After a number of experiments, Mr. Popp arrived at the conclusion that
the simplest mode of heating, if not the most efficient, was at all
events the most suitable, as it was a matter of the first importance
that subscribers should not be troubled with the charge of any
apparatus involving complication or careful management; he therefore
adopted a simple form of cast iron stove lined with fireclay, heated
either by a gas jet or by a small coke fire. It was found that this
apparatus, crude as it was, answered the desired purpose, until some
better arrangement was perfected, and the type was accordingly adopted
throughout the whole system. It was quite recognized that this method
still left much to be desired, and the economy resulting from the use
of an improved form was very marked.

From a large number of trials very carefully carried out by Professor
Gutermuth, it was found that more than 70 per cent. of the total
number of calories in the fuel employed was absorbed by the air and
transformed into useful work. Whether gas or coal be employed as the
fuel, the amount required is so small as to be scarcely worth
consideration; according to the experiments carried out, it does not
exceed 0.09 kilo. per horse power and per hour, but it is scarcely to
be expected that in regular practice this quantity is not largely
exceeded. Professor Weyrauch has also carefully investigated this part
of the subject and fully confirms, if he, indeed, does not go beyond
Professor Gutermuth. He claims that the efficiency of fuel consumed in
this way is six times greater than when burnt under a boiler to
generate steam. He goes so far as to assert that with a good method of
heating the air, not only can all the losses due to the production and
the transmission of the compressed air be made good, but also that it
will actually contain more useful energy at the motor than was