RIETEOROLOGY. o39 



T]ins it appears that the sim's annual intensity upon the whole 

 earth's surface from pole to pole averages 299 thermal days, being 

 five sixths of the value on the equator. 



Though tlie figures in the last column are strictly nnits oi intensity, 

 yet, as shown hy the curves, they also approximately represent annual 

 iemperatnrcfi, except near the poles. Following these indications, the 

 mean annual tem[ierature of the whole earth's surface must be some- 

 what below GGo Fahrenheit. In com})arison with this result, the 

 mean annual tenii)erature found by Professor Dove, from a vast num- 

 ber of observations, may be introduced, which is approximately 58°. 1 

 Fahrenheit. The like value found from the formula of Brewster, is 

 64°. Fahrenheit. 



SECTION VII. 



ON SECULAR CHANGES OF THE SUN S IN'J'ENSITY. 



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 inclination, according to the laws 

 of ])hysical astronomy. 



The recurrence of spots on the sun's disc has lately been discovered 

 to observe a regular jieriodicity. But I heir influence upon tempera- 

 ture appears to be insufiicient for taking account of them. M. K, 

 Wolf, in the Comptes Bendus, 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. Coun- 

 sellor Schwabe, after twenty-six years of observation^ does not think 

 that the spots exert any influence on the annual temperature. And 

 a writer in the Encydopcedia 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 the summer of 1807 the heat was excessive, and the spots 

 of vast magnitude. Warm summers and winters of excessive rigor 

 have happened in the })resence or absence of the spots."* 



Proceeding now to investigation, our first inquiry will relate to 

 changes of the sun's annual intensity upon the earth's surface rtgard< d 

 as one aggregate. 



In the Connaissance dcs Terns, for 1843, Leverrier has exhibitec^i 

 the secular values of most of the elements of the planetary orbits 

 during 100,000 years before and after January 1, 1800. The eccen- 

 tricity of the earth's orbit at the present time being .0168, the value 

 100,000 years ago, and the greatest in that interval was .0473. Sub- 

 stituting these in the formula, we find that the sun's annual intensity 



* Professor Henry was the first to show, by projecting on a screen in a dark room the 

 image of" the sun from a telescope with the eye j.'lass drawn out, tiiat the temperature of the 

 «pets was shghtly less than that of the otiier parts of the si>lar disc. The temperature was 

 indicated by a delicate therinoelectricnl apparatus. Professor Secchi, of Italy, afttrwards 

 obtained ihe same result. — See SiUiman^s Journal, Vol. XLiX, p. 40.'). 



