162 LECTURES TO SCIENCE TEACHERS. 



off at one-eighth of the stroke, you will find that the varia- 

 tion of its tangential force is as much as 45 per cent, in excess 

 of the variation of the tangential force of the engine without 

 expansion. Now that excess of irregularity may be absolutely 

 neutralized by increasing the velocity of the supposed engine 

 from 60 to 69 revolutions. That is to say, supposing the 

 designer of the expansive engine wanted 240 gross horse- 

 power, he might either obtain it and a sufficient approxima- 

 tion to regularity by an engine running at 60 revolutions 

 without expansion, or he might so proportion the gearing 

 that it should run 69 revolutions. Were he so to pro- 

 portion it, it is obvious that the mean work done by the 

 piston at each revolution would be f ths of what he would 

 get in the first case, and taking this reduction of the mean 

 work and the increased effect of the fly-wheel, running at an 

 extra 9 revolutions, you will find that the same size and 

 weight of fly-wheel will cause this highly expansive engine 

 to work with the same approximation to regularity as that 

 with which the non-expansive engine running at 60 revolu- 

 tions would work. I think this fact is not sufficiently borne 

 in mind when engines are designed for driving any particular 

 kind of machinery. 



There is no doubt that the double cylinder engine is the 

 engine of the present day. I do not think its economy really 

 lies in its principle, but that its economy in practice arises 

 from another thing altogether, and that is this, that by making 

 a double cylinder engine you put it out of the power of an 

 ignorant engine-driver to do away with that which you want 

 high expansion ; he must get high expansion ; he is com- 

 pelled to use it whether he likes it or not, whereas with the 

 single cylinder expansive engine he has the power to follow 

 the dictates of his own ignorance, and as a matter of obser- 

 vation I have hardly ever seen such an engine left to the 

 control of an engine-driver but it invariably worked at the 

 lowest possible grade, and, as I have said, I believe that this 

 withdrawal of control is to a large extent the secret of the 

 success of the compound cylinder engine. 



Further with regard to expansion, I wish now to speak of 

 the question of the steam jacket. The steam jacket we owe 

 to Watt, and it was condemned by one of our earliest writers 

 on the steam-engine, Tredgold, who said it was merely adding 

 to the surface to be cooled by the air, but we know now that 



