608 ADDITIONS. 



The temperature of the air decreases uniformly as we ascend above 

 the earth's surface ; but this decrease does not go on continuously. At 

 a certain elevation, varying on different days, the decrease is arrested ; 

 and for a depth of two or three thousand feet of air, the temperature 

 decreases little, or even increases in ascending. Above this, the dimi- 

 nution again takes place at nearly the same rate as in the lower re- 

 gions. This intermediate region of undecreasing temperature extended 

 in the various ascents, from about altitude 4000 to 6000 feet, 6500 to 

 10,000, 2000 to 4500, and 4000 to 8000. This interruption in the 

 decrease of temperature is accompanied by a large and abrupt fall in 

 the temperature of the dew-point, or by an actual condensation of 

 vapor. Thus, this region is the region of the clouds, and the increase 

 of heat appears to arise from the latent heat liberated when aqueous 

 vapor is formed into clouds. 



CHAPTER IV. 

 THEORIES OF HEAT. 



The Dynamical Theory of Heat. 



THAT the transmission of radiant Heat takes place by means of 

 the vibrations of a medium, as the transmission of Sound certainly 

 does, and the transmission of Light most probably, is a theory which, 

 as I have endeavored to explain, has strong arguments and analogies 

 in its favor. But that Heat itself, in its essence and quantity, is Motion, 

 is a hypothesis of quite another kind. This hypothesis has been 

 recently asserted and maintained with great ability. The doctrine 

 thus asserted is, that Motion may be converted into Heat, and Heat 

 into Motion ; that Heat and Motion may produce each other, as we 

 see in the rarefaction and condensation of air, in steam-engines, and 

 the like : and that in all such cases the Motion produced and the Heat 

 expended exactly measure each other. The foundation of this theory 

 is conceived to have been laid by Mr. Joule of Manchester, in 1844; 

 and it has since been prosecuted by him and by Professor Thomson of 

 Glasgow, by experimental investigations of various kinds. It is diffi- 

 cult to make these experiments so as to be quite satisfactory ; for it is 



