460 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
The weak point of Mr. Grirrirus’ determination is the small quantity of water he 
uses, his result depending on the introduction of about 120 grams. into his calorimeter. 
The highest water equivalent with which he werked was about 350 grams., of which 
85 are due to the calorimeter itself. The chief objection to the use of such small 
quantities lies in the great cooling correction. In the experiment quoted by him on 
p- 482, the loss of heat due to radiation and conduction at the end of his experiment 
amounts to about 9 per cent. of his heat supply. If such loss has been wrongly 
estimated to 1 per cent., an error of 745 per cent. would result in the final value. The 
consistency of Mr. GrirritHs’ results shows that if an error occurred due to that 
cause, it must have been systematic, and we may point out how, with such large 
cooling correction, serious errors may arise. In applying the cooling correction it is 
always assumed that the loss of heat depends only on the difference of temperature 
between the calorimeter and enclosure ; but, as has been already pointed out, this is 
not the case as regards conduction. Mr, Grirritus’ calorimeter was suspended by 
three stout glass tubes, through one of which the stirrer was passing. The exhaustion 
in the space surrounding the calorimeter was never sufficient to do away with the con- 
duction of air, so that we may take the larger part of the cooling to be due to 
conduction and convection. The loss of heat in that case must to some extent 
depend not only on the temperature, but on the rate of change of temperature. 
Whether the part which depends on the rate of change is sufficient to produce a 
sensible difference in the result, it is not easy to say, but the error produced would 
with different currents and quantities of water be the same in all cases, and could 
not, therefore, be detected by the inconsistencies thereby introduced into the results. 
The difference between our value of the equivalent and that of Mr. GRIFFITHS is, 
however, of smaller importance than the difference which exists between them and the 
equivalent, as determined directly by Joute, Rowxianp, and Micunescu. JOuLE’s 
latest value, which is the only one which needs consideration, is 772°65 foot-pounds 
at 61°°7 Fahrenheit. The number refers to the degree as measured by JOULE’s 
mercury thermometer. RowLAND adds to this a correction to the air thermometer of 
about 38, and another small correction for a change in the heat capacity of the 
apparatus, which brings the value up to about 776. ‘The correction to the air ther- 
mometer has been obtained by means of a comparison made by JouLE himself with 
one of Rowianp’s thermometers. JouLE’s original thermometers have been tem- 
porarily placed by Mr. B. A. Joute in the hands of Professor ScHusTER, in order that 
an accurate comparison may be instituted between them and modern thermometers. 
A full description of the comparisons made will be given on another occasion. The 
result arrived at shows that the correction is less than that assumed by RowLanp, 
and would bring his value up to only 775 at the temperature indicated. 
GRIFFITHS compares his result with that deduced by RowLanp from Joue’s obser- 
vations. RowLaNp combined the different values obtained by JouLe in his various 
investigations, attaching weights according to his judgment as to their relative 
