creasing or decreasing the amount of heat brought away,
and thus compensate exactly for the heat eliminated by the subject, the
hydrothermal equivalent of the system itself being about 20 calories--on
the other hand the body of the subject may undergo marked changes in
temperature and thus influence the measurement of the heat production to
a noticeable degree; for if heat is lost from the body by a fall of
body-temperature or stored as indicated by a rise in temperature,
obviously the heat produced during the given period will not equal that
eliminated and measured by the water-current and by the latent heat of
water vaporized. In order to make accurate measurements, therefore, of
the heat-production as distinguished from the heat elimination, we
should know with great accuracy the hydrothermal equivalent of the body
and changes in body temperature. The most satisfactory method at present
known of determining the hydrothermal equivalent of the body is to
assume the specific heat of the body as 0.83.[14] This factor will of
course vary considerably with the weight of body material and the
proportion of fat, water, and muscular tissue present therein, but for
general purposes nothing better can at present be employed. From the
weight of the subject and this factor the hydrothermal equivalent of the
body can be calculated. It remains to determine, then, with great
exactness the body temperature.

Recognizing early the importance of securing accurate body-temperatures
in researches of this kind, a number of investigations were made and
published elsewhere[15] regarding the body-temperature in connection
with the experiments with the respiration calorimeter. It was soon
found that the ordinary mercurial clinical thermometer was not best
suited for the most accurate observations of body-temperature and a
special type of thermometer employing the electrical-resistance method
was used. In many of the experiments, however, it is impracticable with
new subjects to complicate the experiment by asking them to insert the
electrical rectal thermometer, and hence we have been obliged to resort
to the usual clinical thermometer with temperatures taken in the mouth,
although in a few instances they have been taken in the axilla and the
rectum. For the best results the electrical rectal thermometer is used.
This apparatus permits a continuous measurement of body temperature,
deep in the rectum, unknown to the subject and for an indefinit