THE STKAM-K\<;i\K. 



273 



TMK STKAM-KNGINE. I. 



UEAT THE BOUHCB OF ITS POWER THE BOILER WAGON 

 BOILER UOILEK FEED Al'hARATUH BAFETY- 



V A i 



THE present might not inappropriately bo termed tho age of 



ht-iim. To its agency wo are in a greater or less degree in- 



for tho manufacture of a largo proportion of tho articles 



we daily employ ; in almost every factory it is now employed as 



:MI'- mover, and fresh applications of its power are con- 

 tinually bring made. I necessary, therefore, for all to 

 be acquainted with tho construction and mode of action of tho 



ngine; and wo shall accordingly endeavour in these 

 lessons to convey briefly such a general idea of it as will enable 

 tho student, by a little observation, to understand the principle 

 of any engine he may meet with. To explain fully the details of 



notion, and tho almost endlvss modifications which have 

 IHVII madi', would require several volumes instead of a few pages. 

 At tho outset it is of tho utmost import- c 



ance clearly to understand that no ma- ^yw o ^^ 



chine can create force it can only modify 

 its action. The original source of tho 

 power of the engine is in the fuel which is 

 consumed in tho furnace. When coal, or 

 any other substance, is burnt, a certain 

 definite amount of heat is produced, and 

 this heat, as ia explained in 

 our lessons on that subject, 

 is capable of performing a 

 definite amount of work. Theo- 

 retically, the combustion of 

 one pound of coal evolves suf- 

 ficient heat to raise a little 

 over 11,000,000 Ibs., or about 

 5,000 tons, to the height of one 

 foot. It is found, however, that 

 even in our best constructed 

 engines scarcely more than one- 

 eighth of this is practically 

 turned to account, and usually 

 the proportion is smaller. In 

 this respect there has been a 

 most remarkable absence of im- 

 provement during the last fifty 

 years, and any person who could 

 devise some plan for utilising 

 the greater portion of the heat 

 actually produced would effect 

 one of the most important dis- 

 coveries ever made in connec- 

 tion with the stuam -engine. 



In the steam-engine heat is 

 employed by virtue of its power 

 in the conversion of water into 

 steam. If wo take a cubic inch 



of water, and apply heat to it, it will first boil, and then gra- J 

 dually become converted into steam. If this steam be carefully | 

 collected in a vessel, and tho space it occupies at the usual 

 pressure of tho air be measured, it will be found to be about 

 1,700 cubic inches, or nearly one cubic foot. Its bulk has 

 therefore been increased to this extent by the influence of tho 

 heat applied. To make this -quite clear, let us suppose that wo 

 have a tube of indefinite length, and having a sectional area of 

 exactly one square inch. Put one inch of water in the bottom 

 of this, and on it place a piston moving easily in the tube, but 

 fitting it air-tight. The air, as we know, presses on this piston 

 with a force of fifteen pounds. Now let us apply heat to tho 

 water, as before, till the whole of it has become converted into 

 steam; the piston will have been raised 1,700 inches, since tho 

 steam occupies that space. It is clear, therefore, that the work 

 thus accomplished by the evaporation of a cubic inch of water 

 has been to raise a weight of fifteen pounds to a height of 

 1,700 inches, or nearly 142 feet. This is equivalent to raising 

 142 x 15, or 2,130 Ibs., 1 foot. Speaking roughly, then, wo may 

 say that the evaporation of a cubic inch of water produces 

 power enough to raise a ton weight to the height of one foot. 

 This general statement should be remembered as it will fre- 

 quently be useful. 



148-N.E. 



Fig. 4. 



f 



If tho steam in the tube be now re-con verted into watnr by 

 being condensed, a vacuum will bo produced, and a precisely 

 hiinilar amount of force will be (railed into action. 



We understand clearly, then, our source of power, and we are 

 thus in a position to inquire into the two essential point* of a 

 Btoam-engine, which are 



1. Tho Boiler (with its furnace), in which the water is con- 

 verted into steam. 



2. The Engine itself, or the mechanism by which the force of 

 the steam is made to accomplish tho required work. 



These wo must consider separately, an in reality they are 

 quite distinct. The boiler, in fact, in not unfrequcntly placed 

 at a considerable distance from the machinery, the steam being 

 conveyed to it by suitable pipes. 



It consists essentially of a large metal vessel to hold the 

 water which is to be converted into steam. This mast be suffi- 

 ciently strong to withstand the pressure of the steam, and 

 must bo so arranged as to allow of the fire being conveniently 

 n and economically applied to it. If the fire 

 is merely placed under the metal vessel, 

 and tho heated air at once allowed to 

 escape, a great loss is sustained, as the 

 smoke and burnt air that ascend the 

 chimney are still at a high temperature. 

 A flue is therefore usually arranged, so 

 that tho hot air from the furnace, after 

 passing along under tho boiler, 

 returns and passes round its 

 sides before entering the chim- 

 ney. 



There ia an almost endless 

 variety in tho forms given to 

 boilers, sometimes one being 

 considered preferable and some- 

 times another, according to the 

 special circumstances of tho case. 

 The two most general forms, 

 however, are those distinguished 

 as the wagon-shaped and the 

 cylindrical. 



The wagon boilers have some- 

 what the shape of a covered 

 wagon ; the bottom is concave, 

 so that the section of the whole 

 resembles Fig. 1. The sides are 

 also bulged slightly inwards, to 

 aid in the construction of the 

 side flues, B, B. A is the furnace, 

 with the ash-pit, c, under it 

 Boilers are now always con- 

 structed of iron plates riveted 

 together. These are bent to 

 the required shape, and holes 

 are drilled or punched in them 

 to receive the rivets, which are 



inserted while red-hot, and at once hammered down. In cool- 

 ing they contract, and force the plates into steam-tight contact. 

 Sometimes the seams are caulked by hammering the edges of 

 the plates with a punch. 



In cylindrical boilers the furnace is usually internal, as repre- 

 sented in Fig. 2. The boiler, in fact, consists of two tubes, one 

 within the other. The water occupies the space between them. 

 The furnace-bars, A, on which the fire is kindled, occupy one 

 end of the inner tube ; the heated air then passes along this 

 tube, returning by flues which surround the boiler, as in the 

 former case. This form is generally known as the Cornish 

 boiler, and not unfrequently two internal furnaces or flues aro 

 employed in place of one (Fig. 3). The advantage of thu 

 construction is that all the heat from the furnaco and ash-pit is 

 imparted directly to the boiler, whereas in other forms a con- 

 siderable portion is absorbed by the brickwork. 



Sometimes, especially in locomotive or marine engines, where 

 space is an important object, multi-tubular boilers are employed. 

 In these there are a large number of internal flues or pipes, 

 usually two or three inches in diameter, through which the heated 

 air passes. It is in this way broken up into a number of small 

 streams, which, as they travel along these tubes, give np to 

 thorn the greater portion of their heat. In fact, so much of the 



