Dr. Maury's account of this phenomenon is asserted with less confidence. The existence of an _isothermal floor_ of the ocean, as he calls it, was first suggested by the observations of Kotzebue, Admiral Beechey, and Sir James C. Ross. "Its temperature, according to Kotzebue, is thirty-six degrees Fahr., or four degrees Cent.; the depth of this bed, of invariable and uniform temperature, is twelve hundred fathoms at the Equator; thence it gradually rises to the parallel of about fifty-six degrees north and south, when it crops out, and there the temperature of the sea from top to bottom is conjectured to be permanent at thirty-six degrees. The place of this outcrop, no doubt, shifts with the seasons, vibrating north and south, after the manner of the Calm belts. Proceeding onwards to the Frigid zones, this aqueous stratum of an unchanging temperature dips again, and continues to incline till it reaches the Poles, at the depth of seven hundred and fifty fathoms; so that on the equatorial side of the outcrop the water above the isothermal floor is the warmer, but in Polar seas the supernatant water is the colder." [Illustration: Fig 4. Thermal Lines of equal Temperature.] * * * * * In the saline properties of sea water Maury discovers one of the principal forces from which currents in the ocean proceed. "The brine of the ocean is the ley of the earth," he says; "from it the sea derives dynamical powers, and its currents their main strength. Hence, to understand the dynamics of the ocean, it is necessary to study the effects of their saltness upon the equilibrium of the waves. Why is the sea made salt? It is the salts of the sea that impart to its waters those curious anomalies in the laws of freezing and of thermal dilatation. It is the salts of the sea that assist the rays of heat to penetrate its bosom." The circulation of the ocean is indispensable to the distribution of temperature--to the maintenance of the meteorological and climatic conditions which rule the development of life; and this circulation could not exist--at least, the character of its waters would be completely changed--if they were fresh in place of salt. "Let us imagine," says M. Julien, "that the sea, now entirely composed of fresh water, of one uniform temperature from the Pole to the Equator, and from the surface to its greatest depths; the solar heat would penetrate the liquid beds nearest to the Equator; it would dilate them, so as to raise them above their primitive level; by the single effect of gravitation, they would glide on the surface towards the polar zones. The absence of all solar radiation would tend, on the contrary, to cool and contract them without this tendency. An exchange would be established from the extremities towards the centre; in other words, a counter current of cold and heavy water, calculated to replace the losses occasioned by the action of solar radiation, would descend from