the small Plate call’d the Nonius, so contrived as 1slide by the other in such manner that its Index Dbe always set on one Part to the Surface of the M ercury, and on the other End pointing to the Divisi 0in the Scale of Inches corresponding thereto. Ag 3ls> ’the Nonius is divided into io equal Parts, whichgether are equal to n of the Divisions of the Sea 1 ®’that is, DE = xi Tenths of an Inch; and coiu®^•quently each small Division of the Nonius is equalI,I ; two of them to 2,2 ; three of them to 3,3 ; ^so on. Whence ’tis easy to observe, that if the Ind®.D points between any two Divisions of the Scalehere between 29,7 and 29,8, we need only lookfee what Division of the Nonius coincides with

35

vstt

to

Division of the Scale, and that will strew how ifTenths of a Tenth, that is, how many Tenths he)"^29,7 in the present Cafe : But you observe thecoincides with a Division of the Scale at the fifth ”vision; consequently, the Mercury stands at 2 9 ’j^Inches in the Scale; and so you proceed with j,greatest Ease to the hundredth Part of an Inch, v/h 1is a great Degree of Exactness. ^

2i. From. what has been said we may easily f £ f ^excellent Use of the Barometer in measuring the H el S,,,sof Places, as Mountains, Towers, Ejfc. For since I j swe shall strew) the specific Gravity of Air (such aS tslnear the Earth’s Surface) is to that of Mercury a s ^jjj12040, ’tis plain 12040 Inches of Air in Heightbalance one Inch Height of Mercury ; confequ eI ) 'J1204 Inches, or 100 Feet, answers to T '- 6 of an I 11Mercury. Therefore if a good Barometer be ect^to the Top of a Mountain, or other high Place? ^Mercury will subside near one Tenth of an In c |j. ^every 100 Feet of perpendicular Ascent, and so Ma proper Index of the whole Height ascended.

22.

made by Dr. Nettleton very exactly, as inbelow.

Alti £l

— But since Mercury is not quite 14 times M 3than Water, the Number 12040 is somewhat too *and therefore a less Height than 100 Feet will an^^f;T 5 of an Inch Descent of Mercury in the BaroM®^;and what that is will be shewn from the Expels

the I