144- Pneumatics.

S I N c E the Gravity of the Air is so va>rious, that at one time it will sustain a P' 1 ' 1 'lar of Mercury 31 Inches high, when ^another it will raise it but to the Heigh *- 0

28 Inch eS(

below them, will be less than the Force of Gravitf

w

c

which they tend downwards.. The Rarefaction °‘ Q ,Air must therefore He bounded, where these two of?site Forces come to balance each other.

3. Now though we cannot possibly define the

of the Atmosphere, yet we may still investigate 7 1 ^much the Air is rarefied at any proposed Altitude $ j;the Earth’s Surface : For doing which, severalhave been proposed ; some of which are very tessi 0 ^and difficult to be understood. I shall here illustra^ pIsaac’s Theorem for that Purpose, which is very c0l f t uand plain. It requires only two different Densities ,Air, at two given Altitudes above the Earth’s Sun^j,to be known, and which we easily obtain by E^Pment as follows. ,< 0 \

4. Take a Vial AEFB, fill’d two Thirds se 1 '„Water to C D ; in which let a long Tube I G i°soat both Ends) be immersed, and closely cemen te jthe Vial at A B, so that none of the included Ahescape. This done, blow a little Air through ^Tube into the Vial, which increasing the Spring 0contained Air, will cause it to raise and support 2 plumn of Water in the Tube, to such a Height H, th*.^Weight, together with that of a Column of Air

on its Surface H, is equivalent to the increased ^P 1of the confined Air.

5. The Vial and Tube thus prepared are to b e

ried ■ ~ “

up to the Top of a Tower, Mountain, or sem 6 stPlace; and in . the Ascent, since the Column (0pressing on the Water at H is constantly shortens ^its Force of Pressure will be diminished, Tlse-Wpii*of the Air in the Vial will therefore keep the , lC i>of Water constantly rising in the Tube ; so that vyou have ascended the Height of about 72 f eet ’ t ),FWater in the Tube will have risen from H to E, /