168 LECTURES TO SCIENCE TEACHERS. 



ing power of these engines at from 200 up to 600 revolutions. 

 To do that it became necessary to take indicator diagrams, 

 and the way I set about it was by limiting the stroke of the 

 indicator, and by employing an extremely strong spring. I got 

 for 120 Ibs. steam only one inch rise on the figure. I then 

 limited the circumferential traverse of the indicator in the 

 same way ; T. provided it with an india-rubber spring, so as to 

 make it speak quickly, and in that manner, by having a 

 diagram not more than an inch in either direction, I obtained 

 most perfect and accurate figures from an engine running 

 at 600 revolutions. You will see them published in the 

 Transactions of the Institute of Naval Architects. By the 

 indicator we are able to tell, not merely the total work 

 that the engine is doing, but whether it is doing it economi- 

 cally and efficiently or not, and if not, are enabled to say 

 where an alteration ought to be made. The indicator gives 

 us the gross indicated horse-power, from which has to be 

 deducted the friction of the different parts of the engine. I 

 may say upon that, that at the Eoyal Agricultural Society, 

 when experimenting we had always used the dynamometrical 

 brake, the actual brake which is shown in this Exhibition. 

 On the last occasion, still using this brake, we put on indi- 

 cators, and were thereby enabled to obtain the ratio of effect 

 on the brake as compared with the indicated horse-power. 

 Speaking from recollection it averaged, taking one engine 

 with another, a difference of 17 per cent. These were non- 

 condensing engines, but I should call your attention to the 

 fact that they were worked in the excellent manner I spoke 

 of yesterday, and yielded results so high, that in one case 

 we got 79*49 million pounds raised one foot for 1 cwt. of 

 coal; a duty that probably is not attained by any single 

 engine in Cornwall, for I am sorry to say that the Cornish 

 engines have very much fallen off in their performances. 



I should like now to say a few words about the injector. 

 We have here, cut open, one of the Giffard injectors so 

 commonly used for feeding boilers ; and I remember distinctly 

 when this instrument was first brought to my attention by 

 Mr. Robinson, of Sharp, Stewart, & Co., I and several en- 

 gineers were together, and, on being told by Mr. Robinson 

 that there was an implement by which the steam from a 

 boiler could be caused to generate a jet of water so powerful 

 as to enter that same boiler against the pressure within it, with 



