128 LECTURES TO SCIENCE TEACHERS. 



in this time. If I deliver in the same time 8 Ibs. weight of 

 steam from the same surface of water under a pressure of eight 

 atmospheres, those bubbles would be only as numerous and as 

 large as were the bubbles of the 1 Ib. of steam escaping from 

 the water at the pressure of one atmosphere, because the steam 

 being eight times as dense it is obvious that you get it off in 

 a more compact form, and that, therefore, the mechanical 

 agitation of the water is no greater under those circumstances 

 than it would be with the lower pressure and the smaller 

 quantity of steam, and that this is so is practically known to 

 every intelligent man who has the conduct of a steam-engine. 

 Such a man is well aware that if a boiler primes, the way in 

 which he can check it is by partially closing the regulator 

 so as to keep the pressure up. We equally know that if the 

 pressure suddenly falls in a boiler from any cause, such as 

 the blowing away of a safety-valve, every drop of water conies 

 out of the boiler with the rush of steam, the fact being, 

 that tranquillity at the surface, which tranquillity is necessary 

 to prevent priming, is to be obtained by proportioning the 

 ratio of the water surface to the number and size of the 

 bubbles of steam (no matter what their density) which have 

 to come through it in a given time. 



The foregoing has been a digression. I will now ask you to 

 consider the question of boiler surface. In the diagram A 

 on the wall with its single flue representing the Cornish 

 boiler, let us consider for the present the flue surface only. 

 Compare with it diagram B where the single flue is 

 replaced by seven flues each J the diameter of the large 

 one in A. It is perfectly certain, that by that means we 

 have 2J times the surface in B we had in A. Then you 

 will see if you divide each one of the seven tubes into 

 seven again, you get 2J times 2J which equals 5|- the 

 original surface. In that way you might say to me, if 

 you were not engineers, why not carry through to infinity 

 and get an infinite surface in the boiler ? There is a very 

 good answer to it ; in fact several answers, any one of which 

 is good. One is that if you have too much surface in the 

 boiler in proportion to the steam to be delivered, you do not 

 have a sufficiency of water between the tubes to insure that 

 there shall always be water in contact with them. Another 

 is that you do not afford a passage between the tubes for the 

 steam generated by the lower tubes to get clearly away to the 



