INTENSITY OF SUN'S HEAT AND LIGHT. 



35 



And taking L between the limits 23° 28' and 66° 32', the like sum for the Tem- 

 perate Zone is 143.46 



Substituting these values in equation (32), dividing by the denominator, and 

 then converting into the same thermal measures, which were employed in the last 

 Section, we obtain these final results: — 



The Sun's Average Annual Intensity. 









Thermal days. 



Thermal months. 



Thermal units. 



" " Temperate Zones ..... 

 " " Torrid Zone 



166.04 

 276.38 

 356.24 

 299.05 



5.45 



9.08 

 11. TO 



9.83 



31.05 

 61.67 

 79.49 

 66.73 



Thus it appears that the Sun's annual intensity upon the whole earth's surface 

 from pole to pole, averages 299 thermal days, being about five-sixths of the value 

 on the equator. 



Though the figures in the last column are strictly units of intensity, yet as shown 

 by the curves, they also approximately represent annual temperatures, except near 

 the Poles. Following these indications, the mean annual temperature of the whole 

 earth's surface must be somewhat below 66° Fahrenheit. In comparison with this 

 result, the mean annual temperature found by Prof. Dove, from a vast number of 

 observations, may be introduced, which is approximately 58°. 1 Fahrenheit. The 



like value found from the formula of Brewster, is J 2 81°. 5 cos 2 L cl L, which is 



64°. Fahrenheit. 



SECTION VII. 



ON SECULAR CHANGES OF THE SUN S INTENSITY. 



In relation to secular variations of intensity, we shall adopt the hypothesis that 

 the physical constitution of the sun has remained constant. The secular changes 

 here considered, therefore, are those which depend solely on position and inclina- 

 tion, according to the laws of physical astronomy. 



The recurrence of Spots on the Sun's disc, has lately been discovered to observe 

 a regular periodicity. But their influence upon temperature appears to be insuffi- 

 cient for taking account of them. 1 A writer in the Encyclopedia Britannica, article 

 Astronomy, states that " in 1823 the summer was cold and wet, the thermometer at 

 Paris rose only to 23°. 7 of Reaumur, and the sun exhibited no spots; whereas, in 



1 M. R. Wolf, in tbe Comptcs Sendus, XXXV, p. 704, communicates his discovery that the minima 

 of solar spots occur in regular periods of 11.111 years, or nine cycles in a century — and that the years 

 in which the spots are most numerous are generally drier and more productive than the others — the 

 latter being more humid and showery. Counsellor Schwabe, after twenty-six years of observation, does 

 not think that the spots exert any influence on the annual temperature. 



