CHAMBERS'S INFORMATION FOR THE PEOPLE. 



and to evaporate it at that temperature { = mechanical 

 equivalent of the total heat of i Ib. of steam at T). 



A, Heat in foot-pounds required to raise temperature of 

 I Ib. of water from 32 to temperature T ( = mechani- 

 cal equivalent of the sensible heat of I Ib. of water or 

 steam at T). 



V, Volume in cubic feet occupied by I Ib. of steam. 



H k, Latent heat of I Ib. steam at T in foot-pounds. 



H ; h ; or (H /*), divided by 772, will respectively 

 give the total, sensible, or latent heat in thermal 

 units. 



The heat in degrees Fahrenheit is given according to 

 Regnault's hypothesis. 



Several of the properties possessed by steam 

 render it peculiarly suitable for use as a source 

 of motion in prime movers. Among them are 

 its elasticity and its easy convertibility from a 

 gaseous to a liquid state. The first of these 

 manifests itself in the tendency of steam, in 

 common with other gases, to expand inde- 

 finitely, undergoing at the same time a corre- 

 sponding diminution of density or pressure. In 

 obedience to a law known as Mariotte's law (from 

 the name of its discoverer), the density or pressure 

 of steam varies inversely as its volume. This 

 means that if a cubic foot of steam be allowed to 

 expand itself into a space of two cubic feet, its 

 pressure will be halved ; if into ten cubic feet, its 

 pressure will be only one-tenth, and so on. In 

 speaking of pressure in this article, we shall mean 

 always (unless the contrary is stated) pressure 

 above the zero of pressures, or a perfect vacuum, 

 and not above the pressure of the atmosphere. 



The easy convertibility of steam into water is 

 a matter of great importance in relation to the 

 economy and efficiency of engines. The reason 

 of this will be found further on in the explana- 

 tion of the theory of condensing engines ; we may 

 merely say here, that by the use of condensers 

 much power is gained to the engine with scarcely 

 any extra cost, which otherwise would be entirely 

 thrown away. 



Having now given some idea of the nature of 

 heat and the properties of steam, we shall proceed 

 to shew the way in which these are utilised by the 

 engineer in the steam-engine. As will be here- 

 after explained in detail, the common mode of 

 employing steam in an engine is by causing it to 

 press alternately on the two surfaces of a movable 

 diaphragm or piston inclosed in a fixed, steam- 

 tight, cylindrical box. In fig. i, A is the piston, 

 and B a section of the box The piston, by means 



Fig. i. 



of a rod E, passing through the end of the box, 

 is made to communicate motion to the rest of the 

 machinery. The steam is first admitted to one 

 end of the cylinder through an opening D, and 

 forces the piston along to the other end. The 

 current of steam from the boiler is then allowed 

 to pass into the other end of the cylinder through 

 the opening C, and forces the piston back again 



418 



to its original position, and so on. But it is 

 obvious that while this return-motion is going on, 

 the steam previously admitted at D must be 

 allowed some exit, or the piston could not be 

 forced back. The manner of this exit constitutes 

 the difference between the two principal classes of 

 engines, according as the steam is allowed simply 

 to rush out into the atmosphere, or is conducted 

 into a separate vessel, and there ' condensed.' 



The simplest way in which steam can be used 

 in a cylinder is at the same time the most waste- 

 ful. It consists in filling each end of the cylinder 

 alternately full of steam direct from the boiler, and 

 having the full boiler pressure, and thus forcing 

 the piston along in exactly the same way as that 

 in which it would have to be forced were water 

 the fluid used instead of steam. We have said 

 this is wasteful; let us examine the reasons. If 

 we imagine the cylinder to have a capacity of 7 

 cubic feet, then, if it be filled entirely with steam 

 from the boiler at 60 Ibs. pressure, it will contain 

 just one pound-weight of steam.* The total heat 

 in this pound of steam, as given in the table, is 

 equivalent to 904,21 1 foot-pounds of energy. When 

 the piston A has reached the end of its stroke, the 

 steam contained in the cylinder is thus in itself a 

 great storehouse of work. But instead of utilising 

 this force, at the moment when the cylinder is full 

 of steam, the opening C is put into communication 

 with the boiler, the opening D with the atmos- 

 phere, and the steam immediately rushes out of 

 the cylinder, and dissipates its contained energy 

 through the air. 



It must be remembered that although the steam, 

 when allowed to go into the atmosphere, is imme- 

 diately reduced to the pressure corresponding to 

 the temperature of the air (which in ordinary 

 cases would be only a fraction of a pound per 

 square inch), still the full pressure of the atmos- 

 phere itself will always be acting on the back of 

 the piston during its stroke, and that therefore, to 

 find the force with which the piston is being 

 pushed along, we must subtract that pressure 

 from the steam-pressure. On the one side of the 

 piston will be the atmosphere with its uniform 

 pressure of nearly 15 Ibs. per square inch, and on 

 the other side the steam pressure of 60 Ibs. The 

 effective pressure thus will be 60 1 5, or 45 Ibs. 

 per square inch only. 



Let us now consider the somewhat more eco- 

 nomical case of an engine in which the steam is 

 first used as described above, but afterwards, 

 instead of being allowed to pass into the atmos- 

 phere, is conducted through a pipe into a closed 

 vessel, and there condensed. The process com- 

 monly called condensation, and associated with 

 the idea of liquefaction, consists in essence merely 

 of the subtraction from steam of a portion of its 

 sensible heat This reduction of temperature has 

 a double effect on the steam : first, the liquefaction 

 of a part of it ; and then, the reduction of the rest 

 to the pressure corresponding to the reduced tem- 

 perature. (It will be remembered that we have 

 stated that steam exists at all temperatures.) It 

 is not possible to do one of these things without 

 the other, and this fact lies at the bottom of a 

 correct conception of what is called by engineers 

 a ' vacuum.' What is commonly called ' vacuum ' 



* These figures are near approximations only, as will be seen 

 from the table. 



