y perfect gas thermometer,
while the mercurial thermometer differs very much from it in some
cases. Thus, Regnault found that when the air thermometer indicated
630 deg. F. above the melting point of ice (or 662 deg. F.), the mercurial
thermometer indicated 651.9 deg. above the same point (683.9 deg. F.), a
difference of 22 deg. F. It is apparent that the air thermometer furnishes
the best standard. As for the particular degree on the scale to be
used for the standard, it is apparent, from the observations above
made, that the temperature corresponding to that at or near the
maximum density of water is more desirable than that at the melting
point of ice. The fact, also, that the specific heats at constant
pressure and at constant volume are the same at the point of maximum
density, as shown by theory, is an additional argument in favor of
selecting this point for the standard. It thus appears that the
solution of this problem, which appears simple and very definite by
Mr. Hanssen's method, becomes intricate and, to a limited degree,
indeterminate when subjected to the refinements of direct experiment.
If the constants used by Hanssen are absolutely correct, then his
result must be unquestioned; but since physical constants are subject
to certain residual errors, one would as soon think of finding the
specific heat of air at constant volume, by using the value of the
mechanical equivalent as one of the elements, and trusting the result,
as he would to trust to the computed value of the mechanical
equivalent without subjecting it to the test of a direct experiment.
We will, therefore, examine the constants used to see if they are the
exact values of the quantities they represent.

He says they are universally accepted as correct; and this may be
true, when used for general purposes, and yet not be scientifically
exact. He uses 0.2377 as the specific heat of air. This is the value,
to four decimals, found by Regnault. Thus, Regnault gives for the mean
value of the specific heat of air

Between -30 deg. C and + 10 deg. C. 0.23771
" 0 deg. C " 100 deg. C. 0.23741
" 0 deg. C " 200 deg. C. 0.23751

And we know of no reason why one of these values should be used rather
than another, except that the mean of a large range of temperatures
may be more nearly correct than that of any other; and if this reason
determines our choice, the number 0.2375 would be used instead of
0.237