356 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In the inverted position, a sufficient quantity of mercury is placed in 

 the piezometer to cover the bottom, and the fluid with which the pie- 

 zometer is surrounded in the pressure chamber is chosen the same as 

 that within the piezometer. On the application of pressure, this fluid 

 is forced in, rising through the mercury, and on release the mercury 

 comes out. The quantity of mercury left is weighed, from which the 

 desired change of volume of the fluid is calculated. Here again it is 

 necessary to know the compressibility of the steel of the piezometer 

 and the mercury. The determination of the compressibility of the 

 mercury in the inverted position is as simple as that for finding the 

 compressibility of water in the erect position. The entire piezometer 

 is filled with mercury, and placed inverted in the water by means of 

 which pressure is transmitted. On application of pressure, water 

 bubbles up through the mercury, and on release of pressure, mercury 

 comes out. The compressibility of the water is involved here only as 

 a correction for the volume of the water forced in. In practice, how- 

 ever, it was found desirable to include a little water initially with the 

 mercury, so as to fill completely all the corners. The object of using 

 the piezometer in the inverted position, aside from the check on ac- 

 curacy afforded by two different arrangements of the apparatus, was to 

 find if possible any effect of the drop clinging to the mouth of the 

 channel. In view" of the fact that the surface tension of mercury is 

 greater than that of water, it seemed plausible that the minimum size 

 of a bubble of water that would detach itself from the mouth of the 

 channel and rise through the mercury would be less than the drop of 

 mercury that would fall through water. No such difference could be 

 found, however, in any of the work. 



It has been noticed that the method demands the knowledge of two 

 compressibilities besides that of the fluid to be investigated. In the 

 case of mercury these two compressibilities are those of steel and 

 water. The correction for the steel was determined independently by 

 measuring the change of length of a rod under pressure and will be 

 discussed more in detail later in this paper. The effect of the water is 

 small at the low pressures, being simply the change of volume of the 

 slight amount of water forced in, but at higher pressures, where the 

 water may lose as much as 20 per cent in volume, the correction for 

 the change of volume of the water will be 20 per cent for the inverted 

 position and 30 or 40 per cent for the upright position. Given, then, 

 the correction for the steel, it is still necessary to run two sets of de- 

 terminations to get the compressibility of either water or mercury, and 

 naturally these two determinations give the compressibility of both 

 water and mercury. The data for the water are to be given in a fol- 



