6 4 4 



THE POPULAR SCIENCE MONTHLY. 



all the air. He then i>roceeded to pump in water, until definite press- 

 ures up to one thousand pounds per square inch had been reached, and, 

 at every one hundred pounds, the weight of water pumped in was de- 

 termined. In this way, after many repetitions, he obtained the de- 

 crease of volume, due to any given increase of pressure. The obser- 

 vations have been plotted into the form of a curve (Fig. 6). The 



'44 



7 er 



Fig. 6. Volume of Cork. 



cufcjzr^ 



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 corrob- 

 orates Mr. Ogston's determination, that the gaseous part of cork con- 

 stitutes 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 au- 

 thor 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 



