=A Cloud Covering Cools by Day and Warms by Night.= One of the principal functions of clouds is to conserve the heat of the sun. A covering of cloud, fog, or dense haze may not only screen off the heat of day, but greatly retard the lowering of temperature at night by reflecting and radiating back to the ground much of the heat that it has lost. =The Temperature of Oceans, Lakes, and Rivers.= The same quantity of heat falling upon different kinds of matter produces different temperatures, depending on the capacity (specific heat) of each kind of matter to absorb or hold heat; this is notably apparent when the matter is land, water, or air; for the same quantity of heat will raise the temperature of a water surface only about one fourth as much as it will a land surface. Water rejects by reflection a considerable amount of the solar rays that fall upon it, while land reflects but a small part; and of that which is received upon the top layer of water much is rendered latent in the process of evaporation and does not impart warmth to the water. Solar rays also penetrate water to a considerable depth and are quite uniformly absorbed by the whole stratum penetrated. These conditions cause large water surfaces and the air immediately over them to have a much lower temperature during the day and a much higher temperature during the night; and also lower temperatures during summer and higher temperatures during winter, than occur over a land surface of the same latitude. =Fresh Water and Salt Water Have Different Freezing Temperatures.= In the ratio of 93.5 to 100 the specific heat of sea water is less than that of fresh water. Sea water is a better conductor of heat, so that it penetrates to a greater depth in salt water in the same period of time than it does in fresh water. Sea water regularly contracts with falling temperature until its greatest density occurs at four degrees below freezing, when it becomes solid ice and expands in the process of freezing; otherwise it would not float. =A Wonderful Phenomenon.= In this respect a most wonderful and unexplainable phenomenon occurs with regard to fresh water. Not only sea water but practically all other forms of matter—liquid, solid, and gaseous—expand with increasing heat and contract with decreasing heat, except fresh water between 39° and 32°, which actually expands with falling temperature. It seems as though the Creator had gone over His work and made revisions and corrections here and there, for unless the law with regard to the contraction of liquids with falling temperatures had been reversed for fresh water between 39° and 32° our rivulets, streams, lakes, and rivers would freeze from the bottom upward and the life of inland water be wholly or partly destroyed.