50 THE VOYAGE OF H.M.S. CHALLENGER. 



mean half a degree colder than the air. Similarly in the Pacific, from lat. 30° to 40° N., 

 the temperature of the surface of the sea was half a degree lower than that of the air. 

 The explanation of these differences is probably to be found in the degree of humidity 

 of the atmosphere, the direction of the wind, and the degree in which descending 

 aerial currents mingle with the winds that sweep across the surface of the ocean. It is 

 evident that a wind, issuing from an anticyclone in which descending currents are strong 

 and decided, necessarily possesses quite different hygrometric and temperature qualities 

 from those of a truly horizontal wind which has traversed a large extent of the ocean. 



The above remarks refer only to those observations which were made strictly on 

 the open sea. Near land great differences, either way, were observed, which varied 

 with season. At Hong Kong, for example, during the latter half of November 1874, 

 the sea was 3° "7 warmer than the air, the low air temperature being occasioned by the 

 lower temperature of the land and the northerly winds prevailing there at this season. 

 On the other hand, at Valparaiso in November and December 1875, the sea was 5° - 8 

 colder than the air, the low sea temperature being probably occasioned by the up- 

 welling to the surface of the colder water of greater depths by the winds blowing off 

 the land on this coast, similar to what Dr. Murray has proved by extensive observations 

 to prevail in the Scottish lochs. 1 



The distribution of temperature over the globe is shown by Maps I. to XXVL, 

 representing the months and the year. The region of highest temperature, which 

 may be taken as comprised between the north and south isothermals of 80°, forms an 

 irregularly shaped zone, lying in tropical and partly in sub-tropical countries. On each 

 side of this warm zone temperature 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 unmistakeably the predominating influence of 

 the sun as the source of terrestrial heat. While, however, the decrease of temperature 

 corresponds in a general way with what may be conveniently termed the solar climate, 

 there are great deviations brought about by disturbing causes, and among these causes 

 the unequal distribution of land and water holds a prominent place. 



January. — During the time of the year when the sun's heat is least felt, and the 

 effects of terrestrial radiation attain the maximum, the greatest cold is over the largest 

 land surfaces which slant most to the sun. Hence the lowest mean temperature that 

 occurs anywhere or at any season on the globe, — 61°"2, occurs in January at Werko- 

 jansk, lat. 67° 34' N. and long. 133° 51' E., in north-eastern Siberia, at a height of 460 

 feet above the sea. In January 1886, temperature fell at this place to — 88°"8, being 

 absolutely the lowest temperature of the air hitherto observed. The lowest mean 

 temperature in America is nearly — 40°, and this cold region is situated a little to the 

 north of the magnetic pole. 



1 " On the Effects of Wiuds on the Distribution of Temperature in the Sea- and Freshwater Lochs of the West of 

 Scotland." Scottish Geographical Magazine, July 1888. 



