Chap. 2.]
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CHAPTER II.
Discovery of atmospheric pressure—Circumstances which led to it—Galileo—-Torricelli—Beautifulexperiment of the latter—Controversy respecting the results—Pascal—his demonstration of the causeofthe ascent of water in pumps—Invention of the air-pump—Barometer and its various applications—Intensity of atmospheric pressure different at different parts of the earth—A knowledge of this necessaryto pump-makers—The limits to which water may be raised in atmospheric pumps known to ancientpump-makers.
In the year 1641, a pump-maker of Florence made an atmospheric, orwhat was called a sucldng pump, the pipe of which extended from 50 to60 feet above the water. When put in Operation, it was of course inca-pable of raising any over 32 or 33 feet. Supposing this to have beenoccasioned by some defect in the construction, the pump was carefullyexamined, and being found perfect, the Operation was repeated, but withthe same results. After numerous trials, the Superintendent of the Grand Duke ’s water Works, according to whose directions it had been made,consulted Galileo, who was a native of the city, and then resided in it.Previous to this occurrence, it was universally supposed that water wasraised in pumps by an occult power in nature, which resisted with con-siderable force all attempts to make a void, but which, when one wasmade, used the same force to fill it, by urging the next adjoining substance,if a fluid, into the vacant space. Thus in pumps, when the air was with-drawn from their upper part by the ‘ sucker,’ nature, being thus violated,instantly forced water up the pipes. No idea was entertained by philo-sophers at this or any preceding period, that we know of, that this forcewas limited ; that it would not as readily force water up a perpendiculartube, from which the air was withdrawn, 100 feet high as well as 20—tothe top of a high building as well as to that of a low one.
When the circumstances attending the trial of the pump at Florencewere placed before Galileo, (his attention having probably never beforebeen so closely directed to the subject) he could only reply, that nature’sabhorrence to a vacuum was limited, and that it “ ceased to operate abovethe height of 32 feet.” This opinion given at the moment, it is believedwas not satisfactory to himself; and his attention having now beenroused,there can be no doubt that he would have discovered the real cause, hadhe lived, especially as he was then aware that the atmosphere did exert adefinite pressure on objects on the surface of the earth. But at that periodthis illustrious man was totally blind, nearly 80 years of age, and within afew months of his death. The discovery is however, in some measure,due to him. It has also been supposed that he communicated his ideason the subject to Torricelli , who lived in his family and acted as hisamapuensis during the last three months of his life.
It was in 1643 that Torricelli announced the great discovery that waterWas raised in pumps by the pressure of the air. This he established byvery satisfactory experiments. The apparatus in his first one, was madein Imitation of the Florentine pump. He procured a tube 60 feet long,and secured it in a perpendicular position, with its lower end in water;then having by a syringe extracted the air at its upper end, he found theWater rose only 32 or 33 feet, nor could he by any effort induce it to