342 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tion from ice I to ice III, using Tammann's "^ notation. This pres- 

 sure is very nearly independent of temperature between —22° and— 30'^. 

 This particular transformation has the advantage that the reaction 

 runs with very great velocity and is accompanied by a comparatively 

 enormous change of volume, 20 per cent, so that in any piece of appa- 

 ratus, once given the two phases, the equilibrium pressure is automati- 

 cally set up almost immediately. This pressure has been found to be 

 2120 kgm. at —23°. For higher pressures, the transition point at 0° 

 from water to a liitherto unknown variety of ice, ice VI may be used. 

 This point has been found by experiment to be 6370 kgm. Equilib- 

 rium is reached more slowly than for the transition I-III, but for 

 work at higher pressures the use of this transition point will give a 

 more accurate calibration. This method of calibration has actually 

 been used, with satisfactory results, for two other coils than the four 

 mentioned above. 



The calibration of the manganin resistance without the use of an 

 absolute gauge may of course be effected by comparison with any 

 other accurately measured pressure phenomenon. For instance, the 

 calibration may be made by comparing the manganin resistance with the 

 mercury resistance, the data for which have been already published. 



Summary of Results. 



Two gauges for high pressures are described in this paper. The 

 first is an absolute gauge which has been used up to 13,000 kgm. 

 The construction of the gauge is described, the correction for dis- 

 tortion is determined, and the reading mechanism is discussed. A 

 procedure is given for freeing the deflections of the springs with which 

 the thrust is measured from hysteresis. All of the factors may be de- 

 termined with sufficient accuracy so that the gauge is accurate to the 

 limit of accuracy of reading, 1/10 per cent at 8000 kgm./cm.^. The 

 second gauge is a manganin resistance. This is shown to be suitable 

 for the purpose, since there is complete freedom from hysteresis and 

 elastic after-effects. The relation between pressure and resistance is 

 shown to be linear up to 12,000 kgm., but the gauge has been used by 

 extrapolation up to 20,500. The accuracy of the readings with this is 

 at least as great as with the absolute gauge. The relative advantages 

 for various kinds of experiment of these two forms of gauge are dis- 

 cussed. Finally, by the use of standard pressures of reference it is 



' Tammann, Kristallisieren und Schmelzen, pp. 315-344 (Barth, Leipzig, 

 1903). 



