ADDRESS. Xlvii 



of an exceedingly refined ethereal medium, in a manner exactly analogous 

 to that in which sound is propagated by the vibration of the air ; and it is 

 now supposed that radiant heat is propagated in a similar manner. This 

 theory of radiant heat, in accordance with the dynamical theory, of which I 

 have been speaking, involves the hypothesis, that the particles of a heated 

 body, or a particular set of them, are maintained in a state of vibration, 

 similar to that in which a sonorous body is known to be, and in which a 

 luminous body is believed to be. At the same time there are remarkable 

 differences between light and heat. We know that light is propagated with 

 enormous velocity, whether in free space or through transparent media; 

 sound also is propagated with great rapidity, and more rapidly through most 

 media than through air. Heat, on the contrary, whatever may be the velocity 

 with which it may radiate through free space, is usually transmitted with 

 extreme slowness througii terrestrial media. There appears to be nothing in 

 light analogous to the slow conduction of heat. Again, the vibrations which 

 render a body sonorous have no tendency to expand its dimensions, nor is 

 there reason to suppose that luminous vibrations have any such tendency on 

 luminous bodies; whereas, with the exception of particular cases, heat does 

 produce expansion. It is principally from this property of heat that it becomes 

 available for the production of motive power, as, for instance, in the expan- 

 sion of steam. These phaenomena of the slow conduction of heat, and the 

 expansion of heated bodies, are proofs of differences between light and heat 

 not less curious than the analogies above indicated. They must, of course, 

 be accounted for by any perfect theory of heat. Mr. Rankine has written 

 an ingenious paper on a molecular theory of heat; but before any such theory 

 can be pronounced upon, it will be necessary, I conceive, to see its bearing 

 on other molecular phaenomena, with which those of heat are in all probability 

 intimately connected. Prof. W. Thomson has also given a clear and com- 

 pendious mathematical exposition of the new dynamical theory of heat, 

 founded on Mr. Joule's principle of the exact equivalence of heat and 

 mechanical effect. This is not, like Mr. Rankine's, a molecular theory, but 

 one which must henceforth take the place of Carnot's theory. 



Before leaving this subject, I may add that Prof. Thomson and Mr. Joule 

 are now engaged in further experiments, which will serve to elucidate the 

 new theory of heat. Some account of the commencement of these experi- 

 ments has already been brought before the Royal Society. 



Many years ago Gay-Lussac made an ascent in a balloon for the purpose 

 of making observations on the air in the upper regions of the atmosphere ; 

 but it is only very recently that systematic observations of this kind have 

 been attempted. Last autumn four balloon ascents were made by Mr. Welsh, 

 under the guidance of the distinguished aeronaut Mr. Green. Attention was 

 chiefly directed to the determination of the pressure, temperature, and moist- 

 ure of tlie air at different altitudes. The decrease of temperature in ascend- 

 ing was very irregular, being changed even, in some cases, to an increase ; 

 but the mean result gives a decrease of 1° Fahr. for every 348 feet of ascent, 

 agreeing within 5 or 6 feet of the result obtained by Gay-Lussac. The latter 

 gentleman ascended 23,000 feet ; the greatest height attained by Mr. Welsh 

 was 22,940 feet. A repetition of similar observations in ascents made from 

 different points of (he earth's surface could scarcely fail to lead to valuable 

 information for the science of Meteorology, 



An immense contribution, of which brief mention was made by my prede- 

 cessor, has been made within the last ievr years to this science, by the publi- 

 cation of Prof. Dove's Isothermal Maps, giving us the temperature of the 



