INTENSITY OP SUN'S HEAT AND LIGHT. 39 



"one-fifteenth is too considerable a fraction of the whole intensity of sunshine, not 

 to aggravate, in a serious degree, the sufferings of those who are exposed to it with- 

 out shelter. The accounts of these sufferings in the interior of Australia, would 

 seem far to exceed what have ever been experienced by travellers in the northern 

 deserts of Africa. The author has observed the temperature of the sxvrface soil in 

 South Africa, as high as 159° Fahrenheit. The ground in Australia, according to 

 Capt. Sturt, was almost a molten surface, and if a match accidentally fell upon it, 

 it immediately ignited." (HerscheVs Astronomy.) 



The phenomenon is of sufficient interest to warrant a glance at the secular values. 

 The eccentricity, 100,000 years ago, has already been stated at .0473 ; and the 

 formula of the proportional general difference of the winter intensities, in the 

 northern and southern hemispheres 1 — 2 e, becomes 1 — .0946; and the maximum 

 difference 1 — 4 e becomes 1 — .1892. Thus the difference of winter intensities 

 between the northern and southern hemispheres, and likewise of summer intensities, 

 was then about three times greater than at the present time. But this wide fluc- 

 tuation of summer and winter intensities, in relation to the two hemispheres, 

 scarcely affected the aggregate annual intensities, as before shown. 



From occasional Historic notices of climate, it has been assumed that the winter 

 season in Europe was formerly colder than at the present time. The rivers Rhine 

 and Rhone were frozen so deep as to sustain loaded wagons ; the Tiber was frozen 

 over, and snow at one time lay forty clays in the city of Rome; but the history 

 of the weather presents winters of equal severity in modern times. 1 In the United 

 States, likewise, since the period of our colonial history, . the indications of an 

 amelioration of climate are not conclusive. The great snow of February, 1717, 

 rose above the lower doors of dwellings, and in the winters which closed the years 

 1641, 1697, 1740, and 1779, the rivers were frozen, and Boston and Chesapeake 

 bays were at times covered with ice as far as the eye could reach; but the like 

 occurs at similar intervals in our day. Mild winters, too, have intervened, and the 

 other seasons are also very variable. The general indications, however, give rise 

 to the question, whether there is a cause of change of climate in the course of 

 the sunl 



About two thousand years- ago, in the time of Hipparchus, 128 B. C, the obli- 

 quity of the ecliptic, or the sun's greatest declination, was 23° 43'. It has now 

 decreased to 23° 27£'; therefore, at the former epoch, the sun came farther north 

 and rose to a higher altitude in summer ; and went farther south and rose only to 

 a lower altitude in midwinter. There is then an astronomic cause of change, of 

 which we propose to determine more precisely the effect. For this purpose, the 

 formula of daily intensity (18) may be written, 



u = [1.90746] / 1 + ecos ( T — P K s j n L sin D {tan H± H). 



1 Thus, in the famous winter of 1*709, thousands of families perished in their houses; the Arabic 

 Sea was frozen over, and even the Mediterranean. The winter of 1740 was scarcely inferior, and snow 

 lay ten feet deep in Spain and Portugal. In 1776 the Danube bore ice five feet deep below Yienna. 



