TEMPERATURE.] METEOROLOGY 135 general excess being from 2 to 3, rising even near Tongatalm to upwards of 4. The climate of the southern part of this extensive region at the seasons visited has a large rainfall, much cloud, and consequently a comparatively small evaporation and sunshine. In June, when the "Challenger" passed the north of Australia, the climate was very dry, the sunshine strong, and the evaporation large, and there the sea was slightly colder than the air. In the Atlantic between lat. 20 N. and 20 S. the sea was everywhere warmer, the mean excess being about a degree ; and in the Pacific between lat. 30 N. and 30 S. the sea was also warmer, the mean xcess being a degree and a half. On the other hand, in the Atlantic from lat. 40 to 20 N. the sea was, on the mean, half a degree colder than the air. This region is remarkable for the high pressure which overspreads it, for the winds and currents which flow out in all directions, for its clear skies, strong sunshine, and consequently large evaporation, by which the temperature of the surface of the sea is lowered, and that of the air resting on it, being open to the heating influence of the sun, is raised. Similarly in the North Pacific from lat 40 to SO the temperature of the surface of the sea was half a degree lower than that of the air. These remarks apply only to the observations made strictly on the open sea. Near land very great differences were observed which varied with season. Thus at Hong Kong during the latter half of November 1874 the sea was 3 7 warmer than the air, the low temperature of the air at this season being caused by the lower temperature of the land and the northerly winds which then prevail; on the other hand, at Valparaiso in November and December of the following year the sea was 5 8 colder than the air during the three weeks the "Challenger" was there, the difference being due to the cold oceanic current which sweeps northwards past that coast, and the rapid increase in the temperature of the air at that time of the year. These results will help us in gaining some knowledge of the temperature of the air over the oceans of the globe in February and August, taken in connexion with a careful examination of the sea temperature of these months represented in figs. 8 and 9. FIG. 10. January Isothermals of the Surface of the Globe. The distribution of temperature over the surface of the globe is shown by figs. 10 and 11, which represent the temperature of the two extreme months January and July for the eleven years 1870 to 1880. The region of highest temperature, which may be regarded as comprised between the north and south isothermals of 80, forms an irregularly shaped zone, lying in tropical and partly in subtropical countries. On each side of this warm zone the tempera ture diminishes towards the poles, and the lines showing successively the gradual lowering of the temperature are, roughly speaking, arranged parallel to the equator, thus showing in an unmistakable manner the predominating influence of the sun as the source of terrestrial heat. While this decrease of temperature corresponds in a general way to what may be called the solar climate, there are great deviations brought about by disturbing causes. Among these disturbing causes the unequal distribution of land and water holds a prominent place. In January the earth presents to the perpendicular rays of the sun the most uniform surface, or the largest water surface, and in July the most diversified surface, or the greatest extent of land. Hence the zone of the earth s surface comprised between the isothermals of 80 is less irregular, and also spreads over an area more restricted, in January than in July. In July the areas enclosed by the isothermals of <LO and 90 are much larger in the Old World than in the New, it being the former which presents the larger land surface to the perpendicular rays of the sun ; and in January, the summer of the southern hemisphere, the most extensive area of high temperature occurs in Africa and the least in Australia, the high-temperature area of South America being intermediate. In contrast to this the belt of temperature exceeding 80 is of least breadth where it crosses the Pacific and Atlantic Oceans, the absolute minimum breadth being in July in the Pacific, the largest ocean, where the disturbing influence of the land is least. During the cold moniLs of the year, when the sun s heat is least and the effects of terrestrial radiation attain the maximum, the greatest cold is over the largest land surfaces which slant most to the sun. Thus the lowest mean temperature that occurs anywhere or at any season on the globe is - 55 8 at Werchojansk (lat. 67 34 N., long. 133 51 E.) in north-eastern Siberia. In Arctic America the lowest isothermal is - 40 - 0. During the winter the ocean everywhere maintains a higher tem perature in all regions open to its influence, as is seen, not only in the higher latitudes to which the isothermals push their way as they cross the Atlantic and Pacific, but also in their irregular courses over and near the Mediterranean, Black, Caspian, and Baltic Seas, Hudson s Bay, the mouth of the St Lawrence, the American lakes, and all other large sheets of salt and fresh water. The disturbing influence of sheets of water on the temperature in all seasons is very
strikingly shown when the isothermals are drawn for every