Does not the expenditure of oil in large motors largely offset the saving in coal?  And then, gas motors are sold at high prices, as are gas generators, and this installation necessarily requires the addition of a large gasometer, scrubbers, etc.  The wear of these apparatus is rapid, and if we take into account the interest and amortization of the capital engaged, we shall find that the use of steam is still more economical.  The obstruction caused by bulky apparatus is another inconvenience, upon which it is unnecessary to dwell.  In a word, the question is a very complex one.  We look at but one side of it in occupying ourselves only with the coal consumed, and we shall certainly expose those who allowed themselves to be influenced by the seductive figures of consumption to bitter disappointment.

To answer such objections Mr. Aime Witz has established a complete parallel between the two systems, in which he looks at the question from a theoretical and practical and scientific and financial point of view.  Considered as a transformation apparatus, a steam motor burning good Cardiff coal in a Galloway boiler with feed water heaters will consume (with a good condensing engine utilizing an expansion of a sixth) from 1,100 to 1,250 grammes of coal per effective horse hour, which corresponds to a rough coefficient of utilization of 9.7 per cent.  A gas generator supplying a gas motor burning Swansea anthracite and Noeux coke, medium quality, will consume 516 grammes of anthracite and 90 of coke to produce 2,370 liters of gas giving 1,487 heat units per cubic meter.  Of the 3,524 heat units furnished to the motor by the 2,370 liters of gas, the motor will convert 18 per cent. into disposable mechanical work.

With the boiler, the gross rendering of the whole is 7 per cent.  With the gas generator it reaches 12.7 per cent.  From a theoretical point of view the advantage therefore rests with the gas generator and gas motor.  In order to compare the net cost of the units of work, from an industrial point of view, it is necessary to form estimates of installation, costs of keeping in repair, interest and amortization.

Figs. 1 and 2 represent, on the same scale, the installations necessary in each of these systems.  The legends indicate the names of the different apparatus in each installation.  The following table shows that, as regards the surface occupied, the advantage is again with the gas generator and gas motor: 

Steam Engine.       Gas Motor. 
Surface covered.       85 sq. m.       72 sq. m. 
Surface exposed.       33    "         43    "
—–­             —–­
Total surface.        118    "        115    "

The estimates of installation formed by Mr. Witz set forth the expense relative to the capital engaged exactly at the same figure of 32,000 francs for a motive power of 75 effective horses.  The expenses of keeping in repair, interest, etc., summed up, show that the cost per day of 10 hours is 47.9 francs for the steam engine and 39.6 for the gas motor, say a saving of 8.3 francs per day, or about 2,500 francs for a year of 300 days’ work.