165. _Q._--Have experiments been made to determine the elasticity of steam
at different temperatures?

_A._--Yes; very careful experiments. The following rule expresses the
results obtained by Mr. Southern:--To the given temperature in degrees of
Fahrenheit add 51.3 degrees; from the logarithm of the sum, subtract the
logarithm of 135.767, which is 2.1327940; multiply the remainder by 5.13,
and to the natural number answering to the sum, add the constant
fraction .1, which will give the elastic force in inches of mercury. If the
elastic force be known, and it is wanted to determine the corresponding
temperature, the rule must be modified thus:--From the elastic force, in
inches of mercury, subtract the decimal .1, divide the logarithm of the
remainder by 5.13, and to the quotient add the logarithm 2.1327940; find
the natural number answering to the sum, and subtract therefrom the
constant 51.3; the remainder will be the temperature sought. The French
Academy, and the Franklin Institute, have repeated Mr. Southern's
experiments on a larger scale; the results obtained by them are not widely
different, and are perhaps nearer the truth, but Mr. Southern's results are
generally adopted by engineers, as sufficiently accurate for practical
purposes.

166. _Q._--Have not some superior experiments upon this subject been lately
made in France?

_A._--Yes, the experiments of M. Regnault upon this subject have been very
elaborate and very carefully conducted, and the results are probably more
accurate than have been heretofore obtained. Nevertheless, it is
questionable how far it is advisable to disturb the rules of Watt and
Southern, with which the practice of engineers is very much identified, for
the sake of emendations which are not of such magnitude as to influence