BRIDGA^IAN. — MERCURY UNDER PRESSURE. 411 



inserted to reduce to manageableness the total quantity of the kerosene, 

 and the yolume of the manganin pressure gauge. The volume occupied 

 by the kerosene increases with pressure both because of the stretching 

 of the cylinder and the shrinking of the copper and the manganin 

 gauge. The stretching of the cylinder was calculated by the theory 

 of elasticity as outlined above. A slight modification of the formula is 

 necessary because the cylinder carries the end thrust of the pressure. 

 The total correction for the stretching of the cylinder is 1.4 per cent at 

 10,000 kgm. The effect of the copper and the small quantity of mica 

 washers in the insulating plug were assumed to be represented by the 

 compressibility O.O5I, the compressibility of the copper alone being 

 O.OeO. This correction amounts to 0.5 per cent at 10,000. Together, 

 the two effects produce a total change on the effective volume of kero- 

 sene of 1.00 cm. ^ per 10,000 kgm., on an initial volume of 28.6 cm. ^ The 

 correction for the thermal dilatation of the lower cylinder was found 

 from the known dilatation of steel at ordinary pressures, assuming it to 

 be independent of pressure. 



In making the actual measurements, endeavor was made to avoid as 

 far as possible the small effect of mechanical hysteresis. The set of 

 readings at —15°, for example, was obtained as follows. The piston dis- 

 placement was measured with increase of pressure from 3500 to 5800 

 kgm. with the lower cylinder at -j-20° ; the low^er cylinder was then 

 cooled to —15°, and the displacement measured from 5800 back to 

 3500 ; and finally the lower cylinder was warmed again to +20° and 

 the displacement measured from 3500 to 5800. The difference between 

 the second curve and the mean of the first and the third, which differed 

 only very slightly, gives at any one pressure the expansion of the cor- 

 responding amount of kerosene on passing from —15° to -1-20°, since 

 the temperature of the upper cylinder was also -|-20°. This difference 

 was found for a number of pressures for the range 3500 to 5800, and 

 a smooth curve drawn through these differences. From this curve was 

 taken the difference corresponding to the freezing pressure of mercury 

 at —15°. This process was performed for four temperatures, —20°, 

 15°, —7°, and 0°, and four points were thus determined on the correc- 

 tion curve. It was not necessary to obtain other points above 0° on 

 the curve, because the correction was already small at 0° and the 

 other end of the curve is fixed by definition as passing through the 

 value zero at 20°. Through these five points, —20° to 20°, another 

 smooth curve was drawn, giving at any temperature the piston displace- 

 ment when the amount of kerosene in the lower cylinder passes from 

 that temperature to -t-20° at the pressure corresponding to the freez- 

 ing pressure of mercury at that temperature. Finally this curve gave 



