If the earth's orbit were an ellipse, the major axis of which remained in the same position, we could readily reckon all the effects which arise from the variations of the great year. But this ellipse is ever changing in form, and in the measure of its departure from a circle the effects on the seasons distributed over a great period of time are exceedingly irregular. Now and then, at intervals of hundreds of thousands or millions of years, the orbit becomes very elliptical; then again for long periods it may in form approach a circle. When in the state of extreme ellipticity, the precession of the equinoxes will cause the hemispheres in turn each to have their winter and summer alternately near and far from the sun. It is easily seen that when the summer season comes to a hemisphere in the part of the orbit which is then nearest the sun the period will be very hot. When the summer came farthest from the sun that part of the year would have the temperature mitigated by its removal to a greater distance from the source of heat. A corresponding effect would be produced in the winter season. As long as the orbit remained eccentric the tendency would be to give alternately intense seasons to each hemisphere through periods of about twelve thousand years, the other hemisphere having at the same time a relatively slight variation in the summer and winter. At first sight it may seem to the reader that these studies we have just been making in matters concerning the shape of the orbit and the attendant circumstances which regulate the seasons were of no very great consequence; but, in the opinion of some students of climate, we are to look to these processes for an explanation of certain climatal changes on the earth, including the Glacial periods, accidents which have had the utmost importance in the history of man, as well as of all the other life of the planet.