a LamptMkrc meter . 1 6£v 
quainted .with works of this kind are but too fenfible how diffi- 
cult it is to have fcrews that {hall he perfectly equal in every 
thread or revolution of each thread ; or pinions and bars that 
{hall be fo evenly divided as perfectly to be depended upon in 
every leaf and tooth to perhaps the two, three, or four tliou- 
fandth part of an inch; and yet, on account of the fnaall 
fcale of thofe micrometers, thefe quantities are of the greateft 
confequenee ; an error of a fingle thoufandtji part inducing in. 
moft.inftruments a miftake of leveral feconds. 
The lad: and greateft imperfection oi all is, that thefe wire, 
micrometers require a pretty ftrong light in the held of view : 
and when ! had double liars to meafure, one of which was very 
obfcure, I was obliged to be content with lets light than is ne-- 
ceflary to make the wires perfectly diflinCl ; and feveral liars 
on this account could not be meafured at all, though other- 
wife not too clofe for the micrometer. 
The iuftrument I am going to defcribe, which I call a 
Lamp-Micrometer, is free from all thefe defeCls, and has, 
moreover, to recommend it, the advantage of a very enlarged 
fcale. The conllruCtion of it is as follows. 
ABGCFE (fig. i ) is a hand nine feet high, upon which a 
femi-circular board qhogp is moveable upwards or downwards, , 
in the manner of fome fire-fcreens, as occafion may require, and 
is held in its fituation by a pegp put into any one of the holes 
of the upright piece AB. This board is a fegment of a circle 
of fourteen inches radius, and is about three inches broader 
than a femi-circle, to give room for the handles rD, rP, to 
work. The ule of this board is to carry an arm L, thirty 
inches long, which is made to move upon a pivot at the center 
of the circle, by means of a bring, which pabes in a groove 
upon the edge of the femi-circle pgohq ; the bring is fabened i 
