1894.] Specific Heat of Water in Electric Units. 29 



have amounted to more than 15 per cent. We consider it unlikely 

 that so large an error occurred always in the same direction. Apart 

 from the cooling correction, however, it is difficult to see how a differ- 

 ence of one-tenth per cent, in our result can be produced unless by 

 the accumulation of a number of small errors. 



The weak point of Mr. Griffiths' determination is the small quan- 

 tity of water he uses, his result depending on the difference in his 

 rate of rise after the addition of about 120 grams into his calori- 

 meter. The highest water equivalent with which he worked was 

 about 360 grams, of which 85 are due to the calorimeter itself. 

 The chief objection to the use of such small quantities of water 

 lies in the great cooling correction. In the experiments quoted 

 by him at p. 482, the loss of heat due to radiation and conduc- 

 tion at the end of his experiment amounts to about 9 per cent, 

 of his heat supply. If such loss had been wrongly estimated to 

 1 per cent., an error of one-tenth per cent, would result in the final 

 value. The consistency of Mr. Griffiths' results shows that if an 

 error occurred due to that cause, it must have been systematic ; 

 arid we may point out how, with such a 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. Griffiths' 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 conduction of air; so that we may take the larger part of the cool- 

 ing 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 

 also 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 are, however, of smaller importance than the difference 

 which exists between them and the equivalent as determined directly 

 by Joule, Rowland, and Miculescu. Joule's latest value, which is the 

 only one which needs consideration, is 772'65 foot-pounds, at 

 61' 7 Fah. 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 3, 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 thermometer has been obtained by 



