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7 Proceedings of the British Association. “ 319 
* 
: considered as almost established, since Wollaston’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 
o the total influence. The chief point ‘insisted on in this branch, 
was, the neglect of Poisson to take into calculation the influence 
of : atmosphere in diminishing the heating power of 
the 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 119, 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- 
hexion 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- 
* 
