128 
THE IOE AGES, PAST AND FUTURE. 
June, 1892. 
rather less 50,000 years ago, but was as much as l-21st 
part 100,000 years ago; it had previously been reduced 
to l-30th part, but at the period of 200,000 years 
ago, the eccentricity had reached the very large amount 
of l-18th part. This period corresponds also to one of 
the periods when the winter in our northern hemisphere 
occurred at the farthest extremity of the orbit, and the greatest 
distance from the sun ; and it has been calculated by Croll 
that the result of the two causes combined would be that the 
average temperature of our mid-winter would be 87° colder 
than at the present time, and would be sufficient to produce 
the Ice Age. 
It has been shown by Ball that the total amount of heat 
received by the earth from the sun during each year is divided 
into two equal portions by the line of the equinoxes, and 
that only the same total amount of heat is consequently 
received during the winter portion of the year, as in 
the summer portion ; and that the tendency to an Ice Age 
depends upon the difference of time of the earth traversing 
these two portions of its orbit. Not only is the length of 
orbit greater in the winter portion, but the time of traversing 
it is increased by the circumstance that the velocity of the 
earth in its orbit diminishes when its distance from the sun 
is increased, and the velocity is increased when the distance 
is diminished ; so that the earth is hurried through the warm 
part and is retarded through the cold part, increasing the 
length of time of exposure to the extra cold. 
At the present time we are so fortunately situated in the 
northern hemisphere, that the winter period from the 
autumn to the spring equinox is shorter by seven days than 
the summer period from spring to autumn equinox; the first 
period being 179 days and the latter 186 days out of the 365 
days making the year. The consequence is that one half of 
the total year’s supply of heat from the sun to the whole 
earth is spread over only 179 days during our winter portion 
of the year, while the other half is spread over 186 days 
during the summer portion, causing the daily supply of heat 
during our winter to be greater than it would be if the 
division were equal. In the southern hemisphere this condi¬ 
tion is, however, reversed, and there is consequently an 
increased cold in winter to the same extent. 
When the extreme conditions combine together of the 
greatest eccentricity of the earth’s orbit, and the outward 
inclination of the earth’s axis, the difference of length 
between the two periods of the year can be exaggerated nearly 
five times and becomes as much as 33 days, or more than a 
whole month, the winter period being 199 days, and the 
