230 I'HILOSOl'HICAL TRANSACTIONS. [ANNO 1782. 



every micrometer hitherto used requires either a screw, or a divided bar and 

 pinion, to measure the distance of the wires or divided image. Those who are 

 acquainted with works of this kind are but too sensible how difficult it is to have 

 screws that shall be perfectly equal in every thread or revolution of each thread ; 

 or pinions and bars that shall be so evenly divided as perfectly to be depended on, 

 in every leaf and tooth, to perhaps the 2, 3, or 4 thousandth part of an inch ; 

 and yet, on account of the small scale of those micrometers, these quantities 

 are of the greatest consequence; an error of a single thousandth part inducing 

 in most instruments a mistake of several seconds. The last and greatest imper- 

 fection of all is, that these wire micrometers require a pretty strong light in the 

 field of view : and when I had double stars to measure, one of which was very 

 obscure, I was obliged to be content with less light than is necessary to make the 

 wires perfectly distinct ; and several stars on this account could not be measured 

 at all, though otherwise not too close for the micrometer. 



The instrument I am going to describe, which I call a lamp-micrometer, is 

 free from all these defects, and has also to recommend it, the advantage of a 

 very enlarged scale. The construction of it is as follows. 



abgcfe (fig. 1, pi. 5,) is a stand 9 feet high, on which a semi-circular board 

 qhogp is moveable upward or downward, in the manner of some fire-screens, as 

 occasion may require, and is held in its situation by a peg p put into any one of 

 the holes of the upright piece ab. This board is a segment of a circle of 14 

 inches radius, and is about 3 inches broader than a semi-circle, to give room for 

 the handles id, ep, to work. The use of this board is to carry an arm l, 30 

 inches long, made to move on a pivot at the centre of the circle, by means of a 

 string, which passes in a groove on the edge of the semi-circle pgohq ; the string 

 is fastened to a hook at o (not expressed in the figure being at the back of the 

 arm l,) and, passing along the groove from oh to q, is turned over a pulley at q, 

 and goes down to a small barrel e, within the plane of the circular board, where 

 a double-jointed handle ep commands its motion. By this contrivance we see 

 the arm l may be lifted up to any altitude from the horizontal position to the 

 perpendicular, or be suffered to descend by its own weight below the horizontal 

 to the reverse perpendicular situation. The weight of the handle p is sufficient 

 to keep the arm in any given position ; but if the motion should be too easy, a 

 friction spring applied to the barrel will moderate it at pleasure. 



In front of the arm l a small slider, about 3 inches long, is moveable in a 

 rabbet from the end l towards the centre, backward and forward. A string is 

 fastened to the left side of the little slider, and goes towards l, where it passes 

 round a pulley at m, and returns under the arm from nm, towards the centre, 

 where it is led in a groove on the edge of the arm, which is of a circular form, 

 upward to a barrel (raised above the plane of the circular board) at r, to which 



