154 LECTUEES TO SCIENCE TEACHERS. 



horse-power is one which we ought to have very readily in 

 mind, because we are apt to be led away by the large figures 

 of millions; it is something like a man's income being stated 

 in francs; we want a ready method of converting the francs 

 into sovereigns. 1 have put here a table showing this con- 

 version : one pound of coal per horse-power per hour would 

 equal '22 If millions of duty of the present day. It is easy 

 to remember that one pound of coal per horse power per hour 

 is equal in round numbers to 222 millions of duty, and 

 that 2 pounds of coal per horse-power per hour are equal to 

 111 millions of duty, and so on. I had occasion to speak 

 yesterday of the difficulty even among engineers of getting 

 out of a fashion. The case of the Cornish engine is a further 

 illustration of this; the Cornish engines were very good in 

 Cornish mines, and thereupon Cornish engines were intro- 

 duced into waterworks. Now except for the purpose of 

 pumping up into a reservoir, where the head of water being 

 nearly constant, the engine is in the same condition as it is in 

 when pumping water to the surface of the ground from a 

 mine, I am prepared to give my opinion that the employment 

 of Cornish engines for waterworks purposes is an entire 

 mistake. They operate by the descent of the weight forcing 

 up the water, and that weight being constant must always 

 be balanced by a constant pressure. To obtain this, what 

 the waterworks' engineer does when he employs a Cornish 

 engine to pump into the distributing mains, is to put up a 

 stand-pipe, and if he calculates the maximum resistance the 

 engine should overcome to be equal to a head of 200 feet, he 

 puts up a stand-pipe 200 feet high, and from the top of 

 the stand-pipe he brings a return pipe which goes to the 

 main. Between eight and eleven o'clock in the morning, when 

 so large an amount of water is taken in London from the 

 mains, there is a very great draught upon them, and it is im- 

 possible to maintain 200 feet in them, in fact, there is not 

 maintained above 150 or 140 feet close to the engine-house ; 

 so that for the purpose of using the Cornish engine you pump 

 water up to the maximum height of 200 feet, and let it im- 

 mediately tumble down on the outside uselessly 40, 50, or 60 

 feet. Thus it is that although such an engine may appear to 

 be working economically when the consumption of coal is 

 compared with the stand-pipe lift of 200 feet, it may never- 

 theless be working very wastef ully when that consumption is 



