Proceedings of the British Association. 319 



considered as almost established, since WoUaston's proofs derived 

 from the two entirely unconnected sources of astronomical and 

 chemical phenomena. He then glanced briefly at the subject of 

 isothermal lines, and passed on to the subject of solar radiation. 

 He examined at great length the researches of Poisson on this 

 subject ; and pointed out what he considered the inadequacy of 

 his speculations on what may be called the astronomical part of 

 the total influence. The chief point insisted on in this branch, 

 was, the neglect of Poisson to take into calculation the influence 

 of the earth's atmosphere in diminishing the heating power of 

 the sun's rays, particularly when they entered it obliquely. This 

 he showed to be most important, by stating the fact, that at Paris 

 the influence of the atmosphere upon rays entering vertically, 

 was to reduce their heating influence by 25 per cent, of what it 

 would have been had they not passed through it ; when they 

 entered so obliquely as to form an angle of 25° with the hori- 

 zon, their heating influence was reduced to one half; and when 

 an angle of 5°, to one twentieth part. If Poisson's views were 

 correct, the total solar influence at Paris would be 24° centigrade ; 

 and as the mean temperature of Paris is 11°, this would leave 

 about 13°, or about 9° of Fah., as the temperature irrespective of 

 the sun's heat ; whereas the mean temperature of the polar parts 

 of the earth, which are so far from being totally deprived of solar 

 influence, that they are alternately under that influence and de- 

 prived of it, is no higher than about 32°. He then proceeded to 

 the consideration of the temperature of the earth below the sur- 

 face, — gave a sketch of the results of former experiments, origin- 

 ating in those made in the caves under the Observatory at Paris; 

 detailed the results of those lately made ; and promised, before 

 the meeting was closed, to bring up this subject again, in con- 

 nexion with the experiments made at Edinburgh. Then he 

 glanced rapidly at the subject of mean temperature, and showed, 

 that while within the tropics it is sufficient to plunge a thermom- 

 eter a foot under the surface of the earth, in order to get by its 

 mean indication the mean temperature of the place, in higher lat- 

 itudes this would not be sufficient ; and he detailed the circum- 

 stances producing the difference, and pointed out the methods 

 and precautions necessary for obtaining it. He then entered on 

 the consideration of the temperature of the space beyond the 

 earth, and stated the probable source of it to be the radiating in- 



