As water is a bad conductor of caloric, the temperature of the seas is in general more constant than that of the air.
The equinoctial ocean seldom attains the maximum warmth of 83°, and has never been known to rise above 87°; while the surface of the land between the tropics is frequently heated to 129°. In the neighbourhood of the line, the temperature of the surface-water oscillates all the year round only between 82° and 85°, and scarce any difference is perceptible at different times of the day.
The wonderful sameness and equability of the temperature of the tropical ocean over spaces covering thousands of square miles, particularly between 10° N. and 10° S. lat., far from the coasts, and where it is not intersected by pelagic streams, affords, according to Arago, the best means of solving a very important, and as yet unanswered question, concerning the physics of the globe. "Without troubling itself," says that great natural philosopher, "about mere local influences, each century might leave to succeeding generations, by a few easy thermometrical measurements, the means of ascertaining whether the sun, at present almost the only source of warmth upon the surface of the earth, changes his physical constitution, and varies in his splendour like most stars, or whether he has attained a permanent condition. Great and lasting revolutions in his shining orb would reflect themselves more accurately in the altered mean temperature of those ocean plains than in the changed medium warmth of the dry land."
The warmest part of the ocean does not coincide with the Equator, but seems to form two not quite parallel bands to the north and south.
In the northern Atlantic, the line of greatest temperature (87° F.) which on the African coast is found but a little to the north of the Equator, rises on the north coast of South America as high as 12° N. lat., and in the Gulf of Mexico ranges even beyond the tropic. The influence of the warmth-radiating land on inclosed waters is still more remarkable in the Mediterranean (between 30° and 44° N. lat.) where during the summer months a temperature of 84° and 85° is found, three degrees higher than the medium warmth of the open tropical seas.