186

Atmospheric Pressure on Z/iquids.

[Book II.

within it. In this experiment the paper merely preserves the liquid sur-face level: it remains perfectly free and loose ; and so far from beingclose to the edge of the glass, it may, while the latter is held in a horizon-tal position, be withdrawn several lines from it without the water escaping;and it may be pierced full of small holes with the same effect.

If' an inverted vessel be filled with any material that excludes the air,and whose specific gravity is greater than that of water, when loweredinto the latter, the contents will descend and be replaced by the water.A bottle filled with sand, shot, &c. and inverted in water, will have itscontents exchanged for the latter. As these substances, however, do notperfectly fill the vessel, and of course do not exclude all the air, theexperiment succeeds better when the vessel contains heavy liquids, asmercury, sulphuric acid, &c. It is said that negroes in the West Indiesoften insert the long neck of a bottle filled with water, into the bung-holesof rum puncheons, when the superior gravity of the water (in this case)descends, and is gradually replaced with the lighter spirit.

In the preceding examples and those in subsequent chapters, it will befound that wherever a vacuity or partial vacuum is formed, the adjacentair, by the pressure above, rushes in and drives before it the object thatintervenes, until the void is filled. If the nozzle of a pair of bellows beclosed, either by the finger or by a small valve opening outwards ; and ashort pipe, the lower end of which is placed in water, be secured to theopening in the under board which is covered by the clapper; then if thebellows be opened, the pressure of the atmosphere will drive the waterup the pipe to fill the enlarged cavity, and by then closing the boards,the liquid will be expelled through the nozzle. Bellows thus arrangedbecome sucking or atmospheric, and forcing pumps. When the orifice ofa syringe is inserted into a vessel of water and the piston drawn up, theair having no way to enter the vacuity thus formed than by the smallorifice under the surface of the liquid, presses the water before it into thebody of the syringe.

As every machine described in this book, and most of those in the nextone, both proves and illustrates atmospheric pressure on liquids, we neednot enlarge further upon it here. There are however some other parti-culars relating to it, which are necessary to be known: first, that itspressure is limited; and secondly, that it varies in intensity at differentparts of the earth, according to their elevation above the surface of the sea.These important facts are clearly established in the accounts given of thediscovery of the air’s pressure, a sketch of which can scarcely be out ofplace here, since it was a pump that first drew the attention of modernphilosophers to the subject, and which thereby became the proximatecause of a revolution in philosophical research, that will ever be consi-dered an epoch in the history of Science.