416 
MR. E. H. GRIFFITHS OH THE VALUE OF 
themselves give us the value of the equivalent appears, to us, to be the onlv wav in 
which this difficulty can be successfully overcome, and, as our results will show, the 
equivalent was determined with the same accuracy as the other quantities.^ ■ 
There does not appear to be any argument in favour of using a small mass of water 
when the value of “J” is to be determined according to Joule’s original method bv the 
friction of water ; but the complexion of the case is completely changed if the mode 
of investigation is altered, for in each different method the accuracy of the result will 
be some function of the variables at our disposal, and, since we cannot apply the 
calculus, these ought to be so selected from general considerations, as to make that 
accuracy a maximum. In the method which we have adopted the variables are the 
mass of water, the resistance of the coil, the difference of potential at its ends, and, 
dependent upon these, is the rate of rise in temperature. 
Now the most satisfactory method of ascertaining the rate of rise in temperatm’e, 
consists in timing the transit of the mercury column across the spider-wire of a 
reading telescope. This wire, being set along the graduations of the thermometer, 
several successive transits are recorded at different parts of the range and the mean 
time is taken at each place. Although the rate of rise ought to be fairly great, in 
order to diminish the importance of radiation losses, if the rate is too rapid the 
observer has not sufficient time to set his cross-wire to the successive graduations, 
and only one observation can be recorded at each part of the range. The confusion 
which would be introduced by attempting to observe every other scale division need 
scarcely be mentioned. It was only by constant practice that one of the observers 
was able to take observations with accuracy at the rate we adopted. 
If, on the other hand, the advance of the column is too slow, the time of apparent 
contact with the telescope wire is uncertain and, at the same time, the phenomenon 
of “ sticking ” is rendered more probable. As during these experiments the strain 
on the observer was very great, mere personal fatigue would be sufficient to prevent 
accurate observations being taken if the duration of the experiment was extended 
beyond an hour. In the case of the experiments on the heat developed by the 
stirrer, it will be seen by reference to the tables that these sometimes lasted three or 
four hours ; but the nature of these experiments not being so complicated, the strain 
was less and the observer had short intervals of rest. 
The rate of rise in temperature thus practically determines the values which we 
must assign to the other quantities at our disposal, for to make the rate great we must 
either diminish the mass or resistance, or else increase the difference of potential. 
The considerations which led us to give to E and II the limiting values assigned to 
* The value of J can be obtained without ascertaining the value of the water equivalent (see p. 481). 
and the determination of the changes in the capacity for heat of water is entirely independent of it. 
Our conclusions are based on observations of differences in time caused by differences in the mass of 
water, and we see no reason why such differences could be ascertained with greater accuracy if the water 
equivalent was diminished. Thetotal times would be decreased,but the differences would remain unaltered. 
