170 LECTURES TO SCIENCE TEACHERS. 



velocity, would obviously concentrate on that -^ square inch 

 the whole of the energy which, if it remained water, would 

 otherwise have been applied on the square inch, and thus 

 by this act of concentration there would be 14 times the 

 power per unit of surface, and it would come with the same 

 effect as if it came from 14 times the height. With such a 

 concentration it would enter into the second tank not merely 

 against a head equal to that from which it came, but against 

 a head very nearly 14 times as great; and you see, therefore, 

 that if you could by magic convert water into mercury, you 

 would then get the power to enter against a very excessive 

 pressure to re-enter its own tank, and much more. Now, 

 instead of taking water issuing from a tank, let us take steam 

 issuing from a boiler, and that steam without magic we can 

 concentrate, because we can condense it, and having con- 

 densed it, we do not concentrate it only 14 times, but 1,696, 

 or 388, or 144 times, or whatever may be the ratio of the 

 particular steam to water. By that concentration, therefore, 

 we have an enormous power, competent not only by itself 

 to re-enter the boiler from which the steam came, but a 

 power so much in excess of this as to be able to take along 

 with it the water employed in producing the concentration, 

 that is the condensation of the steam. In that way the Giffarcl 

 injector does introduce into the boiler the steam that came 

 out of it, and the water that condensed that steam. 



Except in one solitary instance (that of the surface of 

 boilers) I have abstained from giving any proportions ; still 

 less have I had opportunity to indicate the relations between 

 dimensions and horse-power, and I may say that had I had 

 time to dwell upon this branch of the subject of the steam- 

 engine I should have been met with the curious difficulty of 

 not being able to say what in manufacture is meant by a 

 horse-power. But I may tell you now, that, among the five 

 kinds of horse-power which are recognised (for there are five), 

 I, whenever I have used the term horse-power, have referred 

 to the engineer's horse-power of 33,000 Ibs. (roundly, 15 

 tons) raised through one foot in a minute of time. 



Dr. Joule and others have shown us that 772 foot Ibs. 

 represent the equivalent of one heat unit, and we know, 

 there fore,t hat 33,000 foot Ibs. per minute ought to be got for 

 an hourly expenditure of 2,565 heat units. Llangennech coal 

 is capable of giving forth 14,718 heat units per lb., and 



