310 



METEOROLOGY. 



at the former epoch was greater than at present by cue-thousandth 

 part. Now this fraction of 365.24 days, counting the days at twelve 

 hours each in respect to solar illumination, amounts to betiueen four 

 and Jive hours of sunshine in a year ; and by so small a quantity only 

 has the sun's annual intensity, during 100,000 years past, ever ex- 

 ceeded the yearly value at the present time. Nor can it depart from 

 its present annual value by more than the equivalent of five hours of 

 average sunshine in a year for 100,000 years to come. 



The superior and ultimate limit given by Leverrier, to which the 

 eccentricity of the earth's orbit may have approached at some very 

 remote but unknown period or periods, is .0777. At such epoch, the 

 annual intensity is computed, as before, to have exceeded the intensity 

 of the present by thirteen hours of sunshine in a year. On the other 

 hand, the inferior limit of eccentricity being near to zero, indicates 

 only/o?ir minutes of average sunshine in a year, less than the present 

 annual amount. Between these two extreme limits, all annual varia- 

 tions of the solar intensity, whether pastor future, must be included, 

 even from the primitive antediluvian era, when the sun was placed in 

 his present relation to the earth. By the third law of Kepler, on 

 which the equation is based, these results are rigorous for siderial 

 years ; and by reason of the slight but nearly constant excess, the 

 same may be concluded of tropical or civil years. For the annual 

 variation'of the tropical year is only— Od. 000 000 06686. 



The preceding conclusions, it is proper_again to observe, refer to the 

 whole earth's surface collectively. Let us, in the next place, inquire 

 concerning changes of annual inteniyity upon the different latitudes of 

 thi- earth. This variation will be a function of the eccentricity, and 

 the obliquity. For the present, let it be proposed to compute the 

 annual intensity for an epoch 10,000 years prior to A. D. 1800. The 

 eccentricity of the orbit, was then .0187, according to Leverrier ; and 

 for the obliquity of the ecliptic, the most correct formula is probably 

 that of Struve and Peters, quoted in the American Nautical Almanac. 

 It is true their formula may not strictly apply for so distant a period ; 

 but, since the value 24° 43' falls within the maximum assigned by 

 Laplace, it must be a compatible value, though its epoch may be some- 

 what nearer or more remote than 10,000 years. Therefore, compar- 

 ing the computed results with the table for 1850, given in Section V, 

 vas a standard, we find the annual intensity on the equator, at the 

 former period, to have been 1.65 thermal days less than in 1850; the 

 differences for every ten degrees of latitude are as follows: 



Change of the sun's annual intensity 8,200 years B. C, from its value 

 in A. D. 1850, taken as the standard. 



Latitude. 



20® 

 30° 



Difference in 

 thermal days. 



—1. fi.i 

 — L58 

 — L32 

 — .9C 



Latitude. 



Difference in 

 thermal days 



Latitude. 



Difference in 

 thermal days. 



40° 

 GOO 



—0. 22 

 -f- ..18 

 -j-2. 11 



70° 

 80O 



+ 5. 52 

 +7. 18 

 +7. G4 



