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Change then, we see, in all the circumstances which govern the 

 climate of the earth ; therefore change in that too. The phenomena 

 which are most intimately connected with the climatic conditions, 

 and specially with the production of an Ice Age are : — (a) The 

 variations in shape of the earth's orbit, and with that the varying 

 relative distances of the sun in summer and winter, (b) The 

 precession of the Equinoxes together with changes in the direction 

 of the earth's axis. 



The excentricity of the orbit at present is comparatively slight, 

 and is decreasing so that it is becoming more and more circular and 

 will (so vre are told), be as near to a circle as possible in the year a.d. 

 25,780, after which date it will contract again in its minor axis, and 

 become more elliptical. The major axis alters in length to such a 

 very minute extent that it need not be taken into account in con- 

 nection with our subject. It necessarily follows that an alteration 

 in the ellipse itself produces an alteration in the relative position 

 of the foci, at one of which, as we have seen the sun is placed. 

 Such alteration of the shape of the orbit is produced by the influence 

 of the other planets, chiefly by Venus and Japiter. Note then that 

 if the elhpticity increases, the two foci are farther apart from each 

 other i.e. nearer to the earth's orbit, and consequently the sun will 

 be much nearer to us when we are in perihelion than it is at present 

 and much farther off at the the time of aphelion. Since the intensity 

 of the sun's heat and light received by us are proportional, not to 

 the simple distance, but to the square of the distance, such variation 

 will have a considerable efl'ect on the climate of the earth. The 

 difference in distance in summer and winter is nearly 3,000,000 

 miles, but it may amount to as much as 14,000,000 and then the 

 differential climatic effect would be greatest of all. If, as at present 

 the summer happened when the earth were in aphelion it would be 

 a much cooler summer than we now enjoy, and the winter would 

 be very much warmer. But if, on the contrary, our winter occurred 

 in aphelion, there would be a much severer winter and a much hotter 

 summer. And, at the same period, the winter would be much 

 longer (it might possibly be 33 days longer than summer), and we 

 should have in the combination the requisite conditions for a glacial 

 period. Under what conditions, we may ask could there be the great- 

 est difference in the lengths of summer and winter ? This leads us to 

 consider the second variation, viz., the Precession of the Equinoxes. 

 The Equinoxes, i.e., the times of equal day and night all over the 

 world, occur twenty minutes earlier every year, and hence they travel 

 round the whole orbit, and occur successively in every part of it. 

 On the slide we see them approximately in their present position. 

 By and bye the spring Equinox will be in aphelion, the autumnal 

 in perihelion ; the orbit will then be equally divided, and summer 



