400 PROCEEDINGS OF THE AMERICAN ACADEMY. 



ample, by this method, because the changes of volume of the liquid are 

 accompanied by unknown changes of volume of the same order in the 

 containing vessel. But here, where the changes take place at constant 

 pressure, the vessel remains constant during the change, and the only 

 distortion which need be taken into account at all is the change of area 

 of the cylinder, a change which is easy to calculate, and which further- 

 more never in the present work rose above 2 per cent of the total effect. 

 To make this matter clearer, take the example of water. A pressure 

 sufficient to change the volume of water by 20 per cent might intro- 

 duce a change of about 2 per cent in the volume of the very heavy 

 cylinders of this present work. This change of 2 per cent takes no 

 account of the change of volume introduced by the compression of the 

 packing of the various connecting pipes, etc., which are necessary if the 

 pressure is to be measured at all accurately. This change due to 

 the tightening of the packing is evidently incalculable, but may pre- 

 sumably be as large as the other effect. The change of volume as 

 indicated by the advance of the piston is affected, therefore, by a 

 quantity which may very reasonably be supposed to be 4 per cent on 

 20 per cent, that is, 1/5 of the total effect measured. Compare this 

 wnth the change of volume during freezing at constant pressure. Here 

 the change of volume of the rest of the vessel and the packing plays 

 no part whatever, the only correction is 2 per cent for the change in 

 cross-section of the cylinder. That is, a correction of 2 per cent which 

 may be approximately determined, against a wholly unknown correction 

 of 20 per cent. It should be remarked, however, that if once the com- 

 pressibility of the water has been determined in some other way, it may 

 be used in an apparatus of this form to determine the unknown distor- 

 tion, and so the compressibility of other liquids may be determined. 



Description of Apparatus. 



The apparatus consisted essentially of two cylinders, connected by a 

 piece of very heavy nickel steel tubing 12 inches long. The upper cylin- 

 der was placed in a hydraulic press, and the pressure was produced in 

 this by a piston forced down by the ram of the press. The amount of 

 motion of the piston was measured with a micrometer screw reading 

 to 0.0001 inch. There were no valves in any part of the apparatus, as it 

 is difficult to make these capable of withstanding high pressures. The 

 apparatus was initially filled entirely full of liquid after the air had 

 been exhausted wnth an air pump, and the piston was then introduced 

 into the upper cylinder. The dimensions of the various parts were 

 chosen so that the maximum pressure desired could be obtained with a 



