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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 sliahtly 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 m 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 

 occupied by the earth — they are the summer and winter solstices 

 in June and December, giving us the longest and shortest days, 

 and the two eqainoxes 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 ; [b) 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 arc 

 than the smaller one, and as it happens that our summer in the 

 northern hemisphere occurs while this longer arc 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 thpse 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 B. Ball which go so far to strengthen Dr. Oroll'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 not so. Sir 

 Robert Ball proves mathematically that out of the total quantity of 

 heat received by eacii 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 



