The works of the Honourable Robert Boyle, Esq., epitomiz'd by Richard Boulton ... ; illustrated with copper plates.

EXPERIMENT II. Of the Pressure of the Air against the Sides of the Bodies it encompasses. The Pressure of the Air in∣cluded within an Ambient Body explain'd.

* 1.1IF, when the Air is almost drawn out of the Receiver, one endeavours to lift up the Brass Key which is plac'd in the middle of the Brass Cover; it will be as difficult to raise it, as if a considerable Weight were ty'd to it; for the Spring of the Air included in the Receiver being weaken'd, and not able to bear up strong enough against the bottom of the Stopple, as powerfully as before; whoever lifts up the Stop∣ple, must likewise bear up the whole Pillar of External Air, which presses upon the top of the Stopple: But if the Air be gradually permitted to get into the Receiver, the Internal Air being more compress'd, and its Spring encreas'd, it bears up against the Stopple more powerfully, and the Pressure of the Internal and External Air being brought nearer to an Aequilibrium, the Stopple may proportionably be lifted up with less difficulty; till at length, the whole Cavity of the Receiver being fill'd with Air, it may ea∣sily be taken up.

But it's thought strange by some, how the Pressure of the Included, can be any ways pro∣portionable to the Pressure of the External Air; since it is so encompass'd by the sides of

the Receiver, that those Pillars of Air, which would press upon it, if open to the Air, are only icnumbent on the External Superficies of the Receiver; but to unfold this Difficulty,* 1.2 we may consider, That as a piece of Wool•• squeezed in ones Hand, and so enclos'd in a Box, is kept from expanding equally by the sides of the Box, as if it remain'd in ones Hand; so the Air com∣press'd by the Incumbent Asmosphere, and con∣veigh'd in that compress'd State, into the Re∣ceiver, is equally kept from expanding it self by the sides of the Receiver, as if it were still squeez'd together by the weight of the Atmosphere: And if it should further be objected, that if the Air in∣cluded in the Receiver were so confin'd by the sides of it, it's continual Endeavour to expand would break the Glass: It may easily be an∣swer'd, that the expansive Force of the Inter∣nal Air, is counterpois'd by the Compressure of the External, and consequently there being an equal Pressure on both sides, it is preserv'd en∣tire; for a like Reason those slender Bubbles, which Boys usually blow up with Soap and Wa∣ter, continue whole for a good while, the In∣ternal and External Air, being compress'd to a just Aequilibrium.

Notes

• * 1.1

  The Air presses upon the sides of Bodies it encompas∣ses.
• * 1.2

  The Air en∣closed in the Receiver Acts by vir∣tue of its Spring.