THE STEAM-ENGINE. 127 



an explosion occurred, and the engineer at the head of that 

 association is now, as I have said, convinced that it is 

 undesirable ever to expose any part of a boiler subjected 

 to a bursting strain (a strain from within tending to rend 

 it) to fire impinging upon it. 



I will therefore go back to the Cornish boiler, that is to 

 say, to the cylindrical boiler with a single cylindrical internal 

 tube, in one end of which is made the fire, and through the 

 other end of which go the products of combustion. Such a 

 boiler as that contains plenty of water ; there is ample space 

 for the water to circulate, and also a large surface which is 

 free to absorb the heat, and there is ample- surface of water 

 to give off the steam, which is a matter to which too much 

 attention cannot be paid. As an illustration of this subject 

 of the requisite surface for the delivery of the steam, we all 

 know that when we take the cork out of a bottle of soda- 

 water, full nearly to the neck, the gas will rush out and 

 drive the water before it. If we half empty the bottle, 

 recork it, and stand it vertically and let the gas accumulate 

 and then again take out the cork, you will, see a violent 

 agitation, but if you incline it so as to increase the surface of 

 the water and take out the cork there is much less agi- 

 tation. The extra surface has insured tranquillity, as a large 

 surface in a boiler insures tranquil delivery of the steam. 

 The fact being that as the particles of steam in a boiler 

 have to escape at the surface of the water, unless that 

 surface is adequate in proportion to the amount of steam 

 to be generated it is impossible to obtain a quiet delivery 

 of the steam, and in lieu thereof you have a violent 

 ebullition, and you have particles of water carried over 

 with the steam, mixed with it in the form of fine rain; to 

 which we give the name of priming. I may here, perhaps, 

 be asked by you how is it that in a locomotive boiler, which 

 is no larger than one of these Cornish boilers, but which, as 

 we all know, has so much greater evaporating power, we are 

 able to get dry steam at all. The answer to it is this. The 

 locomotive is worked at a very much greater pressure than 

 that at which most of these Cornish boilers work, and the 

 increase of pressure tends to cause tranquillity, and does so for 

 this reason : if I deliver in a given time 1 Ib. weight of steam 

 from the surface of the water at atmospheric -density I must 

 deliver a certain number of certain sized bubbles of steam 



