With heat we can boil water and make steam under a pressure, and with the steam under a pressure we can run an engine, and with the engine make heat by friction, or make electric current that can produce heat. Carry this proposition back to the fuel box, and knowing the amount of heat developed by the burning of a certain quantity of fuel, it is found that, counting the heat that rises in the air through the smoke stack, the heat that is radiated from the boiler, the heat that is carried away in warmed ashes, the heat that exists in the steam after it is exhausted from the cylinder, and all other heat expended whether utilized in driving the machinery or going to waste, the sum total is in every case equal to the heat developed by the fuel box combustion. The most striking thing about all this is that when the steam goes into the cylinder where it is cooled as it expands and drives the pistons, the heat _thus lost by the expanding steam is the exact equivalent of the mechanical energy realized against the piston head_. Not all of the energy that is realized at the piston head is delivered to the driving shaft. Some of it is lost in the friction of the piston rings wearing against the cylinder lining; some, of course, is lost in friction at the journals connecting with the driving shaft. It is usual in counting engine efficiency to count the amount of energy delivered to the belt, or to the driving shaft, and because of the frictional resistance of the pistons working in the cylinder, there is always found a little discrepancy between the energy represented by the cooling of the steam in the cylinder and the energy delivered to the belt, or the driving shaft.