Even more calamitous would be the floods of springtime, for melting snows and falling rains would spread over and erode the cultivated fields of the husbandman instead of being carried away by the open channels of streams, as is largely done now. =The Freezing of Fresh and of Salt Bodies of Water.= The freezing of water does not take place upon the surface of water only, as many suppose. Congelation takes place about millions of minute atoms of matter carried by the water in suspension. Water expands in the process of freezing and each particle of ice, no matter in what part of the body of water it is formed, immediately rises to the surface because of the gain in its buoyance as it changes from the liquid to the solid form. When the surface of water cools by radiation to a cooler air it gains in specific gravity and sinks and warmer water comes up to take its place and in turn be cooled and sink; thus a circulation is established which continues in fresh water until every part of the body of water has fallen to 39° and in salt water to 28°. At these temperatures the two waters reach their maximum density. With the further cooling of salt water particles of ice form and rise to the top, as already described. With the cooling of fresh water below 39° the law that holds good for all higher temperatures is reversed and expansion of volume begins, which continues until 32° is reached. Therefore, fresh water of any temperature between 39° and 32° may float upon water that is considerably warmer; in fact, it has less specific gravity at 32° than at 46°. At 32° that which was a liquid becomes a solid and still further suddenly expands its volume. =The Cold of Ocean Bottoms.= Few have any idea of the enormous volume of cold water that lies upon the surface of the earth, three fourths of which is covered with oceans whose depths average two miles and in many places are five miles. Below one mile in depth these oceans are always at about the freezing point of salt water, which is 28°, except in the tropics, where it is but little warmer, varying between 34° and 36°.