406 Remarks on Mr. J. Taylor's Paper 



steam ; and the consequence is the passing over of a quan- 

 tity of water into the cylindei', to the injury, and sometimes to 

 the destruction of the engine itself. On the other hand, if this 

 space be divided, not in depth but in cubical contents, there 

 is great reason to apprehend the water being allowed to get 

 below the top of the tube : a temporary derangement of the 

 feed-pump might occasion this, without any very great neglect 

 on the part of the engine-man. If this takes place, I scarcely 

 need point out what must be the result. The expansive force 

 of the steam exerted upon the plates rendered soft by the ac- 

 tion of the fire, would bring down the upper surface ; and when 

 once the cylindrical form was lost, a further depression would 

 be rendered easy. It is quite clear that the upper surface of 

 the tube cannot be depressed, without such a strain being 

 thrown upon the ends where they form a junction with the 

 angle iron, as finally to rend them off. I should observe here, 

 that the sort of fracture described by Mr. Taylor in the boiler 

 at East Crennis, would in this case present itself; the angle 

 iron would appear to be wrenched off by a force drawing it 

 inwards. 



Even where the water is not so far reduced in the boiler as 

 to be below the top of the tube, I am by no means inclined 

 to consider this boiler in a safe state. At all times the upper 

 part of the tube is more expanded than the lower, in conse- 

 quence of the water above being hotter than it is below ; and 

 although this may not produce an immediate effect, yet it very 

 probably renders the parts, where the strain is thrown in con- 

 sequence of this unequal expansion, more disposed to give 

 way when a further stress comes upon them. A few inches 

 of water over the tube would certainly prevent the plates from 

 becoming red-hot, and perhaps the leaden plug fi'om melting ; 

 but it would not be sufficient to prevent the strength of the 

 iron being impaired. 



The strength of iron is much impaired before it arrives at 

 the heat of melted lead. I have every reason to believe (and 

 this beUef is founded upon effects which I have myself wit- 

 nessed), that the plates of a boiler urged by an intense fire, 

 and covered with only a thin stratum of water, become very 

 considerably hotter than the steam and water above them. 

 1 account for it in this way: — When the column of water is di- 

 minished to a certain extent, the weight of it is not sufficient 

 to keep it in contact with the plates, the continuous escape 

 of steam bubbles keeping it oft". This effect may be observed 

 in an open pan placed over an intense fire, and containing a 

 thin covering of water or other fluid; the whole appears to 

 be a mass of bubbles, and the bottom of the pan may be oc- 

 casionally 



