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 h 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 Moffat, Mr Lindsay, and especially to Mr Gregg, for their 

 attention and accuracy in conducting them. 



Edinbukoh, 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 a 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 pai't 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 (y), the latitude of the place (/i), the excen- 

 tricity of the earth's orbit (a), and the longitude of the sun's perigee (w). 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, &c., in the British Association Reports for 1840. 

 I Taken from the Second Report on Meteorology, British Association Report, 1840, Art. 88, &c. 



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