A great number of formulae have been proposed for the purpose of calculating the relation between the elastic force of vapour and the temperature of the water producing it ; such are the formula: of Laplace, Ivory, Poisson, Lubbock, Do Prony. The simplest of them is un questionably the one given by Dr. Young, F = (1 4- 04002W; or, as given by Arago and Dulong for the centigrade scale, P= + ; and Regnault found that the results it gave corresponded very nearly with those obtained by actual observation, when the vapour was given off from water heated beyond 212°, but that below that temperature tho results no longer agreed. Regnault adopted, therefore, another formula proposed by Biot, and discussed at length by that philosopher in his Trait6 de Physique,' 1816: but, as this formula contains no less than five constants, and the indications given by the simpler formula of Arago are admitted to be correct for all the pressures dealt with in practice, there can be little reason for adopting the former.
It is important, in calculations with respect to the use of steam as a motive power, to ascertain whether the latent heat of the steam remains the same at all temperatures ; and M. Regnault has added to the other services he has rendered to science by going through a series of direct experiments for the purpose of verifying the laws of Watt and of Southern upon this subject which had previously been received. Regnault found that at the ordinary atmospheric pressure it required 510 thermal units of the French scale to convert I gramme of water into steam (971,636 English thermal units to convert 1 pound avoinlupois); and thus far his experiments (recorded in the ' 316 moires do l'Acadeinie des Sciences,' voL xxi.) agreed with theme of other observers. Watt, however, had laid down the Law " that the quantity of heat necessary to vaporise a given weight of water was the Huns at every degree of the thermometer ; " and Southern had stated " that the latent heat of vaporisation was the same under every degree of pressure;" De Pambour's observations appeared also to show that, up to a pressure of about 65 lbs. on the superficial inch, Wntt's law was substantially correct. Rognault, on the contrary, found that Watt's law was not exact, and that Southern's was still more falla cious ; for he found that when the temperature increased, the total heat also increased, but the latent heat diminished. The results of his experiments were given in the following formula, in which X is the total heat (on the centigrade scale), and t the tempe rature of the steam ; the coefficient 606.5 being the value assigned by
him as the latent heat of vapour at and the specific heat of steam. Regnault ascertained at the same time that between the limits of 0° and 392' there was hardly any difference in the specific heat of water, so that there was no occasion to take that difference into account. In the 3f4tnoire wherein these observations are recorded a table is inserted, showing the tensions, and the total heat of the vapour given off at temperatures increasing by equal increments of 10 degrees centigrade, between 0° and The table in question is also given in Daguin's "rrait6 de Physique,' t. 1., part 2, p. 965. It may be as well to add that the laws of the latent heat of steam are practically applied when buildings are heated by means of it; in these cases the heat absorbed, or rendered latent in one place, is distributed by the pipes which enable the steam to circulate over other parts of a building.
The following table of the densities of vapours is extracted from Daguin's work ; it is based upon the experiments of MM. Gay-Lussac and Dumas, and the density of air is taken as unity.
— — been the nature and date of its origin, it has been reared to its present gigantic stature by the fostering care of different countries, and, with out detracting from or underrating the efforts of others, England may be justly proud of her share of the glory, a share readily conceded by our competitors.
Considering therefore dispute as unprofitable, and the discussion of dates of patents and improvements as uninteresting, we shall incorpo rate all that is requisite of the history of the engine with our account of it.
A deem-engine may be defined generally as an engine by which the forces arising from the properties of elasticity and of instantaneous condensation possessed by steam are transmitted to produce n con tinuous rotatory motion, either of a fly-wheel designed to constitute a reservoir of power for the purposes of driving machinery, or for any other uses that force may be put to.
Admitting this definition, the earlier ateam-engines, as they are commonly called, those of the 3Iarquesa of Worcester (1663), and the improved forms contrived or suggested by others, and even Captain Savery's (1693), which was long employed in this country, were only pumps for raising water : a partial vactuun was formed in close vessels by the condensation of steam within them, the atmospheric pressure raised the water to a certain height ; from whence it was forced higher by the elasticity of the steam admitted to act on its surface.