HISTORICAL ACCOUNT

OF THE

STEAM ENGINE.

CHAP. I.

Nature of Steam-Application of it as a moving power -Hero-Brancas-Marquis of Worcester-Sir Samuel Morland-Papin-Savery-Boaz-Newcomen-Hulls

-Falck-Amontons-Deslandes-Francois.

As the whole power of the Steam Engine depends on the employment of elastic vapour, produced from water at different temperatures, varying from 212°, or the boiling point of Fahrenheit's thermometer, to 300° of the same scale, it may be advisable in the first instance to examine some of the principal phenomena connected with the formation of vapour in its most simple form, and its application to the steam engine will then be sufficiently obvious.

Steam is highly rarefied water, the particles of which are expanded by the absorption of caloric, or the matter of

B

heat. Water rises in vapour at all temperatures, though this is usually supposed to take place only at the boiling point: when, however, the evaporation occurs below 212°, it is confined to the surface of the fluid acted upon, but at that heat, steam is formed at the bottom of the water, and ascends through it, preventing its elevation to a higher temperature, by carrying off the heat in a latent form. At the common pressure of the atmosphere, one cubic inch of water produces nearly 2000 cubic inches of aqueous vapour or steam; but the boiling point, as we have already stated, varies very considerably, and the density of the vapour produced is materially affected by the atmospheric pressure. Thus in a vacuum water boils at about 70°, under common pressure, at 212°; and when pressed by a column of mercury, five inches in height, water does not boil until it is heated to 217°; each inch of mercury producing by its pressure, a rise of about 1° in the thermometer.

According to Dr. Ure's elaborate experiments, the elastic force of this vapour at 212° is such, that it is equivalent to the pressure of a column of mercury 30 inches in height; at 226°.3, to that of 40 inches; at 238°.5, to 50.3 inches; at 257°.5, to 69.8 inches; at 273°.7, to 91.2 inches; at 285°.2, to 112.2 inches; at 312°, to 166 inches; and Mr. Woolf has ascertained that at these temperatures, omitting the last, a cubic foot of steam will expand to about five, ten, twenty, thirty, and forty times its volume respectively; its elastic force, when thus dilated, being in each case equal to the ordinary pressure of the atmosphere.

The following tables are however the result of direct experiment on the elastic force of steam and its power to support a given column of mercury, and although there is some discrepancy in the results which are thus presented, they will furnish important data for the future experimentalist. The latter table has been furnished by the French Royal

Academy, who were requested to report upon the comparative degrees of safety between high and low pressure steam engines.

One pound of Newcastle coal converts about seven pounds of boiling water into steam; and the time required to convert a given quantity of boiling water into steam, is six times that required to raise it from the freezing to the boiling point.

To shew by direct experiment the great expansive force of vapour from water when converted into steam, by the application of heat, it will only be necessary to take a glass tube, at one end of which is a bulb of two inches in diameter, and dropping into it a single spherule of water, the diameter of which will scarcely exceed one tenth of an inch, or about the eighteen hundredth part of the size of the glass bulb, we shall find, that it may very readily be expanded by the application of heat, so as to expel the air from the vessel. That this is actually the case, may be shewn by merely plunging the mouth of the tube into cold water, and suffering the steam to return to its original state, which being effected by the abstraction of a portion

of its artificial heat, the water will rush in from the external vessel, and occupy the place of the steam thus condensed, which could not have taken place had any portion of the air remained in the tube or its bulb.

From these datá, it will be evident, that when steam is merely employed to displace the air in a close vessel, and afterwards to produce a vacuum by condensation, no more heat is necessary than what will raise the water for this purpose to 212°; but if, on the contrary, high-pressure steam is required, a very considerable increase of heat will be essential; and of this kind was the elastic vapour employed in all the early steam engines to which we may now more immediately direct the reader's attention.

Among the numerous competitors for the honour of having first suggested steam as a moving power in mechanics, we must certainly place Hero of Alexandria, Brancas, and the Marquis of Worcester, in the foremost rank.

The simple apparatus suggested by Hero about two thousand years ago, may be readily understood by reference to the annexed figure.