I have spoken of the illuminating power of the gas as of importance; for the richer gases have also more calorific power, and an engine would, of course, require a smaller quantity of them.  The heat-giving power does not, however, vary as the illuminating power, but at a much slower rate; and, adopting the same contrivance as that on which the absolute scale of temperature is formed, I would suggest a formula of the following type:  H = C (I + K), in which H represents the number of heat-units given out by the combustion of 1 cubic foot of gas, I is the illuminating power in candles, and C and K two constants to be determined by experiment.  If we take the value for motive power of the different qualities of gas as given in Mr. Charles Hunt’s interesting paper in our Transactions for 1882, C might without any great error be taken as 22 and K as 7.5.  With Pintsch’s oil gas, however, as compared with coal gas, this formula does not hold; and C should be taken much lower, and K much higher than the figures given above.  That is to say, the heating power increases in a slower progression.  The data available, however, are few; but I trust that Mr. Hartley will on this, as he has done on so many other scientific subjects, come to our aid.

I will now refer to the valuable experiments of Messrs. Brooks and Steward, which were most carefully made.  Everything was measured—­the gas by a 60 light, and the air by a 300 light meter; the indicated horse power, by a steam-engine indicator; the useful work, by a Prony brake; the temperature of the water, by a standard thermometer; and that of the escaping gases, by a pyrometer.  The gas itself was analyzed; and its heating power calculated, from its composition, as 617.5[theta].  Its specific gravity was .464; and the volume of air was about seven times that of the gas used (or one-eighth of the mixture), and was only 111/2 per cent. by weight more than was needed for perfect combustion.  The results arrived at were as follows: 

Per cent. 
Converted into indicated horse power,
including friction, etc.              17.0
Escaped with the exhaust gas.             15.5
Escaped in radiation.                     15.5
Communicated to water in the jacket.      52.0

It will thus be seen that more than half of the heat is communicated to the water in the jacket.  Now, this is the opposite of the steam-engine, where the jacket is used to transmit heat to the cylinder, and not from it.  This cooling is rendered necessary, because without it the oil would be carbonized, and lubrication of the cylinder rendered impossible.  Indeed, a similar difficulty has occurred with all hot-air engines, and is, I think, the reason they have not been more generally adopted.  I felt this so strongly that, for some time after the introduction of the gas-engine, I was very cautious in recommending those who consulted me to adopt it.  I was afraid that the wear and tear would be excessive.  I have,