152. We thus perceive that, firstly, we can get work out of heat when it passes from a higher to a lower temperature, but that, secondly, we must spend work upon it in order to make it pass from a lower temperature to a higher one; and that, thirdly and finally, nothing in the shape of work can be got out of heat which is all at the same temperature level. What we have now said enables us to realize the conditions under which all heat engines work. The essential point about such engines is, not the possession of a cylinder, or piston, or fly wheels, or valves, but the possession of two chambers, one of high and the other of low temperature, while it performs work in the process of carrying heat from the chamber of high to that of low temperature. Let us take, for example, the low-pressure engine. Here we have the boiler or chamber of high, and the condenser or chamber of low, temperature, and the engine works while heat is being carried from the boiler to the condenser--never while it is being carried from the condenser to the boiler. In like manner in the locomotive we have the steam generated at a high temperature and pressure, and cooled by injection into the atmosphere. 153. But, leaving formal engines, let us take an ordinary fire, which plays in truth the part of an engine, as far as energy is concerned. We have here the cold air streaming in over the floor of the room, and rushing into the fire, to be there united with carbon, while the rarefied product is carried up the chimney. Dismissing from our thoughts at present the process of combustion, except as a means of supplying heat, we see that there is a continual in-draught of cold air, which is heated by the fire, and then sent to mingle with the air above. Heat is, in fact, distributed by this means, or carried from a body of high temperature, _i.e._ the fire, to a body of low temperature, _i.e._ the outer air, and in this process of distribution mechanical effect is obtained in the up-rush of air through the chimney with considerable velocity.