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leading to young students in the ordinary diagrams, Drawn in its 
true proportions, the eye could detect no variation from a circle, 
hence it is useful to slivhtly exaggerate it, so that it can be seen to 
be elliptical. A circle has one important point within it called its 
centre; an ellipse has two, each called a focus, and the closer 
together these two points are, the nearer the approach to a true 
circle. The orbit thus being an ellipse, and the sun occupying one 
of the foci, it is evident that the earth’s distance from the sun is 
constantly varying, being in fact never the same for two successive 
days. The difference between the distances in January and July 
is considerable ; we are nearly three million miles nearer tne sun 
in the first week in January than in the first week in July. At 
the former time the earth is said to be in perihelion, at the latter 
in aphelion. In the orbit there are four important positions 
oceupied by the earth—they are the summer and winter solstices 
in June and December, giving us the longest and shortest days, 
and the two equinoxes in March and September, at both of which 
the days and nights are of equal length all over the world. It will 
be convenient for the purposes of explanation that we confine our 
attention to the two latter positions which will divide the year, 
simply into summer (including spring) and winter (including 
autumn). Now notice that the two equinoxes do not divide the 
orbit into two equal parts; one is shown much longer than the 
other, and the earth takes a longer time to traverse it, for two 
reasons—(a) Because the distance is longer ; (4) because the farther 
the earth is from the sun the more slowly it travels. At the 
present time it takes seven days longer to travel over the larger are 
than the smaller one, and as it happens that our summer in the 
northern hemisphere occurs while this longer are is travelled, our 
summer is seven days longer than our winter, the number of days 
being respectively 186 and 179—I mean from one equinox to the 
other. On the opposite side of the globe of course these numbers 
are reversed. The unequal lengths of summer and winter have a 
most important bearing on what we have to say presently. 
And here I must introduce the factors of the problem lately put 
forward by Sir R. Ball which go so far to strengthen Dr. Croll’s 
explanation. The total quantity of heat received by the whole 
earth in the summer and in winter is the same, but taking the 
northern and southern hemisphere separately, it is notso. Sir 
Robert Ball proves mathematically that out of the total quantity of 
heat received by each hemisphere in the year, 63 per cent. is 
received during its summer, and 37 per cent. during its winter. 
This he says is invariably the case under all possible conditions. 
Now since the general climatic conditions of the earth depend 
chiefly and radically upon the heat derived from the sun, we shall 
