MECHANICS AND USEFUL ARTS. 35 



stream; and that as in the river, so in the tube, a series of eddies will be 

 formed, tending to bring all portions of the gas in contact with the sides of 

 the tube. This peculiar motion of gases in a tube may very clearly be 

 observed in the smoke issuing from the funnel of a steamer, the smoke 

 retaining the eddying motion which it had in the funnel for some time after 

 leaving it. 



These considerations led me to consider the mere disturbance of the cur- 

 rents as inadequate to explain the increased evaporation observed, and to 

 attribute it to a very different cause. Gases do not radiate the heat which 

 they contain; so that the only mode in which a gas can communicate its heat 

 to a surface is by contact or convection. This is, in the .present practice, the 

 only mode in which those heating surfaces of a boiler which are not exposed 

 to the radiation of the fire, or flame, can abstract heat from the products of 

 combustion ; but if in a flue or tube a solid body be introduced, it will be- 

 come heated by contact with the gases, and will radiate the heat thus 

 received to the sides of the flue. Now these diffusers, etc., exactly fulfil 

 these conditions; and I, therefore, attribute their effect mainly, if not en- 

 tirely, to the function which they must fulfil in absorbing heat from the 

 gases by contact, and then radiating this heat to the sides of the tubes or 

 flues. And I think it will be admitted, that the amount of heat thus con- 

 veyed to the water may be very important, when it is considered that the 

 temperature of the gases in the tubes of a boiler, at five or six inches 

 from the fire-box tube-plates, is about eight hundred degrees Fahrenheit; 

 and that these radiators will consequently have a temperature of several 

 hundred degrees above that of the surfaces in contact with the water in the 

 boiler, and that a very active radiation must consequently take place from 

 one to the other. This principle once established, the modes of application 

 in practice are, of course, endless; and I will only mention that I do not see 

 any advantage in making these radiating surfaces of such a form as to im- 

 pede the draught, especially in the case of marine boilers, but would 

 rather choose the form which would give the greatest amount of radiating 

 surface, and offer the least impediment to the free passage of the products 

 of combustion through the tubes. Perhaps as effective a form as any for 

 placing in the tubes of boilers would be a simple straight band of metal, or 

 a wider band bent in the direction of its breadth at an angle of sixty degrees. 

 In the case of marine boilers, they should be made so as to draw out easily, 

 to enable the tubes to be swept. 



ON THE COMBUSTION OF WET FUEL. 



The following is an abstract of a paper read before the American Associa- 

 tion, 1800, by Professor B. Silliman, Jr., " On the Combustion of Wet Fuel 

 in the Furnace devised by Moses Thompson." In all ordinary modes of 

 combustion, it is well known that the use of wet fuel is attended with a very 

 great loss of heat, rendered latent in the conversion of water into steam. 

 As the most perfectly air-dried wood still contains about twenty-five per 

 cent, of water, the term wet fuel might seem appropriate to all fuels but 

 mineral coal and charcoal. But, technically, this term is restricted to sub- 

 stances like peat, and those residual products of the arts, which, like wet tan, 

 begasse, and spent dye-stuffs, contain at least one-half, and often more than" 

 half, their weight of water. Until a recent period, the attempt to consume 

 these products as sources of heat has been attended with uneconomical results, 



