220 PROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. 
I shall not follow farther the application of these results, of which a and k are 
the most immediately important. In particular, 1 shall not attempt to find, with 
Poisson, the whole climateric effect of the solar influence which he deduces from 
the quantities a and 6 found above; both on account of the uncertainty of the 
value of 6, and because I have attempted elsewhere to shew that the physical 
assumptions, upon which the great French analyst has founded the determination 
of this quantity, are exceedingly precarious.* 
I have only farther to add, that the extensive reductions and computations 
of which the results have been given in this paper, were performed under my im- 
mediate superintendence by different persons at different times. My thanks are 
due to Mr Broun, Mr Morrat, Mr Linpsay, and especially to Mr Greee, for their 
attention and accuracy in conducting them. 
EDINBURGH, June 1846. 

APPENDIX, 
Containing Remarks on the Connection of the Preceding Observations with the Theory of 
Fourier and Poisson.+ 
“ So far as the effect of SOLAR HEAT is concerned, the @ priori solution of the problem of 
the temperature of any part of the earth’s surface may be thus imagined :—(1.) The whole 
quantity of sunshine which falls on any part of the earth’s surface in the course of a year is 
to be found, and also the law of its variation of force at different seasons. (2.) The part of 
this heat which becomes effective in heating the earth’s crust is to be found by multiplying 
the amount by a constant depending upon the absorbent power of the surface. (3.) This 
quantity of heat thus reduced is propagated towards the interior, according to the laws of 
conduction, which again presuppose the knowledge of two constants proper to each soil, 
namely, the Conductivity and the Specific Heat. 
‘© (1.) The measure of the quantity of sunshine received by any place in a year, and its 
distribution at different seasons, has been a favourite problem with mathematicians. In ul- 
timate analysis, it depends of course on the astronomical elements which affect the progress 
of the seasons, viz., the obliquity of the ecliptic (vy), the latitude of the place (u), the excen- 
tricity of the earth’s orbit («), and the longitude of the sun’s perigee (a). But there are also 
elements quite as important as any of these ; the imperfect transparency of the air and its 
varying thickness, owing to differences of obliquity of the transmitted rays, and the condition 
of opacity depending on the weather. Neither of these is insignificant, neither of them 
compensatory ; both may be considered as functions of the hour-angle and fraction of the 
year, and the second is besides subjected to the most capricious changes. Yet of these ele- 
* See Second Report on Meteorology, Arts. 104, &., in the British Association Reports for 1840, 
+ Taken from the Second Report on Meteorology, British Association Report, 1840, Art. 88, &c. 
