energy when generated in the form of electricity; not only this, but it is still further limited to direct currents, and is, therefore, of no avail in stations sending out alternating currents.

GAS STORAGE AND GAS ENGINES.

For strictly power purposes, the use of gas engines is now, more than ever before, attracting attention, and abroad the gas engine has already found its way into the central station and it has made its bow here also.

On purely theoretical grounds, at least, the gas engine deserves our consideration at this point for the following reasons: In the first place, we may replace the wasteful standby losses of steam boilers under the variable central station load by the ideally simple and economical system of gas storage. The use of the gas engine, therefore, comes legitimately within the province of this paper, since it permits of employing a substitute for the boilers, which may be operated continuously at the most economical load, and yet supply with equal efficiency the most variable load that can possibly be thrown upon it.

Comparing the relative theoretical advantages of the gas engine and the steam engine, Professor Kennedy states that the maximum theoretical efficiency of the steam engine is thirty per cent. Its obtained efficiency is about ten per cent. The theoretical maximum efficiency of the gas engine is eighty per cent. Its efficiency obtained in practice is twenty-five per cent. These facts have led Mr. Thwaite to say that the practical efficiency of the gas engine is equal to the theoretical efficiency of

the steam engine. As a matter of general experience, however, under usual working conditions with large engines, very little if any advantage in efficiency is shown by the gas engine over that driven by steam, but if this statement is disputed, as I am not at all sure that it cannot be successfully, the burden of proof lies with the gas engine. At any rate for our purposes it will be proper to assume that the two are on a par as regards ultimate efficiency, and that so far as this is concerned the two types of engines may be interchanged in any installation which we may consider without affecting the result.

The gas engine has this practical advantage that it may be started and stopped when required, thereby avoiding the standby losses losses so so fatal to economy where boilers are used. These standby losses for boilers are given by Dowson* as ten per cent; that is to say, that boilers that are standing idle and furnishing no steam will require ten per cent of the fuel they will take when working at their rated capacity. The gas plant, however, where gas storage is employed, has no standby losses of this kind, but, in many cases where fuel gas is manufactured to be used on the premises, adequate storage is not provided and in times of light load part of the plant is banked. Under these conditions, Dowson states that the standby losses will not exceed 2.8 per cent. 'The advantages in favor of the gas engine in this respect, even under the most unfavorable circumstances, are, therefore, as 10 to 2.8 or 3.

As to fuel consumption in gas practice furnishes considerable data.

engines, foreign Messrs. Crossly

* "Gas Power," a paper by Emerson Dowson in the Engineering Review, November 30th and December 20th, 1894.

guarantee one brake horse-power hour on a consumption of one pound of anthracite coal, using Dowson gas. Under test conditions*, the gas engine plant at Chateau Say, supplying 650 lamps, consumed 1.2 pounds per indicated horse-power hour. At Leven Tweed Mills, four gas engines, aggregating 200 horsepower, used in six days' trial one and one-quarter pounds anthracite per brake horse-power hour, or one and three-quarters pounds gas coke per brake horsepower hour. At the Godalming Mills, gas engines, aggregating 400 indicated horse-power, averaged one pound of fuel (character not stated) per indicated horse-power. At the Chelsea flour mill a sixty nominal horse-power, twin cylinder gas engine with Dowson gas, under full load test, used 0.87 pounds coal per indicated horse-power. But, as we are all aware, the fuel consumption under test conditions is no criterion to judge by, and especially misleading when we have under consideration an electric lighting plant. Furthermore, as I think has been conclusively shown, the fuel is by no means the only expense attending the production of power. Mr. Dowson places the cost of a Dowson gas plant of eighty horse-power at $20.31 per horse-power, and for a plant large enough to supply 530 horse-power, at $10.37 per horse-power. The wages of firemen for a gas plant are no more than for a steam plant, and a much cheaper fuel may be used. But let us assume that the fuel in both cases costs the same per ton, that the gas plant costs $20 instead of $10, so as to cover the cost of extra land required. Then our account would stand something like this:

*Bryan Donkin, before Incorporated Institute of Gas Engineers.

Cost of coal, 2 pounds, 8,760 hours, at $1.75..
Cost of petty stores..

$15.33

1.64

Cost of driving, stoking and cleaning..

11.67

28.64

Total annual cost per electric horse-power,

$88.28

$103.61

With coal at $3.50 the total cost...

Saving over present methods, with coal at $1.75

a

Mr. Druit Halpin, of England, has devised method of thermal storage which has attracted wide attention abroad, and is likely to attract great attention in this country as soon as its efficiency becomes better understood. We all know that a boiler when supplying an intermittent demand is capable of supplying for a short time energy at a much more rapid rate than its steaming capacity would warrant; that during the period of light load it has stored up a certain amount of energy which it can give out again when called upon to do so. It will have been observed that during the period of comparative rest the pressure in the boiler