the Sun's direct Rays in high than in low Latitudes. 187 



climate, shortly after the rainy season, the transparency of 

 the atmosphere is so perfect as to render it difficult to admit, 

 except on experimental proof, that the air of higher latitudes 

 can be more pervious to the sun's rays. 



Admitting, however, that at Fort Franklin the increased 

 effect of the sun's rays was entirely owing to a greater trans- 

 parency of the atmosphere, the same cause would necessarily 

 have produced a corresponding increase of terrestrial radia- 

 tion. But so far was this from being the case, that Dr. Rich- 

 ardson never saw the effect of terrestrial radiation exceed 

 4 degrees at Fort Franklin, while in London Professor Daniell 

 has observed it amount to 17 degrees. Another fact at vari- 

 ance with the same hypothesis is, that in Bengal the effect of 

 terrestrial radiation is frequently sufficient to freeze water 

 when the temperature of the air is considerably above 32°. 

 I am aware that the observations of Captain Sabine seemed to 

 indicate that the effect of terrestrial, as well as of solar radia- 

 tion was less in low than in high latitudes, but these observa- 

 tions were evidently too few to permit any conclusion being 

 drawn respecting the maximum effect in either case. 



But while the facts respecting terrestrial radiation are irre- 

 concilable with the hypothesis that the atmosphere is more 

 transparent in high than in low latitudes, they admit of an 

 easy explanation on the principles already advanced. For, 

 when the formula v = m! (a*— a*) is applied to the measure- 

 ment of terrestrial radiation, v is negative from t being greater 

 than x ; and as x may be supposed constant when the sky is 

 perfectly clear, it follows that v will be greater when t is large 

 than when it is small, therefore greater in low than in high 

 latitudes. 



The other hypothesis respecting the cause of the increased 

 effect of solar radiation in high latitudes is, that the power of 

 surfaces to absorb radiant heat augments with the obliquity 

 of the incidence of the rays. This ingenious suggestion is 

 ascribed by Dr. Prout # to Professor Daniell ; but there does 

 not appear to be any proof, except from analogy, that the 

 power of bodies to absorb radiant heat varies in the manner 

 assumed. 



Having thus endeavoured to show that the greater effect 

 of the sun's direct rays in high than in low latitudes is not at 

 variance with the received theory of radiant heat, I shall con- 

 clude by offering a few remarks on the constant quantities m 



v 



and a in the formula a 1 -\ = a*. 



m 



By whatever method the increment of temperature v is de- 



* See Dr. Prout's Bridgewatcr Treatise, p. 237. 

 2 B 2 



