52 



MEETING OF THE BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE. 



[1855 



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

 similar to that 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 great rapidity 

 whether in free space or through transparent media ; sound also, 

 is propagated with great rapidity, and more rapidly through 

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

 the velocity with which it may radiate through free space, is 

 usually transmitted with extreme slowness through terrestial 

 media. There appears to be nothing in light analagous to the 

 slow conduction of heat. Again, the vibrations which render a 

 body sonorous seem to 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 princi- 

 pally from this property of heat that it becomes available for the 

 production of motive power, as for instance, in the expansion 

 of steam. These phenomena 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 mole- 

 cular theory of heat; but before any such theory can be pro- 

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

 on other molecular phenomena, with which those of heat are in 

 all probability intimately connected. Prof. W. Thompson has 

 also given a clear and compendious mathematical exposition of 

 the new dynamical theory of heat, fonnded on Mr. Joule's me- 

 chanical 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. Thompson 

 and Mr. Joule are now engagad in further experiments which 

 will serve to elucidate the new theory of heat. Some account of 

 the commencement of these experiments 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 re- 

 gions of tho atmosphere ; but it is only very recently that syste- 

 matic observations of this kind have been attempted. Last 

 autumn four balloon ascents were made by Mr. Walsh, under the 

 guidance of the distinguished aeronaut, Mr. Green. Attention 

 was chiefly directed to the determination of the pressure, temper- 

 ature and the moisture of the air at different altitudes. The 

 decrease of temperature in ascending 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, — agree- 

 ing within 5 or 6 feet with the result obtained by Gay-Lussac. 

 The latter gentlemen ascended 23,000 feet; the greatest height 

 attained by Mr. Welsh was 22,940. A repetition of similar ob- 

 servations in ascents made from different points of the earth's 

 surface could scarcely fail to lead to valuable information for the 

 science of Meteorology. 



An immense contribution, of which mention was made 

 by my predecessor, has been made within the last few years 

 to this science, by the publication of Professor. Dove's Isother- 

 mal Maps, giving us the temperature of the lowest portion of the 

 atmosphere (that which determines the climate of every region) 

 for nearly all accessible points of the earth's surface. An immense 

 number of thermometric observations had been made at fixed 

 stations, or by travellers in almost every part of the globe, but 

 were lying comparatively useless for want of adequate discussion. 

 This task was undertaken some years ago by M. Dove. It was 

 not merely a task of enormous labour, but one requiring great 



critical acuteness and sound philosophical judgment, and these 

 qualifications M. Dove brought to his work, which has resulted 

 in the excellent maps alluded to, accompanied by a considerable 

 amount of letter- press, full of interesting generalizations, and 

 written in the genuine spirit of inductive philosophy. 



His maps present a great number of isothermal lines, — i. e~, 

 lines passing through all those places which, at an assigned 

 perixl of the year, have the same temperature, each line indicat- 

 ing a particular temperature differing a few degrees from those 

 of the adjoining lines. Besides a large map giving these lines 

 for January ami July, the months of extreme winter and summer 

 temperature, there are smaller ones giving similar lines for all 

 the different months. An English edition of these maps has 

 just been published. 



We may easily conceive how a great ocean current of warm 

 water ftom the tropics may affect the temperature of the atmos- 

 phere in the colder regions into which it may penetrate; but it 

 is only since the publication of these maps that we have had any 

 adequate idea of the extent of this influence, or been able to ap- 

 preciate the blessings conferred on the shores of north-western 

 Europe, and especially on our own islands, by the Gulf-stream. 

 This great current, though not always under the same name, ap- 

 pears, as you are probably aware, to traverse the Atlantic in a 

 north-westerly direction till it reaches the West India Islands 

 and the Gulf of Mexico. It is then reflected by the American 

 coast, and takes a north-easterly direction to our own shores, ex- 

 tending beyond Iceland into the North Sea. It is to the enor- 

 mous mass of heated water thus poured into the colder seas of 

 our own latitudes that we owe the temperate character of our 

 climate ; and the maps of M. Dove enable us not only to assert 

 distinctly this general fact, but also to make an approximate cal- 

 culation of the amount to which the temperature of these regions 

 is thus affected. If a change were to take place in the configu- 

 ration of the surface of the globe, so as to admit the passage of 

 this current directly into the Pacific across the existing Isthmus 

 of Panama, or along the base of the Rocky Mountains of North 

 America into the North Sea — a change indefinitely small in 

 comparison with those which have heretofore taken place — our 

 mountains, which now present us to the ever-varying beauties of 

 successive seasons, would become the unvarying abodes of the 

 glacier and regions of the snow-storm ; the beautiful cultivation 

 of our soil would be no longer maintained, and civilization 

 itself must retreat before the invasion of such physical barbarism. 

 It is the genial influence of the Gulf-stream which preserves us 

 from these evils. Among its effects on our climate, I may men- 

 tion one which may not be without its local interest along this 

 coast, especially for those who may wish to visit during the 

 winter for health as well as for pleasure. The temperature of 

 the atmosphere to the north of this island is so ameliorated by 

 the Gulf-stream in the depth of winter, that the isothermal lines 

 for the month of January along the whole eastern coast of Great 

 Britain and the opposite western coast of the Continent, run 

 north and south instead of following their normal east and west 

 direction, thus showing that Scarborough, or any watering-place 

 on the same coast much further to the north, enjoys as tempera- 

 ate a climate in the depth of winter as the coast of Kent. In 

 the early spring, however, it becomes considerably colder than 

 on the latter coast. 



My predecessor, in his address, informed us of an application 

 made to our Government by that of the United States, to adopt 

 a general and systematic mode of observing phenomena of va- 

 rious kinds at sea, such as winds, tides, currents, &c, which may 

 not only be of general scientific interest, but may also have an 

 important bearing on navigation. The plan proposed by Lieut. 



