however, for some time past been thoroughly satisfied that this fear was needless; as I am satisfied that a well-made gas-engine is as durable as a steam-engine, and the parts subject to wear can be replaced at moderate cost.  We have no boiler, no feed pump, no stuffing-boxes to attend to—­no water-gauges, pressure-gauges, safety-valve, or throttle-valve to be looked after; the governor is of a very simple construction; and the slide-valves may be removed and replaced in a few minutes.  An occasional cleaning out of the cylinder at considerable intervals is all the supervision that the engine requires.

The very large percentage of heat absorbed by the water-jacket should point out to the ingenuity of inventors the first problem to be attacked, viz., how to save this heat without wasting the lubricant or making it inoperative; and in the solution of this problem, I look for the most important improvement to be expected in the engine.  The most obvious contrivance would be some sort of intercepting shield, which would save the walls of the cylinder and the rings of the piston from the heat of the ignited gases.  I have just learned that something of the kind is under trial.  Another solution may possibly be found in the employment of a fluid piston; but here we are placed in a dilemma between the liquids that are decomposed and the metals that are oxidized at high temperatures.  Next, the loss by radiation—­15 per cent.—­seems large; but this is to be attributed to the fact that the inside surface of the cylinder is at each inward stroke exposed to the atmosphere—­an influence which contributes to the cooling necessary for lubrication.  The remaining 15 per cent., which is carried away by the exhaust, is small compared with the proportion passing away with the exhaust steam of a high-pressure or the water of a condensing engine.  As the water in the jacket can be safely raised to 212 deg.  Fahr., the whole of the jacket heat can be utilized where hot water is required for other purposes; and this, with the exhaust gases, has been used for drying and heating purposes.

With such advantages, it may be asked:  Why does not the gas-engine everywhere supersede the steam-engine?  My answer is a simple one:  The gas we manufacture is a dear fuel compared with coal.  Ordinary coal gas measures 30 cubic feet to the pound; and 1,000 cubic feet, therefore, weigh 33 lb.  Taking the price at 2s. 9d. per 1,000 cubic feet, it costs 1d. per lb.  The 30 cubic feet at 630[theta] give 19,000[theta] all available heat.  Although good coal may yield 14,000 units by its combustion, only about 11,000 of these reach the boiler; so that the ratio of the useful heat is 11/19.  The thermal efficiency of the best non-condensing engine to that of the gas-engine is in the ratio 4/22.  Multiplying together these two ratios, we get