After the sun’s radiation reaches the outer limits of the earth’s atmosphere its nature and intensity are modified by the composition of the air through which it passes. In general the air itself is very transparent to the small wave-lengths which make up the solar rays, but the presence of fine dust, whether of volcanic or of cosmic origin, has been shown by Humphreys to be a distinct hindrance to their passage, so that volcanic eruptions of an explosive nature, such as that of Krakatoa in 1883, La Soufrière (St. Vincent) in 1902, or Katmai (Alaska) in 1912, may result in a fall of temperature over the world as a whole. The temperature of the earth is determined by the balance between the radiation received from the sun and the terrestrial radiation to space, and a decrease in the latter would be as effective in raising the mean temperature as an increase in the former. The use of glass for greenhouses depends on this principle; for glass is transparent to heat rays of small wave-length, but is largely opaque to the rays of greater wave-length which make up terrestrial radiation. Certain constituents of the atmosphere, especially water-vapour, carbon dioxide and ozone, are effective in this way, and variations in the amount of these gases present may affect the temperature. The angle at which the sun’s rays strike the earth’s surface is a highly important factor. Within the Tropics the sun at midday is nearly vertical throughout the year, and the mean temperature in these regions is correspondingly high; on the other hand, during the long polar night the sun is not seen for half the year, and very low temperatures prevail. There is thus a seasonal variation of the heat received from the sun in middle and high latitudes, the extent of which depends on the “obliquity of the ecliptic,” i.e. the inclination of the earth’s axis to the plane of its orbit round the sun, and any changes in this factor must alter the seasonal variation of climate. Further, since the climate of any place depends so closely on its latitude, it follows that if the latitude changes the climate will change. A ship can change its latitude at will, but we are accustomed to regard the position of the “firm ground beneath our feet” relatively to the poles as fixed within narrow limits. This stability has, however, been questioned from time to time, mainly on evidence derived from palæoclimatology, and theories of climatic change have been based on the wanderings of continents and oceans. Finally, local climate is intimately bound up with the distribution of land and sea, and the marine and atmospheric currents resulting therefrom, and on elevation above sea-level, both of which factors, as we shall see, have suffered very wide variations in the geological past.