THE STEAM-ENGINE. 



T) EFORE entering into the history or practical 

 13 details of our subject, it will be necessary to 

 make some general remarks on the nature and 

 properties of steam, and on the value of the differ- 

 ent kinds of fuel used in producing it. These 

 being understood, the reader will be able more 

 intelligently to follow the descriptions of the 

 steam-engine itself in its construction and 

 operation. 



Steam-engines in their infancy were always 

 known as ' fire ' (that is, heat) engines ; and in 

 point of fact the older term is the more correct, 

 because the water or steam is only used as a con- 

 venient medium through which the force which 

 we call heat is made to perform the required 

 mechanical operations. For general remarks on 

 the theory of heat, &c. our readers are referred to 

 No. 13. In this article, we shall only touch on 

 those points specially connected with our subject. 



Water is one of the few natural substances 

 which exist in the three conditions, solid, liquid, 

 and gaseous, within ordinary limits of tempera- 

 ture. That which causes ice to become water, or 

 water steam, or, conversely, that which causes 

 steam to liquefy into water, or water to congeal 

 into ice, is simply the addition or abstraction of 

 a certain quantity of heat. In other words, when 

 a pound of ice becomes a pound of water, while 

 the apparent difference is only in molecular con- 

 dition, the real difference is, that there is added 

 to the water a large and measurable quantity of 

 an imponderable and impalpable force which we 

 call heat, the nature of which is only now begin- 

 ning to be understood. We have used the word 

 impalpable advisedly, because the water will be of 

 exactly the same temperature immediately after 

 its liquefaction as before, so that a thermometer 

 placed in it would indicate 32, precisely as it 

 would have done had it been previously placed on 

 the ice. Notwithstanding this, the water contains 

 far more heat than the ice. 



Heat exists in two forms, one of which sensibly 

 affects us with warmth or cold, and is called 

 sensible heat ; while the other affects only the 

 particles of the body in which it exists, loosens 

 or altogether destroys their mutual attractions, 

 and is called latent heat. Our present system of 

 measuring heat, whether sensible or latent, as 

 temperature, arises from a perpetuation of old 

 and mistaken ideas of its nature. It is now uni- 

 versally recognised to be a force, and to be capable 

 of measurement in 'foot-pounds,'* just as much 

 as the blow of a steam-hammer. In modern 

 scientific works, quantities of heat are now stated 

 generally either in foot-pounds or in ' thermal 

 units,' t instead of in degrees of temperature. 

 Modern investigations have established the fact, 



* A foot-pound is a familiar standard of work among engineers, 

 and means a force equal to that which would raise one pound 

 avoirdupois one foot high. 



t A thermal unit is the amount of heat necessary to raise through 

 t* F. the temperature of one pound of pure water at or near its 

 temperature of maximum density namely, 39-1* F. 



27 



that the force or energy stored up in one thermal 

 unit is exactly equal to, and convertible into 772 

 foot-pounds of work. This work is therefore called 

 the mechanical equivalent of one thermal unit 



The temperature at which water becomes ice 

 has been fixed as a point on Fahrenheit's scale at 

 32, and that at which it gives off vapour of the 

 same density as the atmosphere at 212, the inter- 

 mediate stages of sensible heat being arbitrarily 

 divided into 180 equal parts. 212 F. is com- 

 monly called the boiling-point of water. This 

 expression, however, must be understood only with 

 the limitation given above, and not as being the 

 point at which water becomes steam. In this 

 latter sense, every point is a boiling-point, for all 

 water, snow, and ice, at every temperature, is 

 giving off vapour into the air. The reduced tem- 

 perature does not prevent the formation of steam, 

 but only decreases its density. Thus at 212 F. 

 water will give off steam of a density equal to that 

 of the atmosphere, or, in other words, exerting 

 a pressure in every direction equal to 14-7 Ibs. 

 per square inch. But if water be exposed to the 

 same temperature in a closed space, the remainder 

 of which is filled with air already saturated with 

 such steam, no more vapour will be given off. 

 The tendency of the particles of water to fly apart 

 is exactly balanced by the pressure of the vapour 

 on its surface. Similarly, water at 32 F. will give 

 off vapour of a pressure equal to 0-085 I DS - per 

 square inch, unless the air above it is already 

 saturated with vapour of that density. It is a 

 most remarkable fact, that while no atmospheric 

 pressure can prevent the water or ice passing into 

 vapour, the previous presence in the air of vapour 

 of the required density (even when so small as in 

 the last-named instance) entirely stops it. 



For every degree of sensible heat, there is a 

 corresponding density of steam produced. This 

 steam contains a fixed amount of latent heat, and 

 exerts a certain uniform pressure on every side of 

 any vessel in which it may be contained. The 

 following table shews the relation between these 

 different values for steam of several different 

 temperatures, and should be carefully studied : 



T. 



32 

 104 



158 



212 

 248 

 293 

 356 

 401 



0-085 

 I -06 



4-5' 

 14-7 

 28-83 

 60-4 



145-8 



250-3 



L. 



1091-1 

 1113-1 

 1129-5 

 1146-1 

 1157-0 

 1170-7 

 1189.9 

 1203-7 



H. 



842872 



859793 

 872484 



885I7S 

 893635 

 904211 

 919017 

 929593 



h. 



o 



55612 

 97411 



139363 

 167460 

 202798 

 252658 

 288634 



V. 



33900 

 312-8 

 80-02 

 26-36 

 140 

 6-992 



3-057 

 1-838 



T, Temperature (or boiling-point) in degrees Fahrenheit 



This corresponds to the sensible heat of the steam. 

 /, Pressure in pounds per square inch of the vapour at 



that temperature. 

 L, Total heat of the vapour at that temperature in 



degrees Fahrenheit, from 32. 

 H, Total heat, expressed in foot-pounds, required to raise 



I Ib. of water from 32* to the given temperature T, 



