THE STEAM-ENGINE. 



405 



pacity of that vessel, and it could not by any means be transferred, as we have 

 supposed the air or gas to be, to a vessel of half the dimensions, since it is 

 inelastic and incompressible. 



The elasticity of gases is likewise varied by varying the temperature to 

 which they are exposed ; thus, in general, if air or any other gas be augment- 

 ed in temperature, it will likewise be increased in elasticity ; and if, on the 

 other hand, it be diminished in temperature, it will be likewise diminished in 

 its elastic force. The more heated, therefore, any air or gas confined in a 

 vessel becomes, the greater will be the force with which it will press on the 

 inner surface of that vessel, and tend to burst it. 



The same body may, by the agency of heat, be made to pass successively 

 through the different states of solid, liquid, and gas or vapor. The most 

 familiar and obvious example of these successive transitions is presented by 

 water. Exposed to a certain temperature, water can only exist as a solid ; as 

 the temperature is increased, the ice, or solid water, is liquefied ; and by the 

 continued application of heat, this water again undergoes a change, and as- 

 sumes the form, and acquires the mechanical qualities, of air or gas : in such 

 a state it is called steam. 



This is a common property of all liquids. If they be exposed for a sufficient 

 length of time to a sufficient degree of heat, they will always be converted 

 into elastic fluids. These are usually distinguished from air and other perma- 

 nent gases, which never are known to exist in the liquid form, by the term 

 vapor, by which, therefore, must be understood an elastic fluid which at com- 

 mon temperatures exists in the liquid or solid state ; by steam is expressed the 

 vapor of water ; and by gases, those elastic fluids which, like air, are never 

 known — at least, under ordinary circumstances — to exist in any other but the 

 elastic form. 



When a liquid is caused, by the application of heat, to take the form of an 

 elastic fluid, or is evaporated, besides acquiring the property of elasticity, it 

 always undergoes a considerable change of bulk. The amount of this change 

 is different with different liquids, and even with the same liquid it varies with 

 the circumstances under which the change is produced. 



When water is evaporated under ordinary circumstances — that is, when 

 exposed to no other external pressure than that of the atmosphere — it in- 

 creases its volume about seventeen-hundred-fold. Thus a cubic inch of 

 liquid water would form about seventeen hundred cubic inches of common 

 steam. If, however, the water be confined by a greater pressure than that 

 produced by the common atmosphere, then the increase of volume which takes 

 place in its evaporation would be less in proportion. 



The steam-engine contrived by Savery in the year 1698, like every other 

 which has since been constructed, consists of two parts, essentially distinct. 

 The first is that which is employed to generate the steam, which is called the 

 boiler ; and the second, that in which the steam is applied as a moving power. 



The former apparatus in Savery's engine consists of two strong boilers, 

 sections of which are represented at D and E in fig. 2 ; D the greater boiler, 

 and E the less. The tubes T and T' communicate with the working appar- 

 atus, which we shall presently describe. A thin plate of metal, R, is applied 

 closely to the top of the great boiler D, turning on a centre C, so that by 

 moving a lever applied to the axis C on the outside of the top, the sliding plate 

 R can be brought from the mouth of the one tube to the mouth of the other 

 alternately. This sliding-valve is called the regulator, since it is by it that 

 the communications between the boiler and two steam-vessels (hereafter de- 

 scribed)- are alternately opened and closed, the lever which effects this being 

 moved at intervals by the hand of the attendant. 



