THE MECHANICAL EQUIVALENT OF HEAT. 
457 
The task of one observer was invariably that of maintaining the potential balance 
by means of the rheostat, and also inverting the currents at regular intervals. The 
attention of the second observer was directed to recording the group of transits about 
each fixed point, the chronograph key being placed at the base of the telescope stand. 
In the intervals he recorded the times of the revolutions of the stirrer, which were 
distinguished on the tapes from the temperature records by double marks. He also 
occasionally recorded the readings of thermometer A (the external temperature), but 
these were as a rule so regular as to render this observation a matter of form. 
Thus, during the progress of an experiment, no notes had to be taken unless some 
exceptional incident occurred. We believe this to be an important matter during 
observations of this description, for the mechanical operation of note-taking greatly , 
distracts the attention! 
The chronograph required rewinding about every twenty minutes, and in the press 
of the other observations this was sometimes forgotten. Some of the omissions in 
the tables are due to this cause. The duration of a whole experiment varied from 
40 to 80 minutes, according to the weight of water present and the E.M.F. used. 
At the end of an experiment, the chronograph tape was counted and the results 
tabulated in the form shown in Tables XXVIII. to XXXIII. These six experiments 
are typical of the rest, and have not been in anyway selected, except that the}'' are 
the first performed on each weight, in Series II., with different E.M.F. s. 
MDCCCXCm.—A. 
8 N 
