MECHANICS AND USEFUL ARTS. 33 



duty per pound of coal is really the most economical that a manu- 

 facturer can use, for the simple reason that the power required 

 merely to drive the engine may be so great as to render the sav- 

 ing in fuel valueless. A case in point suggests itself. An experi- 

 ment was made some time since with a compound engine, the 

 general particulars of which are before us. This engine was of 

 the annular type ; the large cylinder about 35 inches' diameter, 

 the inner cylinder about 15 inches, the stroke of both pistons was 

 the same, about 5 feet, the piston rods both laying hold of the 

 same crosshead, which was connected with an overhead beam. 

 The experiment consisted in shutting the steam off from the 

 inner cylinder and driving with the outer annular piston alone. 

 It was found that the engine, then indicating the same horse- 

 power as before, failed to drive the machinery at the proper 

 speed ; and it was not till the indicated horse-power was aug- 

 mented nearly 40 per cent, that the engine would do the work. 

 On permitting the steam to find its way to the inner c\*liiider as 

 before, the indicated horse-power fell to the original point, the 

 machinery being driven at the proper speed. We shall not pre- 

 tend to explain why this was the case. It is indeed difficult to 

 understand why the fact that the inner cylinder, though open to 

 the atmosphere, took no steam, should so enormously reduce the 

 effective power of the engine. The facts are as we have broadly 

 stated them, and there is no reason to think they would now want 

 explanation if engineers had in times past devoted a little atten- 

 tion to the study of the phenomena of friction in the steam 

 engine. We have no doubt whatever that many so-called eco- 

 nomical engines are doing very bad work indeed, nor that many 

 so-called wasteful engines, as far as coal is concerned, are giving 

 out a far higher duty than is generally believed. The entire 

 subject is wrapped up in mist, a mist which can only be dis- 

 pelled by careful experiments, extending over long periods, and 

 properly and fairly analyzed. That a few engineers have con- 

 ducted experiments on the friction of steam engines and other 

 machines is certain ; but it remains to accumulate in a single 

 volume the statistics which these gentlemen possess, and to put 

 them into a form which may render them generally useful. In 

 pursuance of this object we have for some time past been 

 accumulating data, as yet infinitely far from being complete. But 

 these data have, at all events, done this much, they have satis- 

 fied us that ordinary theories regarding friction in steam engines, 

 based on investigations concerning the coefficients of friction 

 between lubricated surfaces, apply most irregularly and imper- 

 fectly. In other words, there is no theory at present in exist- 

 ence which will enable us even approximately to predicate with 

 certainty what the loss of effect by friction in any given engine 

 may be. In certain cases, calculations made with this object will 

 correspond, with surprising exactitude, with the results obtained 

 through the indicator and dynamometer. But the engineer, rest- 

 ing satisfied with such occasional coincidences, is mistaken in Ids 

 views. In scores of other instances enormous discrepancies \vifl 

 be found to exist between theory and practice, the almost total 



