558 PROFESSOR WILLIAM THOMSON’S ACCOUNT OF 
34. Such are the experimental data on which the mean values of » for the 
successive degrees of the air-thermometer, from 0° to 230°, at present laid before 
the Royal Society, is founded. The unit of length adopted is the English foot; 
the unit of weight, the pound; the unit of work, a “foot-pound;” and the unit 
of heat that quantity which, when added to a pound of water at 0°, will produce 
an elevation of 1° in temperature. The mean value of »« for any degree is found 
citing Ae ope d : 
to a sufficient degree of approximation, by taking, in place of >, and k ; in the 
ex pression 

the mean values of those elements; or, what is equivalent to the corresponding 
accuracy of aproximation, by taking, in place of « and & respectively, the mean 
of the values of those elements for the limits of temperature, and in place of 
d ‘ eae 
= the difference of the values of p, at the same limits. 
35. In REGNAULT’s work (at the end of the eighth Mémoire), a table of the 
pressures of saturated steam for the successive temperatures 0°, 1°, 2°, . . . 230°, 
expressed in millimetres of mercury, is given. On account of the units adopted 
in this paper, these pressures must be estimated in pounds on the square foot, 
which we may do by multiplying each number of millimetres by 2°7896, the 
weight in pounds of a sheet of mercury, one millimetre thick, and a square foot 
in area. 
36. The value of k, the latent heat of a cubic foot, for any temperature ¢, is 
found from 4, the latent heat of a pound of saturated steam, by the equation 
pee 1+ 00366 x 100 
760 © 14 °00366 x z 
where p denotes the pressure in millimetres, and 4 the latent heat of a pound of 
saturated steam ; the values of 4 being calculated by the empirical formula* 
A= (606-5 + 0°305 ¢)— (¢ + -00002 ¢? + 0:000000 2°), 
given by REGNAULT as representing, between the extreme limits of his observa- 
tions, the latent heat of a unit weight of saturated steam. 
. x 036869 . a, 

* The part of this expression in the first vinculum (see ReGnautr, end of ninth Mémoire) is 
what is known as “the total heat’’ of a pound of steam, or the amount of heat necessary to convert 
a pound of water at 0° into a pound of saturated steam at ¢°; which, according to ‘“ Wart’s law,” 
thus approximately verified, would be constant. The second part, which would consist of the single 
term ¢, if the specific heat of water were constant for all temperatures, is the number of thermic 
units necessary to raise the temperature of a pound of water from 0° to ¢°, and expresses empirically 
the results of RegnavuLt’s experiments on the specific heat of water (see end of the tenth Mémoire), 
described in the work already referred to. 

