important than usually believed. While the underlying principles are simple, it is so important a matter that only experts should attempt to design or work out the proper ventilation of public buildings. A fact especially worth mentioning is that in order to cool a room quickly in the evening after a hot day it is of great advantage to have openings in or near the ceiling. Casement windows are far more effective than the ordinary windows, which are usually open only at the bottom. On the other hand, in winter weather it is usually a great disadvantage to have places in the ceiling for the air to leave the room, as is a common practice in some churches and public halls. In that case the fresh air entering from the registers, being the warmest air in the room, rises at once and flows out through the ventilators in the ceiling.

242. Winds and ocean currents. The chief causes of winds are the convection currents caused by the unequal heating of the surface of the earth. The air near the equator rises and flows toward the poles, while the surface layers of air tend to flow toward the equator. These currents are greatly modified by the rotation of the earth, by local disturbances due to unequal heating, by mountain ranges, and the like. If one has watched the currents of water in a large vessel over a hot fire and noticed the turbulent conditions in the water, he can better form a mental picture of the turbulent conditions existing on a vast scale in the earth's atmosphere.

In the daytime the land is, in general, warmer than the surface of the ocean. The large specific heat of water and the fact that part of the sun's heat penetrates more deeply into it prevent as large a rise of temperature of the surface of the water as of the land. Hence the air lying over the land becomes hotter than that over the water, and a convection current is set up, with the surface wind blowing from the water toward the land-the so-called sea breeze. At night, for a somewhat similar reason, the air flows in the opposite direction, giving the land breeze. When either the land or the sea breeze coincides in direction with the prevailing wind, it will be very strong. When the prevailing wind is from the water, the day wind will be strong; at night, on account of the two opposing effects, the breeze may be very weak.

Mountain and valley winds are quite noticeable. In the daytime the wind blows up the mountain; but at night the surface of the mountain cools off quickly and cools the air, causing a downward flow. When this downward breeze converges from two or more mountain sides into a narrow valley, the breeze may become strong. In rolling country, convection currents are more readily produced, with the result that the summer nights are more comfortable than in flat prairie country. (See also section 295.)

The currents of water in the ocean are often well defined, and are explained by the simple principles of convection, the interference of masses of land, the effects due to the earth's rotation, and the action of the prevailing winds.

243. Radiation. The process of transfer of energy known as radiation is very different in character from conduction or convection. A few examples will assist in making clear to the student just what is meant by the term radiation. By some process energy traverses the space between the sun and the earth, although this space is undoubtedly a more perfect vacuum than any that we can make in our laboratories. This energy when absorbed becomes heat energy. The heated filament of an incandescent lamp, even when there is no air or gas in the glass bulb, emits energy which produces heat where this energy is absorbed. One can readily detect this by holding the hand or the face near such a lamp. The same effect is observed near any heated object, such as a steam pipe, a stove, or the glowing coals in a fireplace. In most of these cases the student can easily satisfy himself that the heat is not brought to his hand or face by conduction or by convection currents. The term radiation, when used in a general sense, is a process of transferring energy from a body across space.

It is now generally accepted that that form of propagation of energy which we call heat radiation is a wave-motion. The reasons for this belief will be given in the chapters on light. In some way the rapid vibrations of the electrons and atoms in a hot body are able to produce waves which travel across space to another body and there stimulate vibrations of the electrons and atoms, thus producing heat. Note that the wave-motion is not heat, but produces heat when absorbed.

Many of the facts concerning heat radiation given in the following sections can be easily demonstrated by the use of a simple air thermometer. It consists of a glass bulb, four or five centimeters in diameter, with a long narrow stem. It should be inverted with the end of the stem below the surface of some colored water. The bulb must be blackened. If the bulb is slightly heated, the inclosed air will expand, part bubbling out through the water. On cooling, the colored liquid will rise part way up the stem, where it will be plainly visible. A more accurate instrument is the thermopile, which utilizes the thermoelectric phenomena referred to in section 190.

244. Heat radiation travels in straight lines. It is very easy to show that heat radiation travels in straight lines, for any kind of obstacle placed between a hot body and the blackened bulb of an air thermometer will cut off the heating effect. A fine illustration

of this effect is the fact that when the moon comes between the earth and the sun, producing a solar eclipse, heat as well as light is cut off.

245. Reflection. With a pair of large spherical metal mirrors the radiation from an iron ball hanging in a Bunsen flame can be reflected and focused on the bulb of an air thermometer. The effect is quite decided even when the mirrors are four or five meters apart (Fig. 160).

246. Radiation at low temperatures; law of exchanges. It has been shown that radiation takes place from all bodies, no matter what their temperature may be. Even ice radiates some energy. However, the higher the temperature of a body, the greater the rate at which it radiates energy. If a cold body is brought into a

warm room, it will radiate energy to the other objects in the room; but these other bodies, being warmer, will radiate more energy to it than it gives out. The temperature of the cold body will gradually rise until it comes to the same temperature as the other bodies in the room, when it then will receive radiant energy at exactly the same rate as it emits it. The principle is known as Prevost's law of exchanges.

247. Apparent radiation of cold from ice. If two concave reflectors are set facing each other, and a block of ice is placed at the focus of one mirror and the blackened bulb of an air thermometer at the focus of the other, the temperature of the air thermometer will fall. At first sight this seems to be due to a radiation of cold from the ice. The true explanation is as follows: The ice radiates some energy, which is reflected over to the bulb of the air thermometer. The bulb of the air thermometer radiates some energy which is reflected over to the ice. There is thus an exchange going on. But the bulb of the air thermometer, being at a higher temperature, radiates more energy to the ice than it receives from the ice. It is therefore cooled by its own radiation.

248. The mean temperatures of the earth and other planets. The earth receives from the sun radiant energy, which tends to raise the temperature of the earth. The mean temperature of the earth is determined by the fact that this temperature must be such that the earth will radiate energy at the same average rate at which it receives energy from the sun. Since the rate at which energy is received from the sun is known (at least approximately), it is possible, by using the radiation law of section 254, to compute what the temperature of the earth must be in order to radiate energy at this rate. This computation gives a result near the observed average temperature of the earth.

The planet Venus is much nearer the sun, and hence receives radiant energy at a greater rate than we do. It must therefore be at a higher temperature than the earth in order to radiate energy at the same rate as it receives it. Estimates of its mean temperature based on this kind of reasoning indicate that the mean temperature of Venus is about 80° C., or about 175° F. Mars, on the other hand, is farther away from the sun than the earth. It receives less radiant energy; and in order that it shall radiate energy at the same average rate as it receives energy, its average temperature must be lower than that of the earth. A simple estimate indicates that its mean temperature is about the freezing temperature of mercury; that is, about 40° C. Estimates of this kind are not very accurate, for the type of atmosphere and the amount and kind of cloudiness greatly modify the results.

249. Cooling by radiation. On clear nights objects lying out in the open radiate energy into the space above them and do not receive much energy in return. They therefore cool off rapidly and fall to a temperature lower than that of the air. This is especially noticeable if they are not good conductors, and hence do not receive heat by conduction from the ground below. Straw, boards, and other nonconductors usually get colder than the air on clear nights. Such things as rocks and cement walks are kept warm by conduction of heat from the ground. This effect of the cooling due to radiation is very obvious in the spring and fall when frosts are likely to occur: frost usually forms only on objects which get colder than the air. On clear nights when the temperature of the air goes no lower than 35° F., water in shallow pans placed on boards or on straw away from buildings and trees will usually freeze. On cloudy nights the clouds radiate energy to the earth and thus prevent the temperature of plants and other objects from falling below air temperature.

250. Good and poor radiators. Different kinds of surfaces show very different radiating properties. If one side of a polished vessel is coated with lampblack, the blackened side, when the vessel is filled with boiling water, will radiate more energy than the polished side. This can easily be shown by bringing the vessel near the blackened bulb of an air thermometer and turning first one side and then the other toward the thermometer. If two vessels which are similar, except that one has its surface blackened and the other is polished, are filled with boiling water and allowed to cool, it will be observed that the temperature of the blackened one falls more rapidly than that of the other. A blackened surface is a better radiator than a polished one.

It is a general principle that a good radiator is also a good absorber of radiant energy. This can be easily shown by theoretical considerations. Imagine that a body A which has a good radiating surface is placed inside some inclosure where there are other bodies. A closed room would do. This body A will radiate energy to the room and will also absorb energy which is radiated by the other bodies. It will finally come to the same temperature as the room, and will then absorb energy at the same rate as it radiates