the Swfs direct Rays in high than in love Latitudes. 185 



about the vernal equinox, when the meridian altitude of the 

 sun does not exceed 24° 48', and the mean maximum tempera- 

 ture of the air is 3° # 9 ; whereas in London the sky is clearest in 

 June, about the summer solstice, when the meridian altitude 

 of the sun is 61° 57', and the mean maximum temperature of 

 the air is 69 c, 4. Now, according to Sir John Leslie*, if the 

 atmosphere be equally clear, the number of rays which reach 

 the earth when the altitude of the sun is 61° 57', is to the num- 

 ber when the altitude is 24° 48', nearly in the ratio of 725 : 507. 

 Hence it may be concluded that when the temperature of the 

 air was very low, 507 rays produced a greater calorific effect 

 than 725 rays when the temperature of the air was high. 



The following experiment, though less striking, leads to the 

 same conclusion. 



By the observations of Professor Daniell in London, and 

 of Dr. Richardson at Fort Franklin, it appears that at both 

 places the maximum effect of the sun's rays upon a blackened 

 thermometer was 65 degreesf of Fahrenheit's scale. But in 

 this as in the former experiment, the observation at Fort 

 Franklin was made in March, while that at London was made 

 in June ; it follows therefore, according to the rude method 

 of approximation already adopted, that the calorific effect of 

 the sun's rays when the temperature of the air is 69°*4, is to 

 the effect when the temperature is 3° # 9 in the ratio of 65 : 65 

 x Z! 5 or 65 : 89. 



507 



It would have been desirable to have been able to compare 

 with the foregoing observations of Professor Daniell and Dr. 

 Richardson those of Captain Sabine in tropical climates. 

 But Captain Sabine's observations having been made (at least 

 those at Sierra Leone, and probably also those at Bahia,) du- 

 ring the prevalence of a cooling wind, they cannot be pro- 

 perly referred to as indicating the maximum effect of the sun's 

 rays. Still, when compared with similar observations in 

 higher latitudes, they strongly confirm the result already de- 

 rived from a comparison of the sun's heating power at Lon- 

 don and Fort Franklin. 



But there are some circumstances respecting the foregoing 

 experiments which may seem to detract from their value, or 

 to point to a different conclusion from that which has been 

 drawn from them. 



* Supplement to the Encyclopedia Britannica, article Climate. 



t The maximum effect observed by Captain Back and Lieut. Kendall, 

 at Fort Franklin, was on the 1st of April, and amouuted to 70 degrees; 

 but as the thermometer was sheltered by a glass case, and therefore not 

 under the same circumstances with Professor Daniell's instrument, Dr. 

 Richardson's maximum is adopted. 



Third Series. Vol. 7. No. 39; Sept. 1835. 2 B 



