102 
A. COTTAM-—ON A DIAGE AM FOE 
This Earth of ours comes next, and its orbit would pass through 
Buck’s and Vine’s shops, the old British School in Bed Lion Yard, 
through the new Post Office in Queen’s Boad, by the junction of 
Carey Place and Charles Street, across High Street again at Grant’s 
shop, and just south of Southfield House. The Earth’s motion in 
its orbit is at the rate of 18 miles a second. A person situated at 
the Earth’s equator is, although entirely unconscious of it, being 
whirled through space with three separate motions. There is first 
of all the Earth’s rotation on her axis, which carries the equator 
round at the rate of 1000 miles an hour, there is then the Earth’s 
motion in her orbit at the rate of 18 miles a second, and the Sun’s 
motion through space, in which all the planets accompany him at 
the rate of six miles a second. The Earth’s orbit is at the present 
time only slightly elliptical. The Sun is now about 1,500,000 miles 
from the central point of the ellipse, so that the difference of the 
Earth’s distance from the Sun at the opposite points of its orbit is 
very nearly 3,000,000 miles. The Earth is nearest to the Sun at the 
time of mid-winter of our hemisphere. It has been calculated that 
the eccentricity of the Earth’s orbit varies in long periods—that some 
210,000 years ago the eccentricity equalled about 3^ times its 
present value. And this question is one that should be very inter¬ 
esting to students of Natural History, because it may probably 
explain the occurrence of glacial epochs alternating with periods 
of tropical heat. It is too abstruse a subject to go into now, but 
I may mention that it is fully discussed in Dr. Croll’s treatise, 
entitled ‘ Climate and Time.’ The theory is this, that in the 
periods when the eccentricity of the orbit has been greatest, the 
Earth’s distance from the Sun, instead of varying at opposite seasons 
as it does now by only 3,000,000 miles, would then have varied 
by 10,500,000; and the winter of one hemisphere, instead of being, 
as at present, about one week longer than that of the other, or than 
its own summer, would have been nearly a whole month longer. 
“It may therefore be considered very probable that, under such 
circumstances, glacial epochs would occur, first in one half of the 
world, and then in the other; with intervals of about 10,500 years 
between their respective maxima in either hemisphere. And during 
a period of 200,000 years about ten may have occurred in each 
hemisphere, some more intense than others, according to the value 
of the eccentricity of the orbit for the time being; whilst, in 
alternation with them, a climate of a tropical nature may have 
extended, first in one and then in the other hemisphere, to a much 
greater distance from the equator than at present.”* The ovalness 
of the Earth’s orbit is now diminishing steadily, and will continue 
to do so for about 25,000 years to come. It will then begin to 
increase again until in 50,000 years’ time it will have about the 
same value as at present, and it will not much exceed that value 
for 100,000 years to come. 
The fourth planet is Mars, and he would pass through Watford 
* ‘The Sun, its Planets, and their Satellites,’ hy E. Ledger, p. 192. 
