XXXII. An account of some experiments made with an air-pump on Mr. Smeaton's principle; together with some experiments with a common air-pump

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As the following experiments were made principally to try the performance of Mr. Smeaton's pear-gage, it may be proper to describe it, which I shall do in his own words, taken from the Phil. Trans. for the years 1751 and 1752, vol. XLVII. p. 420. "I have found," says Mr. Smeaton, "the gages that have been hitherto made use of, for measuring the expansion of the air, very unfit to determine in an experiment of so much nicety; I have therefore contrived one of a different sort, which measures the expansion with certainty to much less than the 100th part of the whole. It consists of a bulb of glass, something in the shape of a pear, and sufficient to hold about half a pound of quicksilver: it is open at one end, and at the other is a tube, hermetically closed at the top. By the help of a nice pair of scales I found what proportion of weight a column of quicksilver, of a certain length, contained in the tube, bore to that which filled the whole vessel: by these means I was enabled to mark divisions upon the tube answering to a 1000th part of the whole capacity; which being about one-tent of an inch each, may be estimation, be easily sub-divided into smaller parts. This gage, during the exhausting of the receiver, is suspended therein by a slip wire. When the pump is worked as much as shall be thought necessary, the gage is pushed down till the open end is immerged in a cistern of quicksilver placed underneath; the air being then let in, the quicksilver will be driven into the gage till the air remaining in it becomes of the same density with the external, and as the air always takes the highest place, the tube being uppermost, the expansion will be determined by the number of divisions occupied by the air at the top. The degree to which I have been able to rarify the air in an experiment, has generally been about 1000 times, when the pump is put clean together; but the moisture that adheres to the inside of the barrel as well as other internal parts, upon letting in the air, is in the same succeeding trials worked together with the oil, which soon renders it so clammy as to obstruct the actions of the pump upon a fluid so subtil as the air when so much expanded; but in this case it seldom fails to act upon the air in the receiver till it is expanded 500 times." Thus far Mr. Smeaton's account.

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