But the price of coal gas seemed to be too high for use in these large engines, in which sizes steam is comparatively cheap; and so poorer gas, which, though possessing only about 28 per cent. of the heating power, is still cheaper in proportion than coal gas, when it is made on the spot, was introduced to tide over the difficulty.  Difficulties have been successively overcome, with the result which we have just seen, namely, 1.37 pounds of anthracite per effective horse power, or about half the carbon which a steam engine of the same power of excellent design, and well kept up, would consume.  A 50 horse simplex at Marseilles, in Barataud’s flour mill, is said to have run for the last 2 years on 1.12 pounds of English anthracite per effective horse power; and thus M. Witz says his predictions of 10 years ago, that the gas producer would some day replace the boiler, are being verified in such a way as to surprise even himself.

But the objection is stated, and it is a serious one:  the weight of fuel is not the only thing to be considered.  The steam engine uses coal, the producer requires English anthracite, which is dearer; the gas motor uses a great deal of water and a great deal of oil, which cost money; and gas motors are dear, while gas producers and their adjuncts cost a tidy bit of money, and wear out pretty fast.  Is not steam, after all, more economical in the long run?  Besides, producers are bulky and take up a great deal of space; the weight of fuel is only one element in a complicated problem.

In order to study the grounds of this objection, M. Witz has instituted a comparison between the actual cost of large steam engines and that of gas motors of similar size.

Take a good Galloway or multitubular boiler; for 75 horse power effective the heating surface must be at least 74 square feet.  Using good Cardiff coal, with 4 per cent. of ash, and a heating power of 15,660 Fahr. units; the steam raised will be 8 to 9 pounds per pound of coal, so that 9,400 to 10,577 Fahr. units are utilized in raising steam, or 68 to 76 per cent., which is an excellent result.  Take an engine of 16 inch cylinder diameter, 40 inch stroke, and 66 revolutions, etc.; it will use 22.4 pounds of steam per horse power effective, which represents 2.47 to 2.8 pounds of coal under the boiler.  These 10 pounds of steam carry 11,752 Fahr. units of heat, and produce work equal to 75 horse, or 1,143 Fahr. units of heat; which corresponds to an efficiency of 9.7 per cent.  In a gas motor, on the other hand, we find the materials employed, as per the above data, to contain 8,958 Fahr. units of heat, and to make gaseous fuel in which 6,343 units are available; a return of 70.6 per cent, in the producer.  The motor receives these 6,343, and converts 1,143 of them into work; an efficiency of 18 per cent.  In order to be equivalent from the heat point of view, a steam engine ought to produce a horse power effective per 9.72 pounds of steam at 5 atmospheres; but no such steam engine exists.