THE MECHANICAL EQUIVALENT OF HEAT. 
425 
such circumstances, being sufficient to render the galvanometer uncertain and to pro¬ 
duce irregularities in our results. We propose to repeat these “ lag ” determinations, 
using the naked wire, but placing pentane instead of water within the calorimeter. 
In the meantime we have left our total range uncorrected for the difference in the 
initial and final lag, as our own data are not at present sufficiently accurate for us 
to apply the correction, which is certainly less than 1 in 1400. Its effect would be to 
increase the value of J,* but it would not affect the changes in the specific heat of 
water or the calorimeter. 
The correction will only have to be applied to the final result. 
The exterior temperature was determined by a mercury thermometer A (by Hicks), 
which was similar in most respects to E^, but its range was not so open—about 
27 miUims. = 1° C. It was placed in the calorimeter with E,„ and the readings com¬ 
pared at every 5 millims. of E^. When conducting our experiments we only required 
to know accurately the difference between the internal and external temperature, and, 
for such a purpose, this comparison was sufficient. As A was to be used for the 
reading of stationary temperatures, a careful calibration of the usual kind was made 
by means of the Cavendish dividing microscopes.t 
Our method of experimenting eliminated several of the errors which are associated 
with the use of mercury thermometers. 
(1.) Changes of temperature of stem. 
The exposed portion of the stems of A and E^ was always immediately above the 
surface of the water in our twenty-gallon tank. The water was always maintained at 
a constant temperature (see p. 454) and (as we ascertained by direct measurements) 
the temperature of the air above it varied but slightly. The lower portion of 
the stem passed through a tube immersed in this tank water, and thus the stem 
temperature remained steady when the temperature of the bulb was rising. The 
change in the temperature of even the upper portion of the stem never exceeded 
three or four degrees throughout the year, and the change in the average stem 
temperature must have been much smaller. At the time of determination of the 
values of the ranges, the stem was, therefore, under exactly the same conditions as 
those prevalent during an experiment. 
(2.) The effect of changes of pressure on the bulb. 
The only changes to which it was exposed were atmospheric ones, which, even if 
they affected the elevation, would not affect the range. Again, A and 'Em were so 
* Rowland appears to have paid no attention to the effect of this difference in initial and final lag. 
It is probable, therefore, that tlie correction, if it was possible to obtain it, would cause a greater increase 
in his value than it is likely to effect in ours, since his average rate of rise was three times as great. 
t This calibration is in practical agreement with another calibration of A conducted by platinum 
thermometers; the inequalities as determined by the latter method are, however, always greater than 
those shown by the former. This is to be expected when the length of the mercury thread used in 
the ordinary method of calibration is considered. 
MDCCCXCIIJ.—A. 3 I 
