MUSCLES RECIPROCAL ENGINES 45 



always to have their " steam up " ; their engines must 

 continually be kept running free and using fuel when 

 they are not actually in use. They are therefore always 

 burning fuel, and are thus wasteful and expensive — 

 if we do not use them. Some of them we shall see 

 are always at work, minute after minute, so long as 

 life lasts. 



The first question an engineer asks about any new 

 kind of engine is : What is the standard of its efficiency ? 

 The engine he is in search of is one which will turn every 

 particle of heat or energy contained in oil or coal into 

 effective work. He dreams of inventing an engine which 

 will have a standard of efficiency of 100 per cent. So far 

 he has not succeeded in making one which will turn more 

 than one-fifth (20 per cent.) of the energy of fuel into 

 effective work ; the other four-fifths (80 per cent.) goes 

 up the chimney or is wasted in some other way. Now 

 in this respect the muscular engine, although it has to 

 keep running whether in use or not, is quite good even 

 from an engineer's point of view. Its standard of 

 efficiency is about 25 per cent. It is able to turn one- 

 fourth of its fuel into effective work. 1 



There is one very important difference between an 

 engine constructed of metals and one made out of living 

 flesh which I have scarcely mentioned, and it is a very 

 important difference. The metal engine can be made to 

 work for days or even for weeks at a stretch, whereas the 

 muscular engine can only work well for a limited number 

 of hours. Why should a muscle become tired ? It is 

 supplied through the blood, as we have already seen, with 

 all the elements needed to form an explosive mixture, but 

 before these elements are suitable for the peculiar kind of 

 combustion which goes on in a muscular engine, they 

 have to be worked up or compounded in the ultra micro- 

 scopic combustion spaces of the muscle itself. At present 

 we suppose that, when a muscle has been working for 



1 For methods of estimating the efficiency of muscular work the reader is 

 referred to Prof. J. S. Macdonald's paper in Proc. Roy. Soc, 1917, Series B, 

 vol. lxxxix. p. 394, and Dr M. Greenwood's paper, Proc. Roy. Soc, 1918, Series 

 B, vol. xc. p. 199. 



