made at Fort Franklin in 1825, 1826, and 1827. 251 



poated, under circumstances nearly the same, have a certain 

 degree of comparability with one another ; and Dr Richard- 

 son's curve of diurnal radiation is so regular as to confirm the 

 comparable character of his method. The results obtained in 

 different months, (Table I.) are less satisfactory ; but by pro- 

 jecting both the mean and extreme numbers, I find a coinci- 

 dence which seems to indicate a maximum effect about the 

 month of April, and not in June, as we might expect from ob- 

 servations in other climates. Dr Richardson declares that 

 experience has convinced him that the sky is clearer in spring 

 than in summer in the arctic regions, and has assigned reasons 

 for this difference. (Franklin's Second Journey, Appendix, 

 p. ex.) But without disputing the fact, we must be allowed 

 to doubt (as contrary to general experience) whether the in- 

 creased power of the sun's rays, owing to the shorter tract of 

 air traversed, is not far more effective (a few degrees of ele- 

 vation at low altitudes making a difference of thickness to be 

 traversed quite enormous, varying nearly as the secant of 

 the zenith distance) than any diminution due to slight va- 

 pours raised by the solar heat. We must not, however, rest 

 in mere conjecture. Dr Richardson has pointedly alluded in 

 the preceding paper (p. 245;, and in Franklin's Appendix 

 (p. ex.,) to the presence of snow on the ground* as accompanying 

 the peculiar atmospheric condition which he considers so fa- 

 vourable to the solar action. I am disposed to attribute the 

 effect solely to the mechanical action of the snow in reflecting 

 the solar light to the instrument. Dr Richardson has alluded 

 to a fact well known to those who have used Leslie's photo- 

 meter, that the presence of clouds, when dense, white, and 

 luminous, affects most strongly its indications, and that the 

 scattered light even of a blue sky equals sometimes the direct 

 effect of the sun itself. This paradoxical result shews the 

 necessity that there is for cautious deduction from an instru- 

 ment so curiously sensitive, and as yet so imperfectly studied ; 

 but as these facts cannot be doubted, there can be no difficulty 

 in believing, that the reflection from a boundless field of daz- 



etery case the only certain and accurate measure of the communication of 

 heat." (Thomson's Annals, 1019, xiv. p. 70 



* " It" (the force of the sun's rays) " was much stronger in the spring 

 months, when the ground was covered with snow, than in the sum- 

 mer months, when the altitude of the sun was greater." 



