C 177 ) 
kfs than the Body of the Screw, and goes through the 
Center of the Round Plate without ftiaking; and to pre- 
vent its doing Co, either upwards or downwards, there 
is added a fpringing Plate N, which keeps the Shoulder of 
the Screw clofe to the under fide of the Plate K; over this 
Plate there goes an Index O, and over that an Handle L, 
upon the End of the Screw which comes through theCen- 
ter of the. Plate, which I (hould before have told you, 
is riveted to the Top of the Column B B. The Teeth 
of the Screw muft be of that Size, as to have juft Ten in 
an Inch. The fore fide of the Column muft be divided into 
Inches and Tenths, beginning about the Height of the 
Socket H, where the lower end of the Screw refts, and 
fo continue to the Top of the Column. The Limb of 
the Round Plate muft be divided into an Hundred Parts, 
In the Focus of the Eye-Glafs of the Microfcope is fixed 
an Hair, or very fine Silver Wire, in a Horizantal 
Pofition. ^ ^ 
I come now to fliew the Ufe of the Inftrument com- 
bined wirh the Barometer, in which I (hall be very brief, 
irs Ufe being eafily apprehended by the Defcrsption. 
• Take hold of the Handle, and, booking through the 
Microfcope, turn the Screw till ygu have brought the 
Hair to.touch, as it were, tjie Surface of the Mercury M ; 
then oblerve what Diyifions are cut on the Column, by 
the upper or under' Edge of the Socket, which are 
Tenths of an Inch. See likewife to what Parts the Ini- 
dex points on the Limb of ^the Round Plate, Which yfe 
Himdreds of a Tenth, or ThbulSnd Parts of an Infih^ 
when you perceive the MSrcdry ^aried;^ nife pr^eprel^ 
the, Microicope, till the Hair be brought tb its Surface, 
as before ; then by fabftrading the leffer from the great- 
er of the TwoobfervedNumbers; you wilihave^heJ/a- 
lriition in Inches and Thouiand Parti-^^ t^^% v^\ ^ 
Dd 
