ne is evidently the forerunner of other
engines designed to utilise the force of an emission jet of vapour or
gas. There are very many processes in which gases generated by
chemical combinations are permitted to escape without performing any
services, not even that of giving up the energy which they may be made
to store up when held in compression in a closed vessel.

The reciprocating forms found suitable for steam and gas engines are
hardly adaptable for experiments in the direction of economising this
source of power, one fatal objection in the majority of cases being
the corrosive effects of the gases generated upon the insides of
cylinders and other working parts. As soon as the force of the
emission jet can be applied as a factor in giving motive power, the
fact that no close-fitting parts are required for the places upon
which the line of force impinges will alter the conditions of the
whole problem. In the centrifugal sand pump, as now largely used for
raising silt from rivers and harbours, the serious corrosive action of
the jet of sand and water upon the inside of the pump has been
successfully overcome by facing the metal with indiarubber; but
nothing of the kind could have been done if the working of the
apparatus had depended on the motion of close-fitting parts, as in the
ordinary suction or lift pump.

As an instance of the class of work for which gaseous jets, for
driving turbines or similar forms of motor, may perform useful
services the case of farm-made superphosphate of lime may be cited. By
subjecting bones to the action of sulphuric acid the farmer may
manufacture his own phosphatic manures for the enrichment of his land.
But the carbonic dioxide and other gases generated as the result of
the operation are wasted. Therefore it at present pays better to carry
the bones to the sulphuric acid than to reverse the procedure by
conveying the acid to the farm, where the bones are a by-product.

So bulky are the latter, however, that serious waste of labour is
involved in transporting them for long distances. Calculations made
out by the experts of various state agricultural stations show that,
as a general rule, it is now cheaper for the farmer to buy his
superphosphates ready made than to make them on his farm. The
difference in some cases, however, is not great; and only a
comparative trifle would be needed in order to turn the balance. This
may probably be found in the economic value of the service