The line of maximum temperature, or that which passes through all the points of greatest heat in the ocean, is very irregular, and does not coincide with the terrestrial equator; six-tenths of its extent lies on an average 5° to the north of it, and the remainder runs at a mean distance of 3° on its southern side. It cuts the terrestrial equator in the middle of the Pacific Ocean in 21° E. longitude in passing from the northern to the southern hemisphere, and again between Sumatra and the peninsula of Malacca in returning from the southern to the northern. Its maximum temperature in the Pacific is 88°·5 of Fahrenheit on the northern shores of New Guinea, where it touches the terrestrial equator, and its highest temperature in the Atlantic, which is exactly the same, lies in the Gulf of Mexico, which furnishes the warm water of the Gulf-stream. The superficial water of the Pacific is much cooled on the east by the Antarctic current; it sends a cold stream along the coasts of Chile and Peru, which has great influence on the climate of both countries; it was first observed by Baron Humboldt, and is known as Humboldt’s current. It is more than 14° colder than the adjacent ocean, and renders the air 11° cooler than the surrounding atmosphere. In the Indian Ocean the highest temperature of the surface-water (87°·4) is in the Arabian Sea, between the Strait of Bab-el-Mandeb and the coast of Hindostan; it decreases regularly from south to north in the Red Sea. The superficial temperature diminishes from the tropics with the increase of the latitude more rapidly in the southern than in the northern hemisphere, till towards the poles the sea is never free from ice. In the Arctic Ocean the surface is at the freezing point even in summer; and during the eight winter months a continuous body of ice extends in every direction from the pole, filling the area of a circle of between 3000 and 4000 miles in diameter. The outline of this circle, though subject to partial variations, is found to be nearly similar at the same season of each succeeding year, yet there are periodical changes in the polar ice which are renewed after a series of years. The freezing process itself is a bar to the unlimited increase of the oceanic ice. Fresh water congeals at the temperature of 32° of Fahrenheit, but sea-water must be reduced to 28°·5 before it deposits its salt and begins to freeze: the salt thus set free, and the heat given out, retard the process of congelation more and more below.