a
definite amount of water. This weight is the "water equivalent" of the
apparatus, _i. e._, the weight of water, the temperature of which would
be increased one degree for an equivalent increase in the temperature of
the combined apparatus. It may be determined by calculation from the
weights and specific heats of the various parts of the apparatus. Such a
determination is liable to error, however, as the weight of the bomb
lining can only be approximated, and a considerable portion of the
apparatus is not submerged. Another method of making such a
determination is by the adding of definite weights of warm water to
definite amounts of cooler water in the calorimeter and taking an
average of a number of experiments. The best method for the making of
such a determination is probably the burning of a definite amount of
resublimed naphthaline whose heat of combustion is known.

The temperature of the water in the water jacket of the calorimeter
should be approximately that of the surrounding atmosphere. The
temperature of the weighed amount of water in the calorimeter is made by
some experimenters slightly greater than that of the surrounding air in
order that the initial correction for radiation will be in the same
direction as the final correction. Other experimenters start from a
temperature the same or slightly lower than the temperature of the room,
on the basis that the temperature after combustion will be slightly
higher than the room temperature and the radiation correction be either
a minimum or entirely eliminated.

While no experiments have been made to show conclusively which of these
methods is the better, the latter is generally used.

After the bomb has been placed in the calorimeter, it is filled with
oxygen from a tank until the pressure reaches from 20 to 25 atmospheres.
The lower pressure will be sufficient in all but exceptional cases.
Connection is then made to a current from the dry batteries in series so
arranged as to allow completion of the circuit with a switch. The
current from a lighting system should not be used for ignition, as there
is danger from sparking in burning the fuse, which may effect the
results. The apparatus is then ready for the test.

Unquestionably the best method of taking data is by the use of
co-ordinate paper and a plotting of the data with temperatures and time
intervals as ordinates and abscissae. Such a graphic representation is
shown in Fig. 25.

[Graph: Temperatu