56 Emission and Transmission of Heat 



very great, and the velocity of the air at its exit very high, that 

 the greater part of the central veins would not come into contact 

 with the wall of the conduit, and would preserve their original 

 temperature. But if one provided the conduit with fan wheels 

 mounted on a shaft driven by power, the central veins would be 

 thrown out against the metal of the conduit and one would thus 

 obtain much greater cooling of the gas. The movement of the 

 gas from the centre to the circumference could even be produced 

 by the passage of the gas itself. It would suffice to place in the 

 conduit a number of wheels with vanes inclined as are the vanes 

 of a windmill ; these wheels on which the movement of transla- 

 tion of the gas would impress a certain velocity of rotation, would 

 evidently produce the same effect as wheels with blades set in the 

 plane of the axis of rotation. 



839. The transmission of heat may be increased by still 

 another proceeding which is in some circumstances very efficaci- 

 ous. 



We have seen that in the transmission of heat through a 

 metal plate, it was necessary to distinguish between the absorp- 

 tion by one face, the emission by the other and the transmission 

 through the metal ; and we know that under ordinary circum- 

 stances, the quantity of heat that the plate can transmit is much 

 greater than that which it can absorb or emit. It results from 

 this that if we use plates provided with projecting rods that dip 

 into the two liquids or gases, the one of which is to heat the 

 other, we increase the extent of the surfaces in contact with the 

 two fluids, and in consequence the effect produced ; so much the 

 more since the veins of fluid in contact with the rods will be con- 

 stantly renewed by the motion of translation of the fluids. 



We may take for an illustration a horizontal conduit trav- 

 ersed by highly heated air intended to heat the air, moving in 

 an opposite direction, in a surrounding conduit. If the surface 

 of the inner conduit is provided with rods projecting inside and 

 out, and if they are not arranged in parallel rows, the inner por- 

 tion of the bars will absorb heat throughout their length and this 

 heat will be dissipated by the exterior portions; the current of 

 hot air will be cooled uniformly throughout its section and the 

 heat will be transmitted to all points of the section of the exterior 



