324

TRADE WINDS.

[152.

The velocity of the currents produced by heat, and the rate of cooling effected by them upon a thermometer heated up to a determinate point, vary in different gases, being more rapid the lighter the gas. In hydrogen the rate of cooling is much more rapid than in air, while in carbonic anhydride it is considerably less rapid.

(153) Trade Winds.-The processes of circulation produced by heat in liquids and gases, which have just been described, occur upon a vast scale in the atmosphere and in the ocean. The important phenomena of the trade winds arise from movements which originate from these causes. The temperature of the sur

face of the earth not being uniform, but being highest within the tropics and lowest at the poles, the air near the equator rises in temperature, it becomes expanded, grows less dense, and therefore ascends, its place being supplied by cooler air from the parts adjacent, but nearer to the poles. The heated equatorial air rises to a certain point, and then falls over to supply the place of the cooler air just conveyed from the neighbouring regions. In consequence of these actions, the air upon the surface of the earth is continually moving from the poles towards the equator, and above this current is another proceeding in the contrary direction, from the equator towards the poles. The lower current, which is steadily felt on each side of the equator through at least 30° of latitude, is of the utmost importance to navigation, and forms what are called the trade winds. The upper current does not admit of being so accurately traced, but there is satisfactory proof of its existence. The summits of many inter-tropical mountains, such as the Peak of Teneriffe, 12,180 feet (3712 metres) high, and Mouna Kea, in the Sandwich Islands, 18,400 feet (5608 metres) in height, are sufficiently elevated to reach into the upper current; and at the top of these mountains a strong south-westerly wind blows continually, whilst the north-east trade wind is blowing at the base. If the earth were stationary, these currents would set due north and south. The surface of the globe, however, is revolving from west to east, at the average rate of 980 miles per hour in its equatorial part, and the rapidity of motion gradually diminishes towards the poles, at which point the motion almost vanishes. Air, therefore, which flows towards the equator from the poles, is moving more slowly than those regions of the earth towards which it advances. Since, however, the objects upon the surface partake of the motion of the earth at the particular spot on which they rest, and as therefore the earth's motion is not perceptible, the effect of a wind travelling more slowly in the same

154-]

LAND AND SEA BREEZES-GULF STREAM.

325

direction as that in which the earth is moving would be precisely the same as that of a current blowing in the opposite direction, with a velocity equal to the difference between the rates of the two motions, supposing the earth to be at rest: consequently the wind from the north has a set from the east, which diminishes as it approaches the equator, where the motion of the successive portions of the surface becomes more uniform. From the operation of these causes the north-east is one of the most prevalent winds in our climate. For similar reasons, the equatorial current towards the poles sets in a direction from the west, and retains its course when it comes down to the surface, which it does at and about our latitude, occasioning the westerly winds which prevail in these islands so generally at certain seasons.

The land and sea breezes which occur morning and evening along the coasts of tropical countries, are due to the action of analogous causes. During the early part of the day the surface of the land, from the action of the sun's rays, becomes more heated than the ever-moving ocean; the air above it expands and rises, whilst its place is supplied by cooler air from the oceanthis constitutes the sea breeze: whereas in the evening, after sunset, the land cools more rapidly than the ocean, and the air resting upon it contracts in bulk, and becoming heavier, flows out during the night upon the sea, and produces the land breeze.

(154) Gulf Stream.-Similar currents, of equal constancy and regularity, exist in the ocean, but they are modified in their direction by the general distribution of land and water on the earth's surface. That part of the ocean which is immediately under the tropics, and between the eastern and western hemispheres, for example, becomes highly heated; the water flows off on either side, towards the poles, acquiring a westerly direction as it passes south of the coast of Guinea, and striking the promontory of Cape St. Roque, on the South American coast, is split into two streams; the smaller one continues southwards towards Cape Horn; the larger current maintains a north-westerly course in the Gulf of Mexico, where it receives further accessions of heat, and is gradually changed in its direction; it passes along the southern shores of North America, and finally emerges northward, in the narrow channel between the peninsula of Florida and the Bahama Islands, where it assumes the name of the Gulf Stream. The temperature of this current is found to be somewhat above 5° C. higher than that of the neighbouring ocean. The current passes on, gradually widening and becoming less marked, till it is lost on the western shores of Europe. A less accurately defined

326

GULF STREAM-RADIATION OF HEAT.

[154.

under-current, from the poles, is constantly setting in towards the equator, to supply the place of the heated water which takes the course already described. Besides rendering important aid to the navigator, these currents assist in maintaining an equilibrium of temperature on the earth, moderating the severity of the polar frost, and tempering the sultry heats of the tropics. One cause of the comparative mildness of our own winters is the warmth conveyed to our shores by the Gulf Stream.

It would appear from the results of the deep sea soundings in the Atlantic, that there are two currents of water flowing in opposite directions, a warm superficial current passing from the equator towards the poles, and a cold lower current proceeding from the poles towards the equator.

Radiation of Heat.

(155) A person placed in bright sunshine, or before a blazing fire, must perceive that in addition to the gradual mode of propagation from particle to particle, heat is endowed with the faculty of traversing space, and transparent media such as the atmosphere. This transmission of heat occurs in right lines, with a velocity equal to that of light itself; in fact, in this mode of propagation it follows the same laws as light, and like all radiations it diminishes in intensity as the square of the distance from the active centre increases.

The great supply of heat to the earth from the sun is transmitted by the process of radiation. Some idea of the amount of heat thus received by the earth may be formed from a rough calculation made by Faraday, to the effect that the average amount of heat radiated in a summer's day upon each acre of land in the latitude of London, is not less than that which would be emitted in the combustion of sixty sacks of coal. Dr. Siemens (Lecture to British Assoc. Bradford, Nature, 1873, viii. 443) calculates that the quantity of heat reaching the earth from the sun is capable of evaporating a layer of water 14 feet in depth annually. This is equivalent to 1680 tons per acre annually, or about 92 cwt. in 24 hours.

Heat, in its radiant state, does not raise the temperature of the media which it traverses: a tube full of ether may be held in the focus of a burning mirror without becoming sensibly hotter ; but the moment that the absorption of the rays is caused in any way, as by introducing a bit of charcoal into the liquid, the ether enters into ebullition and is dissipated in vapour.

(156) Reflection of Heat.-Polished objects reflect the greater

157.]

ABSORPTION OF HEAT.

327

part of the heat which falls upon them; the reflected and incident rays are always in the same plane, and the angles which they make with a perpendicular to the reflecting surface are always equal. By means of concave mirrors, the rays of heat, like those of light, may be brought to a focus, and, if sufficiently intense, they will ignite combustible substances placed there. The law of the reflection of heat may be roughly demonstrated by holding a flat sheet of tin-plate in such a position before a common fire that the light of the fire may be reflected from it, whilst the observer is screened from the direct rays; the sensation of heat will be perceptible upon the face the moment that the reflection of the fire is seen. The same effect may be shown in a still more striking manner by means of two similar concave parabolic mirrors (fig. 131, page 330) arranged opposite each other, at the distance of 4 or 5 metres or more. If a lighted candle be placed in the focus of one of the mirrors, the rays will fall upon its concave surface, and thence be reflected in parallel lines to the surface of the second mirror, from which they will be a second time reflected, and will converge at its focus; a luminous spot being formed upon a piece of paper held in this position. If for this paper one of the balls of a differential thermoscope (135) be substituted, the expansion of the air in that bulb will afford evidence that the heat as well as the light is reflected. That the rays take the course described, and which is represented in the diagram, and that they do not act upon the instrument by direct radiation, is shown by interposing a small tin-plate screen between the second mirror and the thermometer: in this case the liquid immediately becomes stationary; while, if the screen be placed between the instrument and the candle, no sensible effect is produced.

If, instead of a candle, a red-hot ball be placed in the focus of the first mirror, paper may be scorched, and gunpowder or phosphorus inflamed in the focus of the second. Heat, however, is emitted in the form of rays from bodies, whether such bodies be luminous or not. A canister of boiling water may be substituted for the candle or the red-hot ball, and the heat which it emits, although less intense, will be concentrated by the opposite mirror equally well.

(157) Absorption of Heat.-Different substances reflect heat unequally. Polished metals possess the power of reflection in the highest degree, but even the metals differ considerably in reflecting power. Melloni, from his experiments, has concluded that of 100 rays, silver reflects 90; bright lead reflects 60; whilst glass reflects but 10.

328

ABSORPTION OF HEAT.

[157.

If the surface of a body be scratched it reflects heat irregu larly, in the same way that a sheet of white paper scatters the light which it reflects; and if the surface be coated more or less completely with lamp-black, the amount of heat which is reflected may be diminished in a degree proportioned to the alteration of the surface. In this case, that portion of the heat which is not reflected is absorbed. When the heat is all reflected, the temperature of the body remains unaltered; but when absorption takes place, the temperature rises in proportion to the quantity of heat which is absorbed.

This difference may be exhibited by placing a lighted taper in the focus of one of the mirrors, and employing in the second focus a differential thermoscope, one ball of which is gilt, and the other ball covered with lamp-black. On placing the gilt ball in the focus, scarcely any motion of the liquid in the stem is perceived; but, on reversing the balls, although the amount of heat which falls on the instrument is no greater than before, the liquid descends rapidly: in the first case, the heat is for the most part reflected: in the second it is absorbed, and the temperature consequently rises.

A similar result may be obtained by taking two bright tin plates, and coating one surface of one of them with lamp-black. On placing them in a vertical position, with a hot iron ball midway between the two plates but not touching either of them, the blackened surface being directed towards the source of heat, it will be found that the blackened plate becomes heated by absorption, while the other remains cool this may be shown by causing a cork to adhere to the outer surface of each plate, by means of a little wax or pomatum; the wax will melt upon the blackened plate, and the cork will fall from it much sooner than from the bright one.

The power of reflection seems to reside almost exclusively in the surface of the body. A film of gold leaf, not exceeding

millimetre in thickness, answers the purpose of a reflector nearly as well as a mass of solid gold; since a sheet of paper partially gilt, if held within a short distance of a mass of red-hot metal, will become scorched, excepting in those parts which are protected by the metallic film. The absorbing power of a substance is complementary to its power of reflecting heat; the best reflectors are the worst absorbents, and vice versa. As is the case with light, so it is found with radiant heat, that, excepting in the case of polished metals, the greater the angle of incidence the more complete is the reflection.

(158) Connexion between Absorption and Radiation.-The experiments of Leslie have proved the existence of an important connexion between the absorbing and the radiating powers of the same substance: they are in all cases directly proportioned to each other. The great diversity of radiating power possessed by different substances may be exemplified by the following experiments. Let a cubic canister of tin-plate have one of its sides