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ical model had been sent to London for repairs. Watt urged its prompt return and a sum of money was voted for this purpose. Watt was at last completely absorbed in the subject of steam. He read all that had been written on the subject. Most of the valuable matter those days was in French and Italian, of which there were no translations. Watt promptly began to acquire these languages, that he might know all that was to be known. He could not await the coming of the model, which did not arrive until 1763, and began his own experiments in 1761. How did he obtain the necessary appliances and apparatus, one asks. The answer is easy. He made them. Apothecaries' vials were his steam boilers, and hollowed-out canes his steam-pipes. Numerous experiments followed and much was learnt. Watt's account of these is appended to the article on "Steam and the Steam Engine" in the "Encyclopædia Britannica," ninth edition.

Detailed accounts of Watt's numerous experiments, failures, difficulties, disappointments, and successes, as one after the other obstacles were surmounted, is not within the scope of this volume, these being all easily accessible to the student, but the general reader may be interested in the most important of all the triumphs of the indefatigable worker-the keystone of the arch. The Newcomen model arrived at last and was promptly repaired, but was not successful when put in operation. Steam enough could not be obtained, although the

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boiler seemed of ample capacity. The fire was urged by blowing and more steam generated, and still it would not work; a few strokes of the piston and the engine stopped. Smiles says that exactly at the point when ordinary experimentalists would have abandoned the task, Watt became thoroughly aroused. "Every ob"stacle," says Professor Robison," was to him the begin"ning of a new and serious study, and I knew he would "not quit it until he had either discovered its worthlessness or had made something of it." The difficulty here was serious. Books were searched in vain. had touched it. A course of independent experiments was essential, and upon this he entered as usual, determined to find truth at the bottom of the well and to get there in his own way. Here he came upon the fact which led him to the stupendous result. That fact was the existence of latent heat, the original discoverer of which was Watt's intimate friend, Professor Black. Watt found that water converted into steam heated five times its own weight of water to steam heat. He says:

Being struck with this remarkable fact (effect of latent heat), and not understanding the reason of it, I mentioned it to my friend, Dr. Black, who then explained to me his doctrine of latent heat, which he had taught some time before this period (1764); but having myself been occupied with the pursuits of business, if I had heard of it I had not attended to it, when I thus stumbled upon one of the material facts by which that beautiful theory is supported.

Here we have an instance of two men in the same university, discovering latent heat, one wholly ignorant of the other's doings; fortunately, the later discoverer only too glad to acknowledge and applaud the original, and, strange to say, going to him to announce the discovery he had made. Watt of course had no access to the Professor's classes, and some years before the former stumbled upon the fact, the theory had been announced by Black, but had apparently attracted little attention. This episode reminds us of the advantages Watt had in his surroundings. He breathed the very "atmosphere" of scientific and mechanical investigation and invention, and had at hand not only the standard books, but the living men who could best assist him.

What does latent heat mean? we hear the reader inquire. Let us try to explain it in simple language. Arago pronounced Black's experiment revealing it as one of the most remarkable in modern physics. Water passed as an element until Watt found it was a compound. Change its temperature and it exists in three different states, liquid, solid, and gaseous-water, ice and steam. Convert water into steam, and pass, say, two pounds of steam into ten pounds of water at freezing point and the steam would be wholly liquified, i. e., become water again, at 212°, but the whole ten pounds of freezing water would also be raised to 212° in the process. That is to say two

pounds of steam will convert ten pounds of freezing water into boiling water, so great is the latent heat set free in the passage of steam to lower temperatures at the moment when the contact of cold surfaces converts the vapor from the gaseous into the liquid state. This heat is so thoroughly merged in the compound that the most delicate thermometer cannot detect a variation. It is undiscoverable by our senses and yet it proves its existence beyond question by its work. Heat which is obtained by the combustion of coal or wood, lies also in water, to be drawn forth and utilised in steam. It is apparently a mere question of temperature. The heat lies latent and dead until we raise the temperature of the water to 212°, and it is turned to vapor. Then the powerful force is instantly imbued with life and we harness it for our purposes.

The description of latent heat which gave the writer the clearest idea of it, and at the same time a much-needed reminder of the fact that Watt was the discoverer of the practically constant and unvarying amount of heat in steam, whatever the pressure, is the following by Mr. Lauder, a graduate of Glasgow University and pupil of Lord Kelvin, taken from "Watt's "Discoveries of the Properties of Steam."

It is well to distinguish between the two things, Discovery and Invention. The title of Watt the Inventor is world-wide, and is so just and striking that there is none to gainsay. But it is only to the

few that dive deeper that Watt the Discoverer is known. When his mind became directed to the possibilities of the power of steam, he, following his natural bent, began to investigate its properties. The mere inventor would have been content with what was already known, and utilised such knowledge, as Newcomen had done in his engine. Watt might have invented the separate condenser and ranked as a great inventor, but the spirit of enquiry was in possession of him, and he had to find out all he could about the nature of steam.

When communicating

His first discovery was that of latent heat. 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 rôle 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°, 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 ideas: 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

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