APPLIED MECHANICS. [russuRi or STBAM IN CYLIMDIX. 



alternations, and consequently the means of generating 

 and condensing the steam, almost unlimited power can 



In many steam-engines advantage is not taken of the 

 power derivable from the condensation of the steam- 

 its mere expansive power is employed ; and, after having 

 done its work, the expanded steam is allowed to escape 

 the atmosphere. This system is adopted for the 

 sake of economy, lightness, and simplicity in the con- 

 struction of the engine ; and such engines are called 

 hi,.ih-prtM*tn oc *on*ondenting: high-prature, because 

 the steam must exert a pressure considerably higher 

 than that of the atmosphere against which it has to act ; 

 or notHxmdenring, because the steam ia not condensed 

 after having done its work. In other steam-engines, 

 called loto-prttmn or condoning engines, although 

 greater power ia derived from the steam, yet the ma- 

 chinery is rather more complex and heavy, and more 

 liiihle to derangement ; also a Urge supply of cold water 

 is necessary to effect the condensation of the steam. Of 

 late years many engines have been advantageously em- 

 ployed, in which the steam is first caused to act as it 

 does in a non-condensing engine ; but instead of being 

 blown off into the air, it is afterwards made to do duty 

 as in a condensing engine. Such are called combined 



Fig. 146. 



t ~" 



engines, because the principles of expansion and con- 

 densation are combined in their action to a greater extent 

 than in most others. 



IX PA NSli IN. Before entering upon questions con- 

 nected with the practical construction of the steam-engine, 

 it will be advisable, in the first place, to discuss theoreti- 

 cally some of the principal facts connected with the ex- 

 pansion and pressure of steam, or generally of elastic 

 fluidti. 



If we suppose a cylindrical vessel fitted with an air- 

 tight piston, and containing within it a certain volume 

 of air, steam, or any gaseous fluid perfectly elastic, wo 

 may represent the amount of pressure which the iluid 

 exerts on the piston at B (Fig. 140) by the length of a 

 vertical line B C. For example, if the area of the 

 piston be 1 square inch, and the pressure on it at B be 

 ]."> II*., we nmy draw a line or ordinate B C 15 inches 

 !:.-:, taking 1 inch for each pound, or we might make it 

 .if inches, or 15 tenths of an inch, using a half or a 

 tenth of an inch to represent 1 Ib. of pressure, or any 

 other proportion that we may find convenient. If now 

 we apply a force to the piston so as to push it along the 

 cylinder to some place B,, and there draw an ordinato 

 B| (', having a length bearing to the pressure on the 

 piston at B,, the same proportion as the length of B C 

 bears to the pressure at B ; and if, farther, wo drew an 

 onlinate at B 2 , and others at any number of interme- 

 diate points, we might, by tracing a curve C a C, C 

 through all the points so determined, exhibit graphically 

 the rate of variation of pressure according to that of 

 volume or density. The law, to which we have already 

 alluded, called Marriotte'a law, U very simple, viz., that 

 the volume multiplied by the pressure is always constant 

 that is to say, the length of A B (which represents 



the volume of gas when the piston is at B), multiplied 

 by the height of BC (which reptcsents the presau 

 B), gives the same product as that of the length of A I ', 

 multiplied by B, C,, or of A B s multiplied by B 2 C a , 

 the temperature remaining the same. * 



If, instead of filling the length ABof the cylinder 

 with fluid, and then forcing in the piston so as to compress 

 it, increasing the pressure in proportion to the increase 

 of density, we supposed the piston to bo at B.,, and the 

 part A B s of the cylinder filled with clastic fluid, it 

 would force the piston towards B with pressure gra- 

 dually diminishing with the density according to the 

 same law, and the curve C 2 C!, C would terminate the 

 ordinates representing pressures along the stroke or dis- 

 tance passed over by the piston. This is precisely the 

 condition under which the piston of a steam-engine is 

 ordinarily worked ; and by taking a practical example 

 we may ascertain what power is developed by the ex- 

 pansion of the steam. We shall suppose that in a steam 

 cylinder of 1 square inch area the piston is close to the 

 end A ; and that by an opening there, steam, having a 

 pressure of 4 atmospheres, CO Ibs. per square inch, is 

 admitted so as to force the piston away from A to the 

 position B 2 , 1 inch from A. The steam-opening thru 

 being closed, the cylinder contains 1 cubic inch of steam 4 

 times the density of steam at atmospheric pressure, 

 and pressing on the piston with a force of CO Ibs. 

 AVhen the piston has reached B,, 2 inches from 



A, the steam has expanded to 2 cubic inches, and 

 is therefore half its former density, or twice that 

 of steam at atmospheric pressure, acting, there- 

 fore, on the piston with the force of 30 Ibs. At 



B, 4 inches from A, the density is one-fourth of 

 that at Bo, and the pressure on the piston is 

 15 Ibs. By reckoning the pressures at a number 

 of intermediate points, we might ascertain an 

 average pressure of nearly 35 Ibs. throughout the 

 stroke, and tlienco calculate the power developed 

 by the movement of the piston to be 35 Ibs. moved 

 through 4 inches, or 140 Ibs. moved through 

 1 inch by the action of 1 cubic inch of steam 

 at a pressure of 4 atmospheres a quantity that 

 would be generated from j Jot' 1 cubic inch of 

 water. 



Now, if we take another case, using the same 

 quantity or weight of steam, but at a different 

 pressure, we shall find a marked difference in the 

 power developed. Let us suppose that steam at a pres- 



The tarn C, C, C 'Fiir. 146) U oiled the kyptrtola, and it en 

 readily be traced geometrically thus : If A B (Fig. 1 17) be the given 



Fig. 1 17. 



Bi 



TOlume or Ifnuth of the cylinder when the prenro li B C, take any 

 other point R< >nd draw an ordinate Hi Q cutting in F a line C K 

 parallel to A B ; Join A F. and prolong it to meet B C in D. and through 

 D drmw D Ct parallel to A II. rutting llj O In Cj ; tben C ij the point 

 of the curre corre>pondm to B,, for A Bj x Bj Cj, or the area of the 

 rectangle A B, Ca H. U equal to A B x B C, or the area of A B C K. M 

 might readily he prored. Any number of pointt, uch ai C, being 

 the curre can be traced through them. (See ante, p. 611, Come 



