OF ARTS AND SCIENCES. 27 



inquire whether the known phenomena of conduction will suggest a 

 better explanation of the protective action of the atmosphere than can 

 be afforded by the observed laws of absorption and radiation. By con- 

 duction, fluids acquire heat most readily when hot bodies are applied 

 to their lower surfaces, in consequence of the convection currents thus 

 established. On the other hand, the application of cold bodies to their 

 lower surfaces, as it does not originate such currents, withdraws their 

 heat only by the much slower process of conduction through their own 

 substance. 



Hence, an undisturbed atmosphere will acquire heat more readily 

 by contact with warm ground than* it loses heat by contact with cold 

 ground. Part of the heat thus acquired might have been conducted to 

 adjacent portions of the ground in the absence of the atmosphere, but 

 another portion would have been directly radiated into space. It now 

 becomes a question whether the atmosphere thus heated will discharge 

 its recently acquired energy by radiation into space as readily as the 

 ground would have done in the absence of an atmosphere. 



Without undertaking to decide this question, it will here be assumed 

 that the heat acquired by the atmosphere from warm ground will not 

 be radiated as readily as it would have been radiated by the ground 

 itself. Since it will not be readily lost by conduction, in the absence 

 of violent agitation of the air, for the reason already given, the hypoth- 

 esis seems admissible that it tends to accumulate, and to increase the 

 stock of energy contained in the atmosphere much more efficiently 

 than can be done by the processes of absorption and radiation. 



If we admit the existence of this tendency, we have next to consider 

 what natural provision can be suggested for checking its effects when 

 they have attained a certain magnitude ; for it is obvious that they do 

 not increase indefinitely. If we suppose atmospheric energy to be 

 manifested only as heat, its accumulation would ultimately be checked 

 by an increasing radiation from terrestrial solids and liquids ; if mani- 

 fested only by expansion, it is perhaps possible that portions of the 

 atmosphere would be driven off into space, carrying away the energy 

 mechanically ; but a more immediate check is afforded by that portion 

 of the accumulated energy which displays itself as atmospheric move- 

 ment. When the winds have attained a certain degree of violence, 

 they disturb the portions of the air which would otherwise remain 

 stagnant over the colder parts of the ground, and the process of heat- 

 ing the atmosphere from beneath gradually ceases to retain sufficient 

 advantage over that of cooling it from beneath to permit a furtbej 

 accumulation of energy. 



