256 PROCEEDINGS OF THE AMERICAN ACADEMY. 



an investigation of several minor points that came up in the course 

 of the work, that may be of interest on their own account. Among 

 these is an experimental determination of the difference of linear com- 

 pressibility of a piece of commercial rolled steel along and perpendicu- 

 lar to the direction of rolling, and some account of the seasoning effect 

 of successive applications of pressure on the elastical behavior of glass. 

 In detail, the paper contains a determination by one method of the 

 compressibility of two kinds of Jena glass, of a piece of commercial 

 aluminum rod, and of several grades of steel ; and by another method, 

 the compressibility of mercury, all up to about 6500 kgm. per sq. cm. 



In determining the compressibility of a solid the method adopted 

 was to measure the change of length of a rod of the substance pro- 

 duced by hydrostatic pressure applied all over the external surface. 

 This method applies, therefore, only to those solids that can be ob- 

 tained in the form of a cylindrical rod or tube. The cubic compressi- 

 bility is found by multiplying the linear compressibility by three. It 

 is a fundamental assumption throughout all the following determina- 

 tions of the compressibility of solids, therefore, that the substance is 

 so homogeneous and isotropic that the compression under hydrostatic 

 pressure is sensibly the same in all directions. Some experimental 

 proof of the justifiability of this assumption has been attempted in the 

 case of a piece of rolled steel boiler plate. 



It is a feature common to all the compressibility methods used in 

 this paper, that the distortion produced by pressure is measured by 

 the displacement of a ring sliding with slight friction on some mov- 

 able part of the apparatus. The method is not continuous reading, 

 therefore, but the apparatus has to be taken apart and readings made 

 after each application of pressure, the reading obtained corresponding 

 to the maximum pressure. A method of this kind is doubtless incon- 

 venient, but it has the advantage of simplicity and directness over 

 any continuous reading method that would be practical over so wide 

 a pressure range. 



In the determination of the compressibility of solids two slightly 

 different methods may be used, according as the solid is of relatively 

 low or high compressibility. The first method, not so accurate as the 

 second, applies to iron and metals of the same order of compressibility. 

 The second applies to substances of higher compressibility, and in- 

 volves directly the compressibility of iron as determined by the first 

 method. 



The first method measures the relative change of length of a rod 

 of the substance and a heavy cylinder of steel. The rod is enclosed 

 in the cylinder, throughout the interior of which hydrostatic pressure 



