242 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the hard glass is more compressible than the soft. That the difference 

 is actually due to the difference of compressibility of the glass and is 

 not an experimental error will receive experimental confirmation later 

 by actual measurement of the compressibility of the glass. Resist- 

 ances in hard as well as in soft glass envelopes may be used as standards, 

 therefore, multiplying, however, the proportional changes of resistance 

 in hard glass by 1.0013 to reduce to soft glass. But it will be noticed 

 from Table V that the ratio of the changes of resistance in the hard 

 and soft glass capillaries varies much more irregularly than the ratio 

 for two capillaries of soft glass (Table I). That this is actually due 

 to irregularities in the deformation of the hard glass will receive con- 

 firmation in the paper on compressibility. The hard glass is not so 

 suitable, then, for the capillary as the soft Jena glass. 



In practical applications of this gauge it will doubtless be incon- 

 venient to work at the temperature above, 25°, and accordingly the 

 temperature coefficient was determined over a range from 0° to 50°. 

 The determination was made by comparing R 7, which was kept at 

 the standard temperature 25°, with R 9, which was maintained during 

 one set of readings at the given temperature over the entire pressure 

 range. Comparisons were made at six different temperatures, 50.35°, 

 43.75°, 36.95°, 30.32°, 15.00°, 0.00°. At each temperature seven 

 readings were made with increasing pressure and two with decreasing 

 pressure to avoid all possibility of hysteresis, no evidence of which was 

 found. In making this comparison it appeared necessary after each 

 change of temperature to season the glass by preliminary subjection 

 to the entire pressure range, the irregularities thus eliminated being 

 greater the greater the temperature range. It was found that pressure 

 may be calculated from temperature and the observed proportional 

 change of resistance by the formula: 



p = ap lO^ 1 " 03 [1 - ai (t - 25°) — h(t- 25°) 2 ], 



where a and /3 have the values previously given, and 



ai = log- 1 7.1253 -10, 

 bi = log- 1 4.4487 -10. 



a t and b x were computed by least squares. It was evident on plotting 

 the various points, that a 1 and b l are variable with the pressure, be- 

 coming less with increasing pressure, but the effect is very slight, and 

 no systematic variation over the entire temperature range could be 

 found. Attempts to introduce such a variation into the general formula 



