336 
PROFESSOR O. REYNOLDS AND MR. W. H. MOORBY 
same in both trials, all error would be eliminated in the difference of the laro-e and 
O 
small trials. After careful consideration, it was arranged that the condensing 
water should be adjusted so that the water in all trials entered the tank at a 
temperature as nearly as possible 85°; it being probable, as the surface exposed 
to the air was nearly the same in the large and small trials, if the differences in 
temperature between the air and the water were the same, the evaporation wmuld 
be the same, or would at least differ by a constant amount. In order to test 
this, it was further arranged that, after the trials were finished, the centrifugal pump 
should be temporarily re-arranged so that it could be used to draw water out of the 
tank and force it round through the condenser and switch, and so back again into the 
tank at rates corresponding to those of the large and small trials, and at the same 
temperature (85°), the water in the tank being at this temperature, the arrange¬ 
ment of the pump being such that, when stopped, all the water in the pipes would 
run back again into the tank. This would practically insure the same loss of water 
by evaporation during one hour’s pumping as during one hour’s trials, and any differ¬ 
ence (ly) thus established between the large and small trials would then be treated 
as a standing correction on the difference of the heavy and light trials. This relative 
correction, taking W as the mean difference of water in the heavy and light trials, 
would be 
w • 
The Standards of Measurement. 
38. In these experiments the expressions obtained for the work done in heatino- 
the water and the l.ieat generated are, respectively, 
27 rN. RW, and SW„ {Tf - 
where R, W, T°, S are respectively length, weight, temperature, and capacity for heat. 
Since these expressions both represent the same absolute quantity of energy, the 
difference in the numerical values of these expressions results only from the differ¬ 
ence in the units in the two expressions. These units may be considered as the unit 
of work and the unit of heat respectively, as it is the inverse ratio of these units, 
measured in absolute quantities of energy, that is expressed by the ratio obtained 
from 
27rNRRb 
SW„(R-Tj)- 
But, as there are no actual standards either of work or heat vdth which quantities 
of work and heat can be respectively compared by a simple measurement, such com¬ 
parisons can only be accomplished by the comparison of the several factors involved 
