Page:Popular Science Monthly Volume 31.djvu/662

644 all the air. He then proceeded to pump in water, until definite pressures up to one thousand pounds per square inch had been reached, and, at every one hundred pounds, the weight of water pumped in was determined. In this way, after many repetitions, he obtained the decrease of volume, due to any given increase of pressure. The observations have been plotted into the form of a curve (Fig. 6). The



base-line represents a cylinder containing one cubic foot of cork, divided by the vertical lines into ten parts; the black horizontal lines according to the scale on the left hand represent the pressures in pounds per square inch which were necessary to compress the cork to the corresponding volume. Thus, to reduce the volume to one half, required a pressure of two hundred and fifty pounds per square inch. At one thousand pounds per square inch the volume was reduced to forty-four per cent; the yielding then became very little, showing that the solid parts of the cells had nearly come together, and this corroborates Mr. Ogston's determination, that the gaseous part of cork constitutes fifty-three per cent of its bulk. The engineer, in dealing with a compressible substance, requires to know not only the pressure which a given change of volume produces, but also the work which has to be expended in producing the change of volume. The work is calculated by multiplying the decrease of volume by the mean pressure per unit of area which produced it. The ordinates of the dotted curve on the diagram with the corresponding scale of foot-pounds on the right-hand side are drawn equal to the work done in compressing a cubic foot of cork to the several volumes marked on the base-line. The author has not been able to find an equation to the pressure-curve; it seems to be quite irregular, and hence the only way of calculating the