Os the Air-Pump.

jar will raife it up into the tube, although the piston of the syringecontinues motionless.—If the tube be about 3? or 33 inches high, thequickiilver will rise in it very near as high as it stands abthat time inthe barometer. And, if the syringe has a small hole, as m, near thetop of it, and the piston be drawn up above that hole, the air willrulh through the hole into the syringe and tube, and the quickiilver willimmediately fall down into the jar. If this part of the appartus be air-tight, the quickiilver may be pumped up into the tube to the fameheight that it stands in the barometer ; but it will go no higher, be-caufe then the weight of the column in the tube is the fame as theweight of a column of air of the fame thickness with the quickiilver,and reaching from the earth to the top of the atmosphere.

7. Having placed the jar Jl, with some quicksilver in it, on thepump-plate, as in the last experiment, cover it with the receiver B-,then pulh the open end of the glase tube d e through the collar ofleathers in the brass neck C (which it fits so as to be air-tight) almostdown to the quickiilver in the jar. Then exhaust the air out of thereceiver, and it will also come out of the tube, because the tube sedose at top. When the gauge m m Iheyvs that, the receiver is well ex-hausted, pulh down the tube, so as to immerse ijts lower end; into thequicksilver in the jar. Now, although the tube be exhausted of air,none of the quicksilver will rife into it, because there > is no air lest inthe receiver to preis upon its surface in the jar. But let the air intothe receiver by the cock k, and the quicksilver will immediately rifein the tube; and stand as high in it, as it was pumped up in the lastexperiment.

Both these experiments Ihew, that the quicksilver is supported inthe barometer by the presiure of the air on its surface in the box, inwhich the open end of the tube is placed. And that the more denleand heavy the air is, the higher does the quicksilver rise; and, on thecontrary, the thinner and lighter the air is, the more will: the quick-silver fall. For, if the handle F be turned ever sq little, it takes someair out of the receiver, by raising one or other of the pistons in itsbarrel; and confequently, that which remains in the receiver is somuch the rarer, and has so much the lese spring and weight; andthereupon, the quicksilver falls a little in the tube: but upon turningthe cock, and re-admitting the air into the receiver, it becomes asweighty as before, and the quicksilver rifes again to the fame height.—Thus we fee the reason why the quicksilver in the barometer fallsbefore rain or fnow, and rifes before fair weather; for, in the formercafe, the air is too thin and light to bear up the vapours, and in thelatter, too dense and heavy to let them fall. N. B. In