Polytechnic Association. 927 



already superabundant. They had formerly in London a form of pipe 

 in which there were pins sticking into the smoke, to take up the heat. 

 That was abandoned ; partly from cost, and partly from the difficulty 

 of cleaning. Any addition to the surface costs something ; and the 

 question is whether this is the most economical mode. It seems to 

 me likely that the same amount of additional surface, put in the form 

 of a greater number of tubes, would answer as well. 



Prof. Van der "Weyde — I should not consider the difference between 

 measuring on the inside or outside of a pipe as amounting to much 

 on a question of this kind ; even when the tubes are small and the 

 thickness considerable. The question is whether the tubes ever get 

 too hot. If they do they take up heat faster than they can radiate it. 

 I say that as a matter of fact they do get too hot. Then this prevents 

 it. You cannot get one of these tubes red hot, while it is surrounded 

 with water. And you cannot prevent the tubes from getting too hot, 

 in this mode, without putting the heat exactly where it is wanted — in 

 the water. 



The President — Suppose the temperature of the fire to be 1,000°. 

 The amount of heat the pipe will take up is in proportion to the 

 difference of temperature between the pipe and the gases of the fire. 

 If the pipe is heated up to 500°, it will take from the fire far less heat 

 than if it is kept nearly as cool as the water. We must remember 

 that water is a bad conductor of heat ; that we heat it by convection. 

 Iron is a very much better conductor of heat than water. These pro- 

 jections or disks come in contact with the largest possible amount of 

 water, and in a manner to secure the most rapid convection of the 

 heat. The pipe is thus kept at the lowest possible temperature, and 

 in a condition to take up the largest possible amount of heat from the 

 gases. 



Mr. J. K. Fisher — Tour reasoning is very good, from the premises ; 

 but I consider the premises unduly assumed. You assume that the 

 pipe gets hot. I know that when pipes get foul, from incrustations, 

 for instance, they frequently do get hot. Sometimes they get too 

 hot because the water has not free access to them. In the case oi 

 a locomotive boiler that burst at Providence, the tubes had originally 

 been three-eighths of an inch apart. In rebuilding it, they left out 

 some of the tubes, and put the rest three-fourths of an inch apart. 

 The result was that it made steam as well as it did before. I have 

 never found evidence that in a well-proportioned and clean boiler, 

 where the water had free access to the tubes, the tubes got sensibly 

 hotter than the water. This is a question which can easily be settled 



