METEOROLOGY. 341 



From this it appears that the annual intensity within theTovrid Zone, 

 ten thousand years afi;o, averaged one thermal day and a half less than 

 now ; while from 35° of latitude to 50°, comprehending the whole area 

 of the United States, it was virtually the same as at the })re8ent day. 

 But ahove 50° of latitude, the annual intensity was then greater in 

 an increaNing rate towards the pole, at which point it was hetween 

 seven and eight thermal days greater than at the present time ; in 

 other words, the poles both north and south, 10,000 years ago, received 

 twenty rays of solar heat in a year, where they now receive but nine- 

 teen. Owing to change in the obliquity of the ecli|)tic, the sun may 

 be compared to a swinging lamp ; at the former period, it apparently 

 moved farther to the north and to the south, passing more rapidly- 

 over the intermediate space. 



The maximum variation of the obliquity of the ecliptic, according 

 to Laplace, without assigning its epoch, is 1° 22' 34", above or below 

 the obliquity 23° 28' in the year 1801.* Now the diiference recog- 

 nized in our calculation almost reaches this limit, being 1° 15'. As 

 the secular perturbations are now understood, therefore, it follows 

 that, since the earth and sun were placed in their present relation 

 to each other, the annual intensity upon the Temperate Zones has 

 never varied ; between the tropics, it has never departed from its 

 present annual amount by more than about y^i oth part, and is now 

 very slightly increasing. The most perceptible difference is in the 

 Polar regions, where the secular change of annual intensity is more 

 than four times greater than on the Equator ; in its annual amount, 

 the Polar cold is now very slowly increasing from century to century, 

 which eiFect must continue so long as the obliquity of the ecliptic is 

 diminishing. And thus, so far as relates to a decreased annual inten- 

 sity, the celebrated " Northwest passage" through the Arctic sea will 

 be even more difficult in years to come than in the present age. 



Having now considered the secular changes of annual intensity upon 

 the earth and its different latitudes, let us next examine the secular 

 changes of intensity in relation to the Northern and Southern hemispheres. 

 The earth is now nearest the sun in winter of the northern hemisphere 

 on January 1st, and farthest from the sun in summer on July 4th. This 

 collocation of times and distances has the advantage of rendering the 

 extreme of summer cooler, and of winter, north of the equator, warmer 

 than it would be at a mean distance from the sun. But south of the 

 equator, on the contrary, it exaggerates the extremes by rendering 

 the summer hotter and the winter colder. Before estimating this 

 difference, we may observe that the perigee advances in longitude 

 11". 8 annually ; by which the instant when the earth is nearest the 

 sun, will date about five minutes in time later every year. The time 

 of perihelion, which now falls in January, will at length occur in Feb- 

 ruary, and ultimately return to the southern hemisphere tlie advan- 

 tage which we now possess. Indeed, it is remarkable that the perigee 

 must have coincided with the autumnal equinox about 4,000 B. C, 

 which is near the time that chronology assigns for the first residence 

 qf man upon the earth. 



* Mecanique Celeste, Vol. II, p. 856, note, Bowditch's translation, 



