[ 6og ] 
tad with the point of the micrometer- fere w more 
perfect. 
7, is the micrometer-fcrew ; V \ is the divided index- 
plate, and W a knobb for the handle. 
The micrometer-fcrew pafles thro’ two folid ferew’d 
holes at D and T. 
The piece T Z is made a little fpringy, and endea- 
vours to pull the ferew backwards from the hole at 
7); of confequence keeps the micrometer-fcrew 
conftantly bearing againft its threads the fame way, 
and thereby renders the motion thereof perfectly 
fteady and gentle. 
X, is the index, having divifions upon it, anfwering to 
the turns of the ferew. This piece points out the 
divifions of the plate, as the face of the plate points 
out the divifors upon the index. 
When the inftrument is us’d, lay hold of the knob 
at i^with one hand, and, moving the feeler up and 
down, with the other move forward the ferew 7, till 
its point comes in contadl with the feeler ; then will 
the plate and index Eand X Ihew the turns, and parts. 
Fig. 2 . reprefents the inftrument immerg’d in its 
ciftern of water, ready for ufe. 
A B , is the ciftern ; C, the cover ; which, when the 
inftrument Jig. i. is rais’d upon blocks, goes on be- 
tween the bar E F and the bafis B C. 
D, a handle to take off the cover, when hot ; E, the 
jnercurial thermometer j jF, the cock to let out the 
water. 
G H , a hollow piece of tin, which fupports feven fpi- 
rit lamps, which are rais’d higher or lower by the 
ferews 1 and K , in order to give the water in the 
ciftern a proper degree of heat. 
4 H 
A TABLE 
