A History of the Growth of the Steam-Engine

by Robert Henry Thurston

Fig. 24.—The Newcomen Model.

The Newcomen model, as it happened, had a boiler which, although made to a scale from engines in actual use, was quite incapable of furnishing steam enough to work the engine. It was about nine inches in diameter; the steam-cylinder was two inches in diameter, and of six inches stroke of piston, arranged as in [Fig. 24], which is a picture of the model as it now appears. It is retained among the most carefully-preserved treasures of the University of Glasgow.

Watt made a new boiler for the experimental investigation on which he was about to enter, and arranged it in such a manner that he could measure the quantity of water evaporated and of steam used at every stroke of the engine.

He soon discovered that it required but a very small quantity of steam to heat a very large quantity of water, and immediately attempted to determine with precision the relative weights of steam and water in the steam-cylinder when condensation took place at the down-stroke of the engine, and thus independently proved the existence of that “latent heat,” the discovery of which constitutes, also, one of the greatest of Dr. Black’s claims to distinction. Watt at once went to Dr. Black and related the remarkable fact which he had thus detected, and was, in turn, taught by Black the character of the phenomenon as it had been explained to his classes by the latter some little time previously. Watt found that, at the boiling-point, his steam, condensing, was capable of heating six times its weight of water such as was used for producing condensation.

Perceiving that steam, weight for weight even, was a vastly greater absorbent and reservoir of heat than water, Watt saw plainly the importance of taking greater care to economize it than had previously been customary. He first attempted to economize in the boiler, and made boilers with wooden “shells,” in order to prevent losses by conduction and radiation, and used a larger number of flues to secure more complete absorption of the heat from the furnace-gases. He also covered his steam-pipes with non-conducting materials, and took every precaution that his ingenuity could devise to secure complete utilization of the heat of combustion. He soon found, however, that he was not working at the most important point, and that the great source of loss was to be found in defects which he noted in the action of the steam in the cylinder. He soon concluded that the sources of loss of heat in the Newcomen engine—which would be greatly exaggerated in a small model—were:

First, the dissipation of heat by the cylinder itself, which was of brass, and was both a good conductor and a good radiator.

Secondly, the loss of heat consequent upon the necessity of cooling down the cylinder at every stroke, in producing the vacuum.

Thirdly, the loss of power due to the pressure of vapor beneath the piston, which was a consequence of the imperfect method of condensation.