342 METEOROLOGY. 



For aBcertaininp; the difference of intensity, we know that the sun's 

 declination goes through a nearly regular cycle of values in a year. 

 The formula shows that the length of the day in the southern hemis- 

 phere is the same as in the northern hemisphere about six months 

 earlier. The ratio of daily intensity of the northern, is to the southern 

 then as 1 to 1' — ^~.. And the like ratio for the summer intensities is 

 as 1 to 1 -|- yV. But Y^g^ is the extreme deviation for a few days only; 

 the mean between this and 0, or -g^-^j, would seem more correctly to 

 apply to the whole seasons of summer and winter. Taking then 3^0*^1 

 of the gieatest and least values of daily intensity, Section IV, for the 

 temperate zone, it appears that winter in the southern hemisjihere is 

 now about 1° colder, and summer 3° hotter than in the northern 

 hemisphere. The intensities during spring and autumn may-be re- 

 garded as equal in both hemispheres. And the summer season of the 

 south temperate zone being hotter, is also shorter by about eight 

 days, owing to the rapid motion of the earth about the perihelion. 



In confirmation of these last deductions, the younger Herschel refers 

 to the glow and ardor of the sun's rays under a perfectly clear sky at 

 noon, and observes, "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 without 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 surface soil in South Africa, as high as 159° Fahrenheit. The 

 ground in Australia, according to Captain Sturt, was almost a molten 

 surface, and il a match accidently 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 differ- 

 ence of the winter intensities, in the northern and southern hemi- 

 spheres, becomes 1 — .0946 ; and the maximum difference becomes 

 1 — .1892, Thus the difference of winter intensities between the 

 northern and southern hemispheres, and likewise, of summer intensi- 

 ties, was then about three times greater than at the present time. 

 But this wide fluctuation of summer and winter intensities, in rela- 

 tion 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, Tlie 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 days in the city of Rome ; but the history of the weather pre- 

 sents winters of equal severity in modern times. Thus, in the famous 

 winter of 1709, thousands of families perished in their houses ; the 

 Arabic Sea was iiozen 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 Vienna. 

 In the United States, likewise, since the period of our colonial history, 

 the indications of an amelioration of climate are not conclusive. The 



