42 INTENSITY OF SUN'S HEAT AND LIGHT. 



SECTION VIII. 



ON LOCAL AND CLIMATIC CHANGES OF THE SUN'S INTENSITY. 



As the principal topics under this head have been anticipated in the former por- 

 tions of the work, they need not here be repeated. The inequality of winter, and 

 especially of summer intensities in the northern and southern hemispheres, has 

 already been discussed in the last Section, and ascribed to the changing position of 

 the sun's perigee. 



Let us now pass to another local inequality, which consists in the difference of 

 daily intensities at two places situated on the same parallel of latitude, but separated 

 by a considerable interval of longitude. This difference arises solely from hourly 

 change of the Sun's Declination, while moving from the meridian of one place 

 westward to the meridian of the other; the Sun in the interval attaining a higher 

 or lower meridian altitude. 



For example, the latitude of Greenwich, near London, is 51° 28' 39". Following 

 this parallel west to a point directly north of San Francisco, in California, the differ- 

 ence of longitude is 122° 28' 2". At the time of the autumnal equinox, the daily 

 change of the sun's declination is 23' 23". Consequently, in passing from the 

 meridian of Greenwich to that of San Francisco, the declination is diminished by 



1 99° 9ft' 9" 

 ^^A x 23'23",orbyT57".3. 



"When the Sun's Declination is 0, at apparent noon at Greenwich, on Sept. 21st, 

 it will be 7' 57".3 S. at noon in the longitude of San Francisco on the same clay; the 

 semi-diameter being 15' 59" or 959" for Greenwich, and 959". 1 for San Francisco. 

 With these elements, let the sun's daily intensity be computed for both places by 

 formulas (13), (18). The result is 50.13 thermal units for Greenwich, and 49.91 

 for the place north of San Francisco, on the same latitude. The difference is .22 

 corresponding to nearly + i° Fahrenheit ; and by so much the intensity upon the 

 zenith of Greenwich is greater, on the same day. 



At the vernal equinox, March 20, the sun's daily change of declination -would be 

 in the opposite direction, and the difference would become — l°~F. The inequality 

 of this species thus compensates itself in theory, leaving the yearly intensity the 

 same for all places having the same latitude. 



For further reference on this point, the daily changes of declination, near the 

 first of each month, are subjoined as follows : — 



January, 5' May, 18' September, 22' 



February, 18' June, 8' October, 23' 



March, 23' July, 5' November, 18' 



April, 23' August, 17' December, 9' 



In this connection, it may be observed that Nervander, Buys Ballot, and Dove 

 have developed a slight inequality of temperature dependent upon the Sun's rota- 

 tion around his axis, and having the same period of about 27 days ; but this result 

 is not confirmed by Lamont, Poggendorfs Annalen for 1852. 



