126 LECTURES TO SCIENCE TEACHERS. 



nally did not return through a central tube, but returned by 

 a flue passing along one side across the end, and away by 

 the other side up the chimney. An improvement on that 

 construction was to make a central tube to add to the 

 surface, and when that was introduced, the products of com- 

 bustion came back along the central tube, then divided and 

 went half on one side and half on the other, by the side 

 flues to the chimney at the back. These waggon boilers were 

 largely used in cases where steam was employed, up to about 

 6 Ibs. pressure, but were not employed for pressure much 

 exceeding this. You will see from the diagram on the wall that 

 the waggon boiler is one of an extremely weak form. There 

 were occasionally stay rods connecting the crown of the arch 

 with the sides, but after all it was, as I have said, an ex- 

 tremely weak form, and was not competent to bear more than 

 the 31bs. to 6 Ibs. pressure which came upon it. Then our 

 Cornish friends showed us how to make boilers with the fire 

 contained within them the boiler which we English engineers 

 look upon as a source of economy ; but we must not authori- 

 tatively pronounce it to be so, because the French, who are 

 a highly scientific nation, dispute it altogether,- and declare 

 that you get such imperfect combustion in the case where 

 the fuel is surrounded by water that it is better to revert to the 

 system of having the fire below the boiler, coupled with the 

 use of lower tubes, " boilers " as they call them ; this favourite 

 French construction of boiler is known by us as the elephant 

 boiler. However, there is no doubt that the English continue 

 to look upon the internal fire as the correct thing, and I may 

 say that boiler insurance engineers also look upon it as being 

 far preferable to the impinging of heat externally upon a boiler 

 shell ; in fact, I think I am right in saying that the gentle- 

 man who stands at the head of that branch of our profession, 

 lays it down as a maxim that there ought not to be direct 

 fire heat applied to any portion of the boiler subjected to 

 internal bursting pressure. There is no doubt whatever 

 that within the last year and a half there have been two 

 instances of fatal explosions of boilers composed of a number 

 of tubes of not more than about 12 inches in diameter, com- 

 petent to bear a cold-water pressure of several hundred 

 pounds on the square inch. One of these explosions was 

 at a pressure, I believe, of only 50 or 60 Ibs. These 

 boilers were insured by an association I have in my mind ; 



