Page:A biographical dictionary of eminent Scotsmen, vol 7.djvu/287

Rh appear trifling, and not at all out of the course of his ordinary business, but which was nevertheless productive of results, that not only gave immortality to his name, but impressed a great and lasting change on the commerce and man. ners of his own country, and also of a great portion of the world. We here allude to a circumstance that shall shortly be mentioned, that led to the improvements of Watt on the steam engiite; and the events of his life are so intimately interwoven with the history of the perfection of this extraordinary machine, that it will be necessary, in a brief and popular way, to describe the leading principles of its action.

The steam engine, at the time of which we speak, was constructed after the plan invented by Newcomen. The chief use to which these engines were applied, was the pumping of water from coal mines, one end of the pump rod being attached to a long lever, or beam supported in the middle. To the other end of this lever was attached the rod of a piston, capable of moving up and down in a cylinder, after the manner of a common syringe. The weight of the pump rod, &c., at the one end of the beam, having caused that end to descend, the other end was necessarily raised, and, the piston rising in the cylinder, steam was admitted from the bottom to fill the vacuity. But when the piston arrived at the top, cold water was injected at the bottom, and by reducing the temperature of the steam, condensed it, forming a vacuum. In this state of things, the atmosphere pressing on the top of the piston, forced it down, and raised the pump rod at the other end of the beam. This operation being continued, the pumping of the mine was carried on. Such was the form of the steam engine, when Watt first found it; and such is its construction at many coal mines even in our own day, where the economy of fuel is not a matter of any importance.

Anderson, the professor of natural philosophy, in the course of the winter of 1763, sent a model of Newcomen's engine to Mr Watt in order to be repaired. This was accordingly done, and the model set in operation, and with this an ordinary mechanic would have been satisfied. But the mind of the young engineer had two years before this time been occupied in researches into the properties of steam. During the winter of 1761, he made several very simple yet decisive experiments, for the most part with apothecaries' phials, by which he found that a cubic inch of water will form a cubic foot of steain, equal in elasticity to the pressure of the atmosphere, and also that when a cubic foot of steam is condensed by injecting cold water, as much heat is given out as would raise six cubic inches of water to the boiling point. To these important discoveries in the theory of steam, he subsequently added a third, beautifully simple, as all philosophical truths are, and valuable from its extensive application to practical purposes: he found that the latent heat of steam decreases as the sensible heat increases, and that universally these two added together make a constant quantity which is the same for all temperatures. This matter is commonly misrepresented, and it is stated not only in accounts of the steain engine, but also in memoirs of Mr Watt, that the discoveries of Dr Black regarding the properties of heat and steam laid the foundation of all Watt's inventions. Dr Black himself gave a correct statement of the matter, and frequently mentioned with great candour, that Mr Watt discovered unaided the latent heat of steam, and having communicated this to the doctor, that great chemist was agreeably surprised at this confirmation of the theory he had already formed, and explained that theory to Mr Watt; a theory which was not made public before the year 1762. During the same year Watt made some experiments with a Papin's digester, causing the piston of a syringe to move up and down by the force of steam of