The method requires but a very slight correction for temperature. Air, alone or mingled with oxygen, or a mixture of air and oxygen, can be easily tested with.

=Maintenance of Plants.=--If it should be necessary to retain a consulting engineer to install an engine capable of filling all requirements, it is also necessary to select a careful attendant in order that the engine may be kept in good condition. It is a rather widespread belief that a gas-engine can be operated without any care or inspection. This belief is all the more prevalent because of the employment of street-gas engines, which, by reason of their simplicity of construction and regularity of fuel supply, often run for several hours, and even for an entire day, without any attention whatever. But this negligence, particularly in the case of engines driven from producers, is likely to produce disastrous results. Although engines of this type do not require constant inspection during operation, still they require some attention in order that the speed may be kept at a fixed number of revolutions. Moreover, the care of the engine, the cleaning of the valves and of the various parts which are likely to become dirty, and the examination and cleaning of pipes, should be accomplished with great care and at regular intervals. This task should be entrusted only to a man of intelligence. A common workman who knows nothing of the care with which the parts of an engine should be handled is likely to do more harm than good.

The factory owner who follows the instructions which have been given in this book will avoid most of the stoppages and the trouble incurred in engine and generator installations, and may count upon a steadiness of operation comparable with that of a steam-engine.