He found the temperature of maximum density of sea-water, whose density at 20°C. was 1·0273, to be −3°·67C. (25°·4F.), and the temperature of freezing-point −2°·55C. (27°·4F.).[56] Somewhere between 25° and 26° F. may therefore be regarded as the temperature of maximum density of sea-water of average saltness. We have no reason to believe that the ocean, from the surface to the bottom, even at the poles, is at 27°·4F., the freezing-point. The actual slope resulting from difference of specific gravity, as we shall presently see, does not amount to 10 feet. Herschel’s estimate was, however, made on insufficient data, both as to the rate of expansion of sea-water and that at which the temperature of the ocean at the equator decreases from the surface downwards. We are happily now in the possession of data for determining with tolerable accuracy the amount of slope due to difference of temperature between the equatorial and polar seas. The rate of expansion of sea-water from 0°C. to 100°C. has been experimentally determined by Professor Muncke, of Heidelberg.[57] The valuable reports of Captain Nares, of H.M.S. _Challenger_, lately published by the Admiralty, give the rate at which the temperature of the Atlantic at the equator decreases from the surface downwards. These observations show clearly that the super-heating effect of the sun’s rays does not extend to any great depth. They also prove that at the equator the temperature decreases as the depth increases so rapidly that at 60 fathoms from the surface the temperature is 62°·4, the same as at Madeira at the same depth; while at the depth of 150 fathoms it is only 51°, about the same as that in the Bay of Biscay (Reports, p. 11). Here at the very outset we have broad and important facts hostile to the theory of a flow of water resulting from difference of temperature between the ocean in equatorial and temperate and polar regions. Through the kindness of Staff-Captain Evans, Hydrographer of the Admiralty, I have been favoured with a most valuable set of serial temperature soundings made by Captain Nares of the _Challenger_, close to the equator, between long. 14° 49′ W. and 32° 16′ W. The following Table represents the mean of the whole of these observations:—