INTENSITY OF SUN'S HEAT AND LIGHT. 



19 



curves through the series of points thus determined, as shown in the accompanying 

 diagram (Plate I). 



The Equatorial curve will be observed to have two maxima at the Equinoxes in 

 March and September, and two minima at the Solstices in June and December. 

 Since the earth is nearer the sun in March than in September, the curve shows a 

 greater intensity in the former month, other things being equal. 



In the latitude of 10°, the Sun will not be vertical at the summer solstice, but 

 only when the Declination is 10° N., which happens twice in the year. The curve 

 corresponds in every particular with the known course of the sun. Above the 

 latitude of 23° 28', the tropical flexure entirely disappears; and there is only a 

 single maximum at midsummer. 



For comparison with the curves of Intensity, I have also traced curves of Tempera- 

 ture observed at Calcutta, in lat. 22° 33' N. ; at New Orleans, in lat. 29° 57'; at 

 Philadelphia, in lat. 39° 57'; at London, in lat. 51° 31'; and at Stockholm, in lat. 

 59° 20'. The values for Stockholm represent the averages for every five days 

 during fifty years, as given in the Encyclopedia Metropolitana, article Meteorology. 

 The curve for Philadelphia is adjusted from the daily observations made at the 

 Girard College Observatory from 1840 to 1845, under the direction of Prof. Bache. 

 The rest are interpolated graphically from the mean monthly temperatures. 



'Retardation of the Effect. — In the Temperate Zone the Temperatures will be 

 seen to attain their maximum about one month later than the sun's intensity would 

 indicate. At Stockholm it is somewhat more than a month; and, during this 

 interval the earth must receive, during the day, more heat than it loses at night; 

 and, conversely, after the winter solstice, it loses more heat during the night than 

 it receives by day. In illustration of this point, and to approximately verify the 

 formula, I here insert a former computation of the sun's Intensity for the 15th day 

 of each month, on the latitude of Mendon, Mass., and the results are found to 

 agree very nearly with those observed at that place about one month later, as fol- 

 lows: (The observed values are taken from the American Almanac for 1849, and 

 are derived from fifteen years' observations.) 



Computed values. 



Observed values. 



Difference. 



Jan. 15 . 



5040 



23°.3 



24°.3 



Feb. 15 



+ 1°0 



Feb. " 











7142 



33°. 1 



33°.5 



Mar. ' 









+ .4 



Mar. " 











9764 



45°.2 



45°.8 



April ' 









+ -6 



April " 











12574 



58°.3 



55°.0 



May ' 









—3°. 3 



May " 











14482 



67°.l 



64°. 5 



June ' 









—2°. 6 



June " 











15346 



71°.l 



71°. 8 



July ' 









+ -1 



July " 











15085 



69°.9 



68°. 9 



Aug. ' 









— 1°.0 



Aug. " 











13437 



62°.3 



61°.0 



Sept. ' 









— 1°.3 



Sept. " 











10860 



50°.3 



48°.5 



Oct. ' 









— 1°.S 



Oct. " 











8080 



37°.5 



38°.9 



Nov. ' 









+ 1°.4 



Nov. " 











5638 



26°. 1 



27°.7 



Dec. ' 









+ 1°.6 



Dec. " 











4510 



20°. 9 



26°.0 



Jan. ' 









-t-5°.l 



It may be proper to observe that the formula was divided by sin L, a constant 

 factor ; and the numbers in the second column were then successively computed : 

 their sum, divided by twelve, gave 10163 as the mean, to be compared with 47°.l, 

 the observed mean at Mendon. Then as 10163 : 47°.l : : 5040 : 20°.3, Jan. 15, etc. 



