DEW. 363 



And now Wells was approaching the solution of the 

 problem he had set himself; for it followed from his obser- 

 vations, that such obstructions as the propped board and the 

 pasteboard roof kept in the heat. It followed also, from the 

 observed effects of clear skies, that clouds keep in the heat. 

 Now, what sort of heat is that which is prevented from 

 escaping by the interference of screens, whether material or 

 vaporous? There are three processes by which heat is 

 transmitted from one body to another, these are, conduc- 

 tion, convection, and radiation. The first is the process by 

 which objects in contact communicate their heat to each 

 other, or by which the heat in one part of a body is gradually 

 transmitted to another part The second is the process by 

 which heat is carried from one place to another by the abso- 

 lute transmission of heated matter. The third is that pro- 

 cess by which heat is spread out in all directions, in the 

 same manner as light. A little consideration will show that 

 the last process is that with which we are alone concerned ; 

 and this important result flows from Dr. Wells' experiments, 

 that the rate of the deposition of dew depends on the rate at 

 which bodies part with their heat by radiation. If the process 

 of radiation is checked, dew is less copiously deposited, and 

 vice versa. 



When we consider the case of heat accompanied by 

 light, we understand readily enough that a screen may inter- 

 fere with the emission of radiant heat. We use a fire-screen, 

 for instance, with the object of producing just such an inter- 

 ference. But we are apt to forget that what is true of 

 luminous heat is true also of that heat which every substance 

 possesses. In fact, we do not meet with many instances in 

 which the effect of screens in preventing the loss of obscure 

 heat is very noteworthy. There are some, as the warmth of 

 a green-house at night, and so on ; but they pass unnoticed, 

 or are misunderstood. It was in this way that the explana- 

 tion of dew-phenomena had been so long delayed. The 

 very law on which it is founded had been practically applied, 

 while its meaning had not been recognized. " I had often 



