Historic Inventions

by Rupert Sargent Holland

He picked out the trade of a maker of mathematical instruments, and went to London to fit himself for it. He was apprenticed to a good master and made rapid progress, but the climate of London was bad for his health, and as soon as his term of instruction was finished he went back to Scotland. There he found it difficult to get employment, but at last he obtained permission to open a small shop in the grounds of the University of Glasgow, and to call himself “Mathematical-instrument-maker to the University.”

When the Newcomen engine was given to Watt to repair he studied it closely, and soon reached an important conclusion. A great amount of heat was lost whenever the cold water was let into the cylinder to condense the steam, and this loss vastly increased the expense of running the engine, and cut down its power. He saw that to prevent this loss the cylinder must be kept as hot as the steam that entered it. This led him to study the nature of steam, and he had soon made some remarkable discoveries in regard to it. He found that water had a high capacity for storing up heat, without a corresponding effect on the thermometer. This hidden heat became known as latent heat.

It was of course a matter of common knowledge that heat could be obtained by the combustion of coal or wood. Watt found that heat lay also in water, to be drawn out and used in what is called steam. If you change the temperature of water you find that it exists in three different states, that of a liquid, or water, that of a solid, or ice, and that of a gas, or steam. If water were turned into steam, and two pounds of this steam passed into ten pounds of water at the freezing point the steam would become liquid, or water, again, at 212° of temperature, but at the same time the ten pounds of freezing water into which the steam had been passed would also have been raised to 212° by the process. This shows that the latent heat of the two pounds of steam was sufficient to convert the ten pounds of freezing water into boiling water. That is the latent heat which is set free to work when the steam coming in contact with the cold changes the vapor from its gaseous to a liquid state. The heat, however, is only latent, or in other words of no use, until the temperature of the water is raised to 212°, and the vapor rises.

Mr. Lauder, a pupil of Lord Kelvin, writing of Watt’s “Discoveries of the Properties of Steam,” describes his results in this way: “Suppose you take a flask, such as olive oil is often sold in, and fill it 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°, 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°. 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 idea: suppose you take 100 lbs. of water at 60°, and blow one pound of steam into it, making 101 lbs., its temperature will now be about 72°, a rise of 12°. Return to your 100 lbs. of water at 60° and add one pound of water at 212° the same temperature as the steam you added, and the temperature will only be raised about 2°. 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.”

Newcomen, in making his steam-engine, had simply made additions to Papin’s model. Watt had already done much more, for in trying to find how the engine might be made of greater service he had discovered at the outset the principle of the latent heat of steam. He knew that in Newcomen’s engine four-fifths of all the steam used was lost in heating the cold cylinder, and that only one-fifth was actually used in moving the piston. It was easy to see how this loss occurred. The cylinder was cooled at the top because it was open to the air, and was cooled at the bottom in condensing the steam that had driven the piston up so as to create a vacuum which would lower the piston for another stroke. Watt knew that what he wanted was a plan by which the cylinder could always be kept as hot as the steam that went into it. How was he to obtain this? He solved it by the invention of the “separate condenser.” This is how he tells of his discovery. “I had gone to take a walk on a fine Sabbath afternoon, early in 1765. I had entered the green by the gate at the foot of Charlotte Street and had passed the old washing-house, when the idea came into my mind that as steam was an elastic body it would rush into a vacuum, and if a communication were made between the cylinder and an exhausted vessel it would rush into it, and might be there condensed without cooling the cylinder. I then saw that I must get rid of the condensed steam and injection-water if I used a jet as in Newcomen’s engine. Two ways of doing this occurred to me. First, the water might be run off by a descending pipe, if an offlet could be got at the depth of thirty-five or thirty-six feet, and any air might be extracted by a small pump. The second was to make the pump large enough to extract both water and air.... I had not walked farther than the golf-house when the whole thing was arranged in my mind.”

This was the discovery that gave us practically the modern steam-engine, with its countless uses in unnumbered fields. Newcomen’s engine was limited to the pressure of the atmosphere, Watt’s could use the tremendous force of steam under higher and higher pressure. He led the steam out of the cylinder and condensed it in a separate vessel, thereby leaving the cylinder hot. He closed the cylinder top, and prevented the loss of steam. The invention may seem simple enough as we study it, but as a matter of fact it was the attainment of this result of keeping the cylinder as hot as the steam that enters it that has given us our steam-engine.