Polytechnic Association. 929 



by a pipe will be in proportion to the difference of temperature, I 

 think it is in a geometrical ratio and not a mere proportion, so that I 

 think there will be a greater difference in favor of the extended sur- 

 face than lie calculated. 



With regard to the deposit and incrustation, I believe that always 

 occurs where there is an eddy formed, or where there is a sudden 

 access of heat ; so that the deposit would be at the bottom of the 

 boiler, and these spaces would be kept clear. 



Prof. P. H. Yan der Weyde — When the projections are on the 

 inside of the pipe they take up heat from the smoke, and the pipe is 

 hotter than it would be without them, but it cannot give the heat 

 readily to the water. One-quarter of an inch of water will totally 

 obstruct the radiation of heat. The plates projecting into the water, 

 on the other hand, enable the water to take up the heat from the pipe. 

 In illustration of the impossibility of radiating heat through water, 

 we may take a differential thermometer so sensitive as to be moyed 

 by the approach of the hand, and cover the bulb with a thin layer of 

 water, then pour on ether and set it on fire, and the thermometer will 

 not start by a hair's breadth. Or we may freeze water in the lower 

 part of a tube, and, holding it obliquely over a flame, boil water in 

 the upper part of the tube for a quarter of an hour without melting 

 the ice. 



Mr. Slangsby — I think there can be no doubt that more heat will 

 be taken from the fire by this means than with a tube in an ordinary 

 form. The only difficulty I see is in the construction of the boiler. 

 I do not see how the tubes can be removed. 



The President — That is a difficulty, but it has been overcome. 

 They are screwed on at one end and the other is put on with a 

 thimble. 



Dr. L. Bradley — I always, when I can, act as mediator between 

 contending parties. I propose to bring these two theories together, 

 and to take the heat from the fire by projections within the smoke- 

 tubes, and to impart it to the water by projections on the outside of 

 the tubes. I think that would be better than either alone; but I 

 apprehend there would be difficulty in casting the pipes. 



The President — That plan had also been thought of by myself, and 

 Dr. Bradley has stated the real objection to it. 



Manna. 



Dr. J. V. C. Smith read a paper on the manna described as gathered 

 by the Israelites in the wilderness, maintaining that it was entirely 

 [Inst.] 59 



