*3

Pneumatics.

and the more so as the Weight ofincumbent Air is greater ; and henceEee the Denjity of the Air is greatest atEarth’s Surface, and decreases upwardst;jj ^* e ° me t r ical Proportion to the Altitudes^ en in arithmetical ProgreJJion. Now it isthat the Air is four Times more rare

at

Hof e -p.

'yhi^ • e nsity is always proportional to the Forcesbty- ^nipresses it, which is also proved by the fol-6 _ ? Experiment.

A^T et: Mercury be poured into an inflected Tube Plates|i eil ’ °pen at both Ends, to a small Height as BC. XXIXsopping the Orifice D very close with a Cork or Fig. 4 .j'icg] " 0 , measure the Length of confined Air DC veryAs 1 3nd pom- Mercury into the other Leg A B, till*he above the Surface of .that in C D be equal to'Ms J'Bht at which it stands in the Barometer. ThenHi/ a ]a the Air in the shorter Leg will be compressedl he tvh^° rce twice as great as at first when it possessedJ^ e Space CD ; for then it was compressed only^oight of the Atmosphere ; but now it isl e hh S ^ tfi^t Weight, and the additional equal°f a Column of Quicksilver. Let E be now3ce os the Mercury in the Leg C D, and upon^ E, the Space into which the Air is now( > S , c f it will be found to be j ust half the former, 7. b that is, DE=iDC.f^bjt eriCe it appears that the Spaces S=DC, and.'^ergj^ phich a given Quantity of Air possesses, underd c all v . ^ossures p and P, are as those Pressures reci-' b, that is, S"; s P ; p. And because the Densitiesip ate r ere t s' e Quantity of Matter is given (An. LVI.

' b eCi procalIy as the Magnitudes of Bodies, viz.s - ^ ■ therefore the Densities of the Air are as

th,

k c 0m ‘ 0 j tner,For

ro o - — ces directly, viz. a' : D :: p : P-6 the T Per T ot the Air is the Principle to which wettvention and Contrivance of several very use-ful