186 Mr. Dalmahoy on the greater calorijic Effect of 



1st, Dr. Richardson remarks that the indication of the 

 photometer bore a larger ratio to that of the blackened ther- 

 mometer in March than in May, and this may, perhaps, sug- 

 gest a doubt as to the accuracy of one or both of these instru- 

 ments. But if it be considered that the photometer is directly 

 affected only by solar radiation, while the blackened thermo- 

 meter is influenced both by solar and terrestrial radiation, it 

 will appear probable that the variation alluded to arose from 

 no defect, at least in the photometer. 



2ndly, Further, the apparent greater effect of solar radia- 

 tion at Fort Franklin may be ascribed to the reflection of in- 

 direct rays from the snow. But as observation seems to 

 prove the effect of the sun's rays to be greater in temperate 

 than in tropical climates independently of the presence of 

 snow, it follows that the cause assigned above is not sufficient 

 to account for the whole difference of effect at Fort Franklin. 

 Moreover, judging from the effect of the sun in melting the 

 snow when the temperature was much below the freezing 

 point, there is reason to doubt whether snow, as the objection 

 assumes, reflects the heating rays in great quantity*. 



Since, therefore, the experiments which have been adduced 

 (though less satisfactory than if they had exhibited the mo- 

 mentary effect of solar radiation) seem liable to no important 

 objection, it may be fairly inferred from them that, at equal 

 altitudes of the sun, the heating power of its direct rays is 

 greater when the temperature of the air is low than when it 

 is high, — a conclusion the same as has been already drawn 

 from the equation v = m (af—af). 



But this result will necessarily appear anomalous when tried 

 by the equation v = m a x , and can only be reconciled with 

 it by assuming that in high latitudes, either the atmosphere 

 is more pervious to the sun's rays, or the power of bodies to ab- 

 sorb radiant heat is greater, than in low latitudes. 



The grounds upon which Professor Daniell has supported 

 the former of these hypotheses, I believe, are now generally 

 allowed to be untenablef . Dr. Richardson has suggested that 

 at Fort Franklin, in the month of March, the air may be ren- 

 dered more transparent than in lower latitudes, by containing 

 much less than its due quantity of moisture. With respect 

 to this explanation it may be remarked, that in a tropical 



* Regarding the effect of colour, see article Climate, p. 194. [See also 

 Dr. Stark's researches on the Influence of Colour on Heat, Phil. Trans. 

 1833, p. 285. et $eq., of which an abstract will be found in Lond. and Edinb. 

 Phil. Mag., vol. iii. p. 458. — Edit.] 



[f We observe, however, that Mr. Royle appears to subscribe to this 

 hypothesis : see our last Number, pp. 135, 136. — Edit.] 



