right ascension apparatus is constructed thus. Against the sides of the 

 tube, and 2 feet 6 inches from the mouth of it, are fixed the centres of two 



4 h2 



