218 KEPORT— 1873. 



ill practice. The engine at a certain pressure and a certain cut-olT and at a certain 

 velocity may be calculated to do its work at the greatest economy in steam ; but 

 if the exigencies of trade require more work to be got out of the engine, these con- 

 ditions are at once disturbed, and the theoretical mechanic tells me that I am not 

 upon the best footing. The precaution must also be taken that the engine must 

 in all cases be above its work ; but whether above or below its work a theoretical 

 drawback is involved : if underweighted to begin with, the load, as a rvde, is 

 bit by bit increased until it is overweighted ; and this, not that the manufac- 

 turer is ignorant, but that he has sacrificed theoretical advantages to the exigencies 

 of his trade. 



Where steadiness of motion is required, it seems preferable to use a pair instead 

 of a single engine ; and it is advantageous al^o to use the steam expansively up to 

 a certain point, although by so doing the mechanical result obtainable from the 

 engine is proportionately diminished. Where high pressures are used it is better 

 to have a compound engine, using the steam throughout the double stroke by 

 means of a smaller cylinder exhausting into a larger one. To arrange differently 

 involves a gi-eat waste of metal in the engine, and very heavy pressure upon the 

 bearings, especially when close to the dead centres ; for the engine must be con- 

 structed to resist the iiiaximmu strain, even though it be during an inconsiderable 

 portion of the stroke only. I am acquainted with a case where high pressure, high 

 rate of expansion, combined with great steadiness of motion were required ; and in 

 order that this threefold object should be accomplished, a pair of condensing-engines 

 Avere compounded with a pair of high-pressure engines, the four engines working 

 all in a block, each engine receiving only its own strain, and all coupled together 

 by means of the pinions upon the line-shaft. Here we have four engines dividing 

 amongst them, with the most satisfactory results, the work which might be done 

 by a single engine of larger dimensions. The strain is equalized over the different 

 cranks, fly-wheel, shafts, segments, wheels, and bearings ; there is the most exqui- 

 site steadiness of motion, gi-eat economy of fuel, and a complete absence of break- 

 downs and accidents. These four engines have now been working in combination 

 several years without accident or breakdown ; and this, in itself, is no slight advan- 

 tage. Greater economy in steam might be realized by cutting off earlier in the 

 stroke, and more work might be got from the engine by cutting off later ; but it is 

 not always easy, neither may it be desirable, to alter existing arrangements. The 

 fact which has already been noted in tlie boiler reappears in the steam-engine — 

 that by economizing fuel to the utmost less work is got out of the boiler, so by 

 economizing steam to the utmost, less Avork is got out of the steam-engine. If the 

 pressure upon the piston be 60 lbs. to the inch continued throughout the entire 

 strolce, the maxiiuum amoimt of work is got from the engine ; but in this case there 

 is no gain from expansion : but if the initial pressure be 00 lbs., and the cut-off be at 

 one eighth of the stroke, the gain from expansion is considerable ; but the average 

 pressure is 2.3 lbs. onlj^, being considerably less than half the work which the 

 engine is able and which it was constructed to perform. Every part of the engine 

 will have been made to stand safely the maximum pressure of 60 lbs., without 

 which it would break to pieces at once ; the difference between this and the 

 minimum is so much strength thrown away. Thus we find that, like every thing 

 else in the world, economy itself must be purchased, and sometimes at too great 

 a cost ; for even as regards expansion there is a limit at which it ceases to be 

 profitable. 



The steam-engine may be made into a valuable adjunct to the boiler as an instru- 

 ment for the generation of heat, by retaining and restoring to the boiler a consi- 

 derable portion of the waste heat, which has already done its work as a motive 

 power. Primarily this is done by using a portion of the injection-water ; but the 

 beneficial result may be considerably enhanced by causing this water to travel 

 through a series of copper pipes which receive the impact of the steam on its 

 passage from the cylinder to the condenser. A considerable amount of waste heat 

 may thus be recovered and utilized, and the injection- water itself is also correspond- 

 ingly economized. 



Compounding under its difierent aspects is here recommended ; nor is there any 

 thing in it opposed to scientific or mechanical simplicity. Compound the boiler 



