REPORT ON ATMOSPHERIC CIRCULATION. 49 



publication of Humboldt's isothermal lines. But as regards the ocean, which comprises 

 three-fourths of the earth's surface, the monthly and annual distribution of temperature 

 in the atmosphere over it can scarcely be said to have been yet seriously looked at. 



In these circumstances, the thanks of the climatologist is specially due to the 

 Signal Officer of the United States for the monthly averages for the North Atlantic, 

 which were published for several years in the International Bulletin, and to the 

 Meteorological Council of London for monthly averages for the Red Sea. The required 

 data have thus been available in this work for drawing the isothermals for these import- 

 ant parts of the ocean. A comparison of these means, Table IX., pp. 228-9 and 254-9, 

 and of the Challenger mean air temperatures, Table I., with the temperatures of the sea 

 for the same positions and months, shows that it is absolutely necessary, in the advance 

 of meteorology, that the determination of the monthly temperatures of the air over 

 the ocean be undertaken and carried out. The differences observed between the 

 temperature of the surface of the sea and that of the air over it, so far as a comparison 

 can yet be made in the North Atlantic and Red Sea, point to a much greater prevalence 

 of ascending and descending movements in the atmosphere than is generally supposed. 

 As regards the other oceans, the isothermals of the temperature of the atmosphere must 

 in the meantime continue to be drawn essentially from observations made on the islands 

 and along the coasts of these oceans. 



Some interesting results are arrived at by comparing the temperatures of the 

 ocean and air observed by the Challenger. The whole of the observations have been 

 sorted into 174 groups according to geographical position, and the differences entered 

 on a chart of the route of the expedition. In the Southern Ocean, between latitudes 

 45° and 60°, the temperature of the sea was lower than that of the air. The mean 

 difference was 1°"4, due probably to the temperature of the air being higher owing 

 to the prevailing W.N.W. winds, and that of the sea lower owing to the numerous 

 icebergs. To south of lat. 60° the sea was about 2° warmer than the air, owing 

 perhaps to an increased prevalence of southerly, and hence colder winds in these 

 high latitudes. 



The temperature of the sea exceeded that of the air from June 1874 to March 

 1875, or during that part of the cruise from Sydney to New Zealand, then to the 

 Fijis and through the East India Islands to Hong Kong, and thence to the Admiralty 

 Islands. During the whole of this time, except when near the north of Australia, the 

 sea was much warmer than the air, the excess generally being from 2° to 3 , rising near 

 Tongatabu to upwards of 4°. In passing the north of Australia in September, in which 

 season the wind is off the land and the air therefore dry and sunshine strong, the sea 

 was 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 excess being a degree and a half. 



On the other hand, in the Atlantic from lat. 20° to 40° N., the sea was on the 

 (rnrs. chem. chaix. esp. — part v. — 1889.) 6 a 



