18 



INTENSITY OF SUN'S HEAT AND LIGHT. 



again on August 3d. Therefore, during this long interval of eighty-five days, com- 

 prehending nearly the whole season of summer, the Sun's vertical intensity over the 

 North Pole is greater than upon the Equator. To this subject we shall again recur 

 in a subsequent Section. 



Fifthly, having glanced at these particular cases of the formula, let a more com- 

 plete survey be made for the northern hemisphere. And the same will equally 

 apply to the southern hemisphere, allowing for the reversal of the seasons and 

 change of the Sun's distance. In equation (14), when H exceeds 6\ and when 

 the declination D is south, a change of sign would be introduced ; but the proper 

 trigonometric signs will be observed simply by using the upper sign in summer, or 

 when the declination is north, and the lower sign during the rest of the year, in 

 the annexed formula of daily Intensity : — 



u = [5.94210] A 2 sin L sin D (tan II ± H). (18.) 



Here brackets include the logarithm of the co-efficient k ; is to be taken in 

 seconds of arc; H is the actual length of the semi-diurnal arc to radius 1, and 

 tan H is the natural tangent. The subjoined table has been computed in this man- 

 ner, for intervals of fifteen days, and expresses the results in units of intensity. In the 

 last three columns for the Frigid Zone the braces include values for the days when 

 the sun shines through the whole twenty-four hours ; the blank spaces indicate 

 periods of constant night. 



The Sun's Diurnal Intensity at every Ten Degrees of Latitude in the Northern Hemisphere. (Plate I.) 



A. D. 1853. 



Lat. 0°. 



Lat. 10°. 



Lat. 20°. 



Lat. 30°. 



Lat. 40°. 



Lat. 50°. 



Lat. 60°. 



Lat. 70°. 



Lat. 80°. 



Lat. 90°. 



Jan. 1 . . . 



77.1 



67.2 



55.8 



42.8 



30.1 



16.5 



5.1 









" 16 







78.1 



68.9 



58.2 



45.8 



32.7 



19.3 



7.2 









" 31 







79.6 



71.7 



61.9 



49.7 



38.6 



25.0 



11.9 



1.4 







Feb. 15 







81.0 



74.7 



66.6 



55.6 



45.1 



31.9 



19.0 



6.4 







Mar. 2 







81.6 



78.0 



71.3 



62.9 



52.7 



41.1 



27.9 



14.5 



2.1 





" 17 







82.0 



80.2 



76.0 



69.6 



61.1 



50.2 



37.1 



25.5 



11.6 





April 1 







80.8 



81.4 



79.5 



75.3 



68.9 



60.2 



49.9 



38.0 



25.6 



20^5 



" 16 







79.0 



81.7 



82.0 



79.5 



75.1 



68.6 



61.1 



51.4 



4to 



44.6 



May 1 







76.9 



81.5 



83.7 



83.6 



80.8 



77.1 



70.9 



64.6 



64.3 



65.3 



" 16 







74.7 



80.8 



84.7 



86.7 



85.7 



83.3 



79.7 



76.8 



80.3 



81.5 



" 31 







73.0 



80.1 



85.1 



87.8 



88.9 



87.8 



85.7 



86^8 



91.0 



92.4 



June 15 







72.0 



79.6 



85.2 



88.4 



90.1 



89.9 



88.8 



91.7 



96.1 



97.6 



July 1 







72.0 



79.5 



85.0 



88.5 



90.4 



89.5 



88.4 



90.8 



95.1 



96.6 



" 16 







73.0 



79.8 



84.7 



87.5 



87.6 



86.5 



84.1 



84.3 



88.3 



89.7 



" 31 







74.7 



80.4 



83.9 



85.1 



84.5 



81.6 



77.3 



13A 



76.2 



77.4 



Aug. 15 







76.7 



80.8 



82.7 



82.4 



79.8 



74.7 



68.2 



60.9 



59.2 



60.1 



" 30 







78.5 



80.7 



80.6 



77.7 



72.1 



65.5 



57.3 



47.7 



38^8 



38.9 



Sept. 14 







79.8 



79.8 



77.5 



72.6 



65.6 



58.8 



46.9 



34.5 



21.9 



14.7 



" 29 







80.5 



78.4 



73.8 



67.0 



57.8 



47.0 



36.2 



22.5 



9.0 





Oct. 14 







80.7 



76.4 



69.7 



61.0 



50.2 



38.2 



25.7 



12.6 



1.0 





" 29 







79.9 



73.5 



65.0 



54.6 



42.5 



30.1 



17.5 



5.2 







Nov. 13 







78.8 



70.7 



60.8 



49.8 



37.1 



23.8 



11.0 



0.9 







" 28 







77.5 



68.3 



57.3 



45.3 



31.8 



18.9 



6.8 









Dec. 13 







76.9 



66.9 



55.4 



43.0 



30.3 



16.3 



4.9 









To indicate the law of the Sun's Diurnal Intensity to the eye also, I have taken 

 the relative units in the table as ordinates, and their times for abscissas, and traced 



