REVERSED AND NON-REVERSED SPECTRA. 



89 



Since the preceding datum corresponds to both increasing and decreasing 

 pressures, the work done must be reckoned per 2 atmospheres or it will be 

 io 5 ergs. Taking the specific heat of oil as 0.5 and the mechanical equivalent 

 as 42 X io 6 , the rise of temperature of the oil should be 



10 



: = 5 X io" 3 C., nearly 



Hence the residual temperature discrepancy found, o.ooi C., would be but 

 one-fifth of the full temperature discrepancy to be anticipated i.e., four- 

 fifths of the heat would have dissipated by conduction, etc., during the wait- 

 ing between successive compressions. 



The residual temperature is thus adequate to account for the full discrep- 

 ancy, and if a tube of this kind is to be used as a pressure-gage, the tube should 

 be made of "invar" or other metal without thermal expansion. True, a 

 water-jacket surrounding the tube would improve the apparatus, but the 

 thermal increments in question are so small that the device would not be 

 trustworthy. If, however, a temperature discrepancy is shown by the optic 

 gage it should also be shown by the more sensitive Bourdon gage, which is 

 not the case. Thus a residual effect of temperature is improbable. The 

 optical difficulties are slight and could be overcome by suspending the yoke, 

 which in figure 55 rolls loosely on the cylinder a, b, t', from steel pivots bearing 

 on jeweled cups. Elastic and particularly slow viscous yieldings to persistent 

 pressure are thus the probable reason for the errors. This also accounts for 

 the displacement of the fiducial reading. 



Two further experiments were now made in which the ellipses were centered 

 before each observation (table 18). In the second set (series 12) the tube 

 was attached to the yoke and the latter to the hangers by soft adhesive wax, 

 applied in the molten state. This proved quite adequate. 



TABLE 18. 



The zeros were regained and the results were marked improvement on the 

 preceding series. The same mean elongation is found for increasing and de- 

 creasing pressure; but the values are not identical in the two series. The 

 following data give the details of series 1 1 : 



Pressure 100 200 300 400 500 600 500 400 300 200 100 atm. 



Micrometer reading 50 43 34 28 20 15 21 28 36 43 50 cm./io 4 



A few units in the cm./io 4 place are thus uncertain. The graph is shown 

 in figure 58. 



