for volcanoes and hot springs, has not been perceptible at the surface since first the rocks grew solid. Even in the Azoic or Archæozoic Age there are traces in ice-worn rocks and the like of periods of intense cold. Such cold waves have always been going on everywhere, alternately with warmer conditions. And there have been periods of great wetness and periods of great dryness throughout the earth.
A complete account of the causes of these great climatic fluctuations has still to be worked out, but we may perhaps point out some of the chief of them. Prominent among them is the fact that the earth does not spin in a perfect circle round the sun. Its path or orbit is like a hoop that is distorted; it is, roughly speaking, elliptical (ovo-elliptical), and the sun is nearer to one end of the ellipse than the other. It is at a point which is a focus of the ellipse. And the shape of this orbit never remains the same. It is slowly distorted by the attractions of the other planets, for ages it may be nearly circular, for ages it is more or less elliptical. As the ellipse becomes most nearly circular, then the focus becomes most nearly the centre. When the orbit becomes most elliptical, then the position of the sun becomes most remote from the middle or, to use the astronomer's phrase, most eccentric. When the orbit is most nearly circular, then it must be manifest that all the year round the earth must be getting much the same amount of heat from the sun; when the orbit is most distorted, then there will be a season in each year when the earth is nearest the sun (this phase is called Perihelion) and getting a great deal of heat comparatively, and a season when it will be at its farthest from the sun (Aphelion) and getting very little warmth. A planet at aphelion is travelling its slowest, and its fastest at perihelion; so that the hot part of its year will last for a much less time than the cold part of its year. (Sir Robert Ball calculated that the greatest difference possible between the seasons was thirty-three days.) During ages when the orbit is most nearly circular there will therefore be least extremes of
- See Sir R. Ball's Causes of the Great Ice Age, and Dr. Croll's Climate and Time. These are sound books to read still, but the reader will find many of their conclusions modified in Wright's The Quaternary Ice Age, which is a quarter of a century more recent.