214 MECHANICAL EXAMINATION OF ATMOSPHERIC AIR.

SECT. I.

MECHANICAL EXAMINATION

OF

ATMOSPHERIC AIR.

JDY mechanical examination we would be understood to meansuch properties as are connected with sensible changes of mo-tion, and which indicate the presence and agency of moving ormechanical powers. They are therefore strictly the subject ofmathematical discussion; admitting of measure, number, anddirection; notions indeed purely mathematical. But as thosemechanical properties characterise, or are peculiar to all kinds ofelastic aeriform or gazeous fluids, no objection occurs to us againstthe propriety of considering them here briefly.

The mechanical properties of atmospheric air, are GRAVITY,ELASTICITY, INVISIBILITY, and DILATABILITY.

GRAVITY OE ATMOSPHERIC AIR.

Experiment I.

To prove die weight of the air, a pair of common bellowsmay serve; thus, for instance, if we stop the nozle and securethe valve-hole closely, having first blown or squeezed all the airout of them, we shall find that a considerable force is requisitefor separating the boards. They are kept together by the pres-sure of the air which surrounds them, in die same manner as ifdiey -were immersed in water.