150 Experimental Investigations 



perature could take place only in the single case of the 

 surface of the cylinder being completely isolated, so as 

 to be no way affected by the action of surrounding 

 bodies, which is absolutely impossible. 



The circumstances under which the experiments were 

 made are very different from those here taken for 

 granted. The bodies we subject to experiment are con- 

 stantly surrounded on all sides by the air and other 

 bodies which act on our instruments continually, and 

 often in a very perceptible manner ; and we can never 

 hope to isolate-a cylinder so completely that the appar- 

 ent progress of heat in its interior shall perceptibly 

 obey the law we have just discovered. In common 

 cases it deviates widely from this law. 



As the causes of this deviation are well known, we 

 will see whether there be no means of appreciating their 

 effects. 



The surface of the cylinder being surrounded by the 

 atmospheric air and other bodies, all which are of a 

 known and sensibly constant temperature, we may de- 

 termine the comparative effects of these bodies on the 

 different parts of the surface of the cylinder. 



In those parts of the cylinder which are hotter than 

 the air and other surrounding bodies, the surface of the 

 cylinder will be cooled by the action of these bodies ; 

 but if one of the extremities of the cylinder be colder 

 than the atmospheric air, those parts of the cylinder 

 which are colder than the circumambient fluid will be 

 heated by its influence and that of the surrounding 

 bodies. 



We will begin with examining the case where the 

 coldest extremity of the cylinder is at the same tem- 

 perature as the surrounding air. Let us suppose, 



