308 PHILOSOPHICAL TRANSACTIONS. [aNNO 1793. 



is merely for the convenience of reversing the instrument : for though it might 

 be used out of the meridian, and for azimuths ; yet since it is designed principally 

 for meridian passages, when it is in its place the whole is clamped firmly to the 

 bottom frame by 4 clamps, which confine it to the circle on which it rides : and 

 this method of turning proves itself to be steady, by the levels on the bottom 

 plate never altering in the least on screwing the clamps. The 4 pillars, and their 

 braces, explain themselves. They stand over the bell-metal circle ; and the 

 clamps are placed near the foot of each, for greater steadiness, since they carry 

 the y's for the pivots of the transit. 



Mr. W. then gives a description of the other parts of the instruments, with 

 references to several engraved representations. 



There is a level for adjusting the axis. The circle was ordered to have 10 

 radii ; that when the telescope is horizontal, and pointing to a meridian mark, 

 there might be a vacancy between the cones, above or below, for introducing a 

 level. In the brace between the pillars, over the moveable y, the bottom bar is 

 omitted ; in order to give the better room for passing the level, without inclining 

 it, or running any hazard of striking it, From the lower bar of the opposite 

 brace, over the fixed y, there stands out a forked piece of brass, to receive the 

 leg of the level, and direct it to its place ; as also for keeping it upright when the 

 foot stands on the pivot, and just allowing a very little shake, -so as not to cramp 

 it. By this contrivance the level is easily handled, and reversed, without danger 

 of disturbing it or the instrument. 



The circle itself is 2 feet diameter at the divisions ; being "23-^ inches at the 

 edge. The undivided circle, on the side of the telescope next to the open end 

 of the axis, serves for strength and uniformity ; and to it is applied the clamp for 

 elevation. That clamp is so made, as to allow the circle to run freely all round ; 

 not bearing at all against it, but supporting itself, and yet being easily remove- 

 able. It has no command over the circle whatever, when handled with care, 

 excepting in the altitude of the telescope, by an adjusting screw when the clamp 

 is set : and as that screw has a milled head at each end, it is as conveniently 

 turned from the one as from the other side of the instrument to bring the hori- 

 zontal wire to bisect the object. 



The telescope is of 1 inches aperture and 33 focal length. The object-glass 

 does not slide within the tube ; but screws into the end of a piece of false tube, 

 of 4 inches length, which slides on the outside of the principal tube, and is fixed 

 in its place, by 3 screws and collars running in grooves, when its distance from 

 the wires is adjusted. In this way, we have the whole aperture of the tube ; and 

 no greater length than is absolutely necessary for use; which, in such an instru- 

 ment, appeared to be an advantage. 



The wires are not in one cell ; but in 2 distinct cells, with their faces towards 



