314 Mr. J. Rose-Innes on the Practical Attainment of 
freezing-point of the hydrogen thermometers was in both 
cases 1 metre of mercury. 
An inspection of the above Table shows us that the 
difference of reading between two hydrogen thermometers is 
quite appreciable ; it is as high as 0°'009 C, for instance, at 
20° C. Hence, at this temperature, one at least of the two 
thermometers must be wrong in its reading by a fairly large 
figure in the third place of decimals. This suggests that 
even where the two thermometers agree, as in the neighbour- 
hood of 40° C, we cannot be certain there is no error : it is 
just as likely that both thermometers happen to exhibit at 
this place the same divergence from the thermodynamic 
scale, the divergence being of the same order of magnitude 
as that actually proved to exist at 20° C. 
We readily find from the tables of last Section that the 
thermodynamic correction at 20° C. for the hydrogen thermo- 
meter, under the conditions of pressure observed by M. 
Chappuis, is less than o, 002 0. Such a quantity is con- 
siderably smaller than the error proved to exist in at least 
one of the two hydrogen thermometers. A similar remark 
applies to the thermodynamic correction at 30° C. ; and it is 
quite likely, though not actually proved, that the correction 
is smaller than the experimental error at other temperatures 
between the freezing- and boiling-points. Hence the thermo- 
dynamic corrections to the readings of a constant-volume 
hydrogen thermometer are usuallv neglected. 
As the standard instrument for thermometric purposes it 
is usual to take just such an instrument as those examined 
by M. Chappuis, viz., a constant-volume hydrogen thermo- 
meter with a pressure of 1 metre of mercury at the freezing- 
point. But since we do not really know which of his two 
thermometers is the more correct in its indications, we may 
inadvertently have let our choice fall upon the more faulty 
one for the standard instrument. Hence we cannot trust 
our standard thermometer in the third place of decimals for 
anything more than thermoscopic purposes. And since the 
difference of reading of the two hydrogen thermometers 
at 20° 0. is greater than the thermodynamic correction 
to the constant-volume nitrogen thermometer at the same 
temperature, we cannot be certain that the usual standard 
thermometer is everywhere superior in the accuracy of 
its actual readings, to the constant-volume nitrogen ther- 
mometer, 
