=157. Convection Currents in Nature.=--Winds are produced by differences in the _pressure_ or _density_ of the air, the movement being from places of high toward places of low pressure. One of the causes of a difference in density of the air is a difference in temperature. This is illustrated by what are called the _land_ and _sea breezes_ along the sea shore or large lakes. During the day, the temperature of the land becomes higher than that of the sea. The air over the land expands and being lighter is moved back and upward by the colder, denser air from the sea or lake. This constitutes the _sea breezes_ (Fig. 139). At night the land becomes cooler much sooner than the sea and the current is reversed causing the _land breeze_. (See Fig. 140.) [Illustration: FIG. 139.--Sea breeze.] [Illustration: FIG. 140.--Land breeze.] The _trade winds_ are convection currents moving toward the hot equatorial belt from both the north and the south. In the hot belt the air rises and the upper air flows back to the north and the south. This region of ascending currents of air is a region of heavy rainfall, since the saturated air rises to cool altitudes where its moisture is condensed. The _ocean currents_ are also convection currents. Their motion is due to prevailing winds, differences in density due to evaporation and freezing, and to the rotation of the earth, as well as to changes in temperature. =158. The heating and ventilation of buildings= and the problems connected therewith are matters of serious concern to all who live in winter in the temperate zone. Not only should the air in living rooms be comfortably heated, but it should be continually changed especially in the crowded rooms of public buildings, as those of schools, churches, and assembly halls, so that each person may be supplied with 30 or more cubic feet of fresh air per minute. In the colonial days, the _open fire place_ afforded the ordinary means for heating rooms. This heated the room mainly by _radiation_. It was wasteful as most of the heat passed up the chimney. This mode of heating secured ample _ventilation_. Fire places are sometimes built in modern homes as an aid to ventilation. Benjamin Franklin seeing the waste of heat in the open fire places devised an iron box to contain the fire. This was placed in the room and provided heat by conduction, convection, and radiation. It was called _Franklin's stove_ and in many forms is still commonly used. It saves a large part of the heat produced by burning the fuel and some ventilation is provided by its draft. [Illustration: FIG. 141.--Heating and ventilating by means of a hot-air furnace.]