His first discovery was that of latent heat. When communicating this to Professor Black he found that his friend had anticipated him, and had been teaching it in lectures to his students for some years past. His next step was the discovery of the _total_ heat of steam, and that this remains practically constant at all pressures. Black's fame rests upon his theory of latent heat; Watt's fame as the discoverer of the total heat of steam should be equally great, and would be no doubt had his role of inventor not overshadowed all his work. This part of Watt's work has been so little known that it is almost imperative to-day to give some idea of it to the general reader. Suppose you take a flask, such as olive oil is often sold in, and fill with cold water. Set it over a lighted lamp, put a thermometer in the water, and the temperature will be observed to rise steadily till it reaches 212 deg., where it remains, the water boils, and steam is produced freely. Now draw the thermometer out of the water, but leaving it still in the steam. It remains steady at the same point--212 deg. Now it requires quite a long time and a large amount of heat to convert all the water into steam. As the steam goes off at the same temperature as the water, it is evident a quantity of heat has escaped in the steam, of which the thermometer gives us no account. This is latent heat. Now, if you blow the steam into cold water instead of allowing it to pass into the air, you will find that it heats the water six times more than what is due to its indicated temperature. To fix your ideas: suppose you take 100 lbs. of water at 60 deg., and blow one pound of steam into it, making 101 lbs., its temperature will now be about 72 deg., a rise of 12 deg. Return to your 100 lbs. of water at 60 deg. and add one pound of water at 212 deg. the same temperature as the steam you added, and the temperature will only be raised about 2 deg. The one pound of steam heats six times more than the one pound of water, both being at the same temperature. This is the quantity of latent heat, which means simply hidden heat, in steam. Proceeding further with the experiment, if, instead of allowing the steam to blow into the water, you confine it until it gets to some pressure, then blow it into the water, it takes the same weight to raise the temperature to the same degree. This means that the total heat remains practically the same, no matter at what pressure. This is James Watt's discovery, and it led him to the use of high-pressure steam, used expansively. Even coal may yet be superseded before it is exhausted, for as eminent an authority as Professor Pritchett of the Massachusetts Institute of Technology has said in a recent address: