The theory which connects fluctuations of climate on a geological scale with changes in the composition of the earth’s atmosphere is due to Tyndall and Arrhenius, and was elaborated by Chamberlin. The theory supposed that the earth’s temperature is maintained by the “blanketing” effect of the carbon dioxide in the atmosphere. This acts like the glass of a greenhouse, allowing the sun’s rays to enter unhindered, but absorbing the heat radiated from the earth’s surface and returning some of it to the earth instead of letting it pass through to be lost in space. Consequently, any diminution in the amount of carbon dioxide present would cause the earth to radiate away its heat more freely, so reducing its temperature. But it is now known that the terrestrial radiation which this gas is capable of absorbing is taken up equally readily by water-vapour, of which there is always sufficient present, and variations of carbon dioxide cannot have any appreciable effect. Brief mention may be made here of a theory put forward by Humphreys, who attributed glaciation to the presence of great quantities of volcanic dust in the atmosphere. It would require an enormous output of volcanic dust to reduce the temperature sufficiently; but in any case the relation, if any, between vulcanicity and temperature during the geological ages is rather the reverse of that supposed by Humphreys, periods of maximum volcanic action coinciding more frequently with high temperatures than with low. Perhaps the best comment on Humphreys’ theory is that in 1902 F. Frech produced its exact opposite, warm periods being associated with an excess of vulcanicity and cold periods with a diminution. The theory which attributes climatic changes in various countries to variations in the position of the poles has been adduced in two main forms. The first is known as the Pendulation Theory, and supposes the existence of two “oscillation poles” in Ecuador and Sumatra. The latitude of these points remains unchanged, and the geographical poles swing backwards and forwards along the meridian of 10 E. midway between them. Varying distances from the pole cause changes of climate, and the movements of the ocean, which adjusts itself to the change of pole more rapidly than the land, causes the great transgressions and regressions of the sea and the elevation and subsidence of the land.