190

Limits of Atmospheric Pressure

[Book II,

proved the same important truth, viz. that tlie pressure of the atmospherewas that mysterious power, which under the name of nature’s abhorrenceto a vacuum had so long eluded the researches of philosophers. The sub-ject was taken up in England by Boyle, who pursued it with unremittedardor, and whose labors have immortalized his name ; but it was Germany that bore off the most valuable of the prizes which the discovery offeredto philosophers. The Torricellian experiment gaverise to the air pump;and in 1654, a Prussian philosopher, a mathematician and a magistrate,Otto Guerricke, of Magdeburgh, made public experiments with it at Ratis-bon, before the emperor of Germany and several electors. Some authorsascribe the Invention of the pump to Candido del Buono, one of the mem-bers of the Academie del Cimento at Florence , and intimate that the firstessays with it were only made by Guerricke.

The apparatus of Torricelli , i. e. the glass tube and basin of mercury,was named a baroscope, and afterwards a barometer, because it measureithe pressure of the atmosphere at all elevations ; hence to it, engineersin all parts of the earth may have recourse, to determine the perpendicularlength of the pipes of atmospheric pumps.

Änother application of the barometer was the natural result of Perrier’sfirst experiment on the Puy de Dome . As he ascended that mountainwith it, the mercury kept falling in exact proportion to the elevation towhich the instrument was carried; hence it is obvious, that when thetube is properly graduated, it will measure the height of mountains, andall other elevations to which it can be carried. By it, aeronauts deter-mine the height to which they ascend in balloons. The observations ofPerrier were continued daily from 1649 to 1651, during which he per-ceived that the height of the column slightly varied with the temperature,wind, rain, and other circumstances of the atmosphere ; and hence theinstrument indicated changes of weather, and became known and is stillused as a “ weather glass.” The extent of these variations is aboutthree inches, generally ranging from twenty-eight to thirty-one, and areprincipally confined to the temperate zones. In tropical regions, thepressure is nearly uniform, the mercury Standing at about thirty inchesthroughout the year. These facts have an important bearing on our sub-ject; for an atmospheric pump or siphon, with a perpendicular pipe thirty-four or thirty-five feet long, might operate during certain States of the at-mosphere, while in others it could not; and in some parts of the earth itwould be altogether useless.

It will appear in the sequel, that the physical properties of the atmos-phere which we have enumerated, must necessarily be understood, inOrder perfectly to comprehend the action of the machines we have to de-scribe. As regards the aerial pressure, its limits and Variation at differentaltitudes, we need only remark, that a sucking pump or a siphon, whichraises water thirty-three feet in New-York and Buenos Ayres, Londonand Calcutta, St. Petersburgh and Port Jackson in New Holland, couldnot, in the city of Mexico , elevate it over twenty-two feet; and at Quito ,and Santa Fe de Bogota in South America , and Gondar the Capital öfAbyssinia twenty feet, on account of the great elevation of these cities;(from the same cause, the pressure of the atmosphere on Mont Blanc isonly about half that on the plains) and if Condamine and Humboldt ,when on the summit of Pinchincha, had applied one to raise water there, oron the side of Antisana, at the spot where, from the great rarity ortenuityof the air, the face of the latter philosopher was Streaming with blood, hisattendant fainted, and the whole party exhausted, it would not have raisedwater over twelve or fourteen feet; (the mercurv in the barometer feil