TION OF
HEAT), and also for determining the variation of the specific heat of
water. In the latter case two steady currents of water at different
temperatures, say 0 deg. and 100 deg. are passed through an equalizer,
and the resulting temperature measured without mixing the currents,
which are then separately determined by weighing. This is a very good
method of comparing the mean specific heats over two ranges of
temperature such as 0-50, and 50-100, or 0-20 and 20-40, but it is not
so suitable as the electric method described below for obtaining the
actual specific heat at any point of the range.

S 3. _Method of Cooling._--A common example of this method is the
determination of the specific heat of a liquid by filling a small
calorimeter with the liquid, raising it to a convenient temperature, and
then setting it to cool in an enclosure at a steady temperature, and
observing the time taken to fall through a given range when the
conditions have become fairly steady. The same calorimeter is afterwards
filled with a known liquid, such as water, and the time of cooling is
observed through the same range of temperature, in the same enclosure,
under the same conditions. The ratio of the times of cooling is equal to
the ratio of the thermal capacities of the calorimeter and its contents
in the two cases. The advantage of the method is that there is no
transference or mixture; the defect is that the whole measurement
depends on the assumption that the rate of loss of heat is the same in
the two cases, and that any variation in the conditions, or uncertainty
in the rate of loss, produces its full effect in the result, whereas in
the previous case it would only affect a small correction. Other sources
of uncertainty are, that the rate of loss of heat generally depends to
some extent on the rate of fall of temperature, and that it is difficult
to take accurate observations on a rapidly falling thermometer. As the
method is usually practised, the calorimeter is made very small, and the
surface is highly polished to diminish radiation. It is better to use a
fairly large calorimeter to diminish the rate of cooling and the
uncertainty of the correction for the water equivalent. The surface of
the calorimeter and the enclosure should be permanently blackened so as
to increase the loss of heat by radiation as much as possible, as
compared with the losses by convection and conduction, which are less
regular. For accu