at as 4 per cent, either high or low.

While the use of such computations will serve where approximate results
only are required, that they are approximate should be thoroughly
understood.

Calorimetry--An ultimate or a proximate analysis of a fuel is useful in
determining its general characteristics, and as described on page 183,
may be used in the calculation of the approximate heating value. Where
the efficiency of a boiler is to be computed, however, this heating
value should in all instances be determined accurately by means of a
fuel calorimeter.

[Graph: B.T.U. per Pound of Combustible
against Per Cent of Fixed Carbon in Combustible

Fig. 23. Graphic Representation of Relation between Heat Value Per Pound
of Combustible and Fixed Carbon in Combustible as Deduced by Wm. Kent.]

In such an apparatus the fuel is completely burned and the heat
generated by such combustion is absorbed by water, the amount of heat
being calculated from the elevation in the temperature of the water. A
calorimeter which has been accepted as the best for such work is one in
which the fuel is burned in a steel bomb filled with compressed oxygen.
The function of the oxygen, which is ordinarily under a pressure of
about 25 atmospheres, is to cause the rapid and complete combustion of
the fuel sample. The fuel is ignited by means of an electric current,
allowance being made for the heat produced by such current, and by the
burning of the fuse wire.

A calorimeter of this type which will be found to give satisfactory
results is that of M. Pierre Mahler, illustrated in Fig. 24 and
consisting of the following parts:

A water jacket A, which maintains constant conditions outside of the
calorimeter proper, and thus makes possible a more accurate computation
of radiation losses.

The porcelain lined steel bomb B, in which the combustion of the fuel
takes place in compressed oxygen.

[Illustration: Fig. 24. Mahler Bomb Calorimeter]

The platinum pan C, for holding the fuel.

The calorimeter proper D, surrounding the bomb and containing a definite
weighed amount of water.

An electrode E, connecting with the fuse wire F, for igniting the fuel
placed in the pan C.

A support G, for a water agitator.

A thermometer I, for temperature determination of the water in the
calorimeter. The thermometer is best supported by a stand independent of
the calorimeter, so that it may not be moved by tremors in the parts of
the calorimeter, which w