Sea-water is a bad conductor of heat, therefore the temperature of the ocean is less liable to sudden changes than the atmosphere; the influence of the seasons is imperceptible at the depth of 300 feet; and as light probably does not penetrate lower than 700 feet, the heat of the sun cannot affect the bottom of a deep sea. It has been established beyond a doubt that in all parts of the ocean the water has a constant temperature of about 39°·5 of Fahrenheit, at a certain depth, depending on the latitude. At the equator the stratum of water at that temperature is at the depth of 7200 feet; from thence it gradually rises till it comes to the surface in S. lat. 56° 26ʹ, where the water has the temperature of 39°·5 at all depths; it then gradually descends till S. lat. 70°, where it is 4500 feet below the surface. In going north from the equator the same law is observed. Hence, with regard to temperature, there are three regions in the ocean: one equatorial and two polar. In the equatorial region the temperature of the water at the surface of the ocean is 80° of Fahrenheit, therefore higher than that of the stratum of 39°·5; while in the polar regions it is lower. Thus, the surface of the stratum of constant temperature is a curve which begins at the depth of 4500 feet in the southern basin, from whence it gradually rises to the surface in S. lat. 56° 26ʹ; it then sweeps down to 7200 feet at the equator, and rises up again to the surface in the corresponding northern latitude, from whence it descends again to a depth of 4500 feet in the northern basin. The temperature of the surface of the ocean decreases from the equator to the poles. For 10 degrees on each side of the line the maximum is 80° of Fahrenheit, and remarkably stable; from thence to each tropic the decrease does not exceed 3°·7. The tropical temperature would be greater were it not for the currents, because the surface reflects much fewer of the sun’s rays which fall on it directly, than in higher latitudes where they fall obliquely. In the torrid zone the surface of the sea is about 3°·5 of Fahrenheit warmer than the air above it; because the polar winds, and the great evaporation which absorbs the heat, prevent equilibrium; and as a great mass of water is slow in following the changes in the atmosphere, the vicissitude of day and night has little influence, whereas in the temperate zones it is perceptible.