SECT. II.] PRELIMINARY DEFINITIONS. 29 



whatever at constant temperature 1, common to all its points, 

 and that the surface in question is in contact with air maintained 

 at temperature : the heat which escapes continuously at the 

 surface and passes into the surrounding medium will be replaced 

 always by the heat which proceeds from the constant cause to 

 whose action the body is exposed; thus, a certain quantity of heat 

 denoted by h will flow through the surface in a definite time (a 

 minute). 



This amount_ ^ of a flow continuous and always similar to 

 itself, which takes place at a unit of surface at a fixed temperature, 

 is the measure of the external conducibility of the body, that is 

 to say, of the facility with which its surface transmits heat to the 

 atmospheric air. 



The air is supposed to be continually displaced with a given 

 uniform velocity : but if the velocity of the current increased, the 

 quantity of heat communicated to the medium would vary also : 

 the same would happen if the density of the medium were 



iucrease ~ 



31. If the excess of the constant temperature of the body 

 over the temperature of surrounding bodies, instead of being equal 

 to 1, as has been supposed, had a less value, the quantity of heat 

 dissipated would be less than k. The result of observation is, 

 as we shall see presently, that this quantity of heat lost may be 

 regarded as sensibly proportional to the excess of the temperature 

 of the body over that of the air and surrounding bodies. Hence 

 the quantity h having been determined by one experiment in 

 which the surface heated is at temperature 1, and the medium at 

 temperature 0; we conclude that hz would be the quantity, if the 

 temperature of the surface were z, all the other circumstances 

 remaining the same. This result must be admitted when z is a 

 small fraction. 



32. The value h of the quantity of heat which is dispersed 

 across a heated surface is different for different bodies; and it 

 varies for the same body according to the different states of the 

 surface. The effect of irradiation diminishes as the surface 

 becomes more polished; so that by destroying the polish of the 

 surface the value of h is considerably increased. A heated 



