GRAVITY OF ATMOSPHERIC AIR.

215

Experiment II.

In like manner, if we stop the end of a syringe, after itspiston has been pressed down quite to the bottom, and thenendeavour to draw it up, we shall find a considerable forcenecessary, viz. about 15 pounds for every square inch of thesection of the syringe. Exerting this force we can draw up thepiston and hold it there; but the moment we cease pulling, thepiston rushes down again. We feel something as it were draw-ing in the piston, but it is solely the weight of the incumbentair pressing it; and this obtains in every position of the syringe;because the air is a fluid and presses in every direction; nay, itpresses on the syringe as well as on the piston; for if the pistonbe suspended by its ring on a nail, it will require a weight todraw down the syringe, equal to that which is necessary to drawup the piston, and as soon as the syringe is freed from the weightit will spring up to its place again.

That the air presses equally in all directions becomes obviousfrom the following experiments:

Experiment III.

In a tall phial let an orifice be made about three inches fromthe bottom; stop this orifice. Through a cork in the neck of thephial, insert a long tube open at each end, and let the lower endbe below the orifice in the side of the phial. The mouth of thephial being closed up about the tube, pour water into the tubetill it is full. Upon opening the orifice, the water will be dis-charged, till its surface in the tube is level with the orifice; af-ter which it will cease to flow, because the external lateral pres-sure of the air, balances the perpendicular pressure upon thewater in the tube.