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and variables to the north and south respectively. This theoretical deduction is verified by the results of observation, which show that the mean of the barometer is higher at the tropics than at the equator ; in other words, there is an almost constant deficiency of pressure in the equatorial regions.
Fig. 2 (6) gives a representation of a vertical section of these two gradients. A and B representing regions of higher barometer
north and south of E, the equator a region of lower barometer, A E being the gradient for north-easterly winds, and B E for south-easterly.
If, for the sake of illustration, we suppose (Fig. 2, a) the barometer to stand at a uniform height within the region in question, then to produce these gradients the barometer must fall at E. A depression being formed there, a motion of the air from A and B will take place towards it, causing the barometer to rise at A and B, both actions producing the same result. It will be seen further on, that this depression is followed by a compression, as it may be called, which will have the effect of causing the barometer to rise at E, and that again will cause a fall at A and B. This will render the gradients less steep. But this compression being succeeded by another depression, the same variation will again Occar.
Whatever difference of opinion there may be as to the causes at work, it may be safely asserted that there is none as to the region in which they are most effectively in operation, and that is in the equatorial regions. We are thus driven to the conclusion that these gradients are mainly due to a fall of the barometer or the formation of a depression in the vicinity of the equator.
That the general circulation is owing to a depression, followed
by a compression moving from east to west, may be shown thus:
Let P (Fig. 3) represent a particle or body of air moving towards the centre of a depression D—the barometer being higher at P than at D. When the particle has reached the position P: the depression moving towards the west is now at D', and the particle will therefore move towards it into the position Pļ, but the depression being now at Dạ, the particle will move to P3; in plain terms, the motion of the depression will cause the body of air to describe a semicircle-moving from west through north to east.
The compression by which, according to the law of inertia, this depression must be followed, will complete the circulation of the body of air P. For if we consider that while the particle is in the position P3 the following compression is in the position D, the barometer being now higher at D than at P3 the particle will move into the position P4. As the compression proceeds westwards to D' and D2 the particle will move towards Ps and at last regain P, its original position. In short it will describe a semicircle moving from east through south to west.
In a similar manner it might be easily shown that a depression moving westwards, followed by a compression, will cause the air south of the equator to rotate in the reverse direction.
This result is in strict agreement with what was observed in the experiment with the water in the basin--the circulation being clearly the effect of a hollow, followed by an elevation.
It may now be naturally asked, is this experimental and theoretical
deduction confirmed by known facts ? and it may be confidently affirmed that it is. Within the tropics, and especially near the equator, the barometer rises and falls with so much regularity, that it has been said “ that the hour of the day may be inferred from its height.” It rises from about 4 h.m. to 10 h.m.; falls from 10 h.m. to 4 h.a; rises again from 4 h.a. to 10 h.a.; and falls from 10 h.a. till 4 h.m. These changes are graphically represented in Figure 4.
4h.m. The circle, P, P1, P2, P3, represents a vertical section of the earth at the equator, and the surrounding elliptical envelope the atmosphere. The arrows show the direction in which the wave form is moving. There are two depressions, and two compressions, and four sections, in which the barometer is either rising or falling, according as the waves are advancing or receding.
It is worthy of notice that the sun is followed by a depression, and not by a compression_thus showing that its effect on the atmosphere is to produce the former.
The greater depression reaches its minimum about 4 h.a., followed by the greater compression, which attains its maximum about 10 h.a.
We have now seen by theory, experiment, and observation, that? the general circulation is caused by a depression, followed by a compression moving westwards; and that these are found succeeding each other with great regularity within the tropics.
We are now, therefore, face to face with the inquiry, What is the origin of this depression ? The following compression and the succeeding smaller and secondary depression and compression
need not at present engage our attention, as they could easily be shown to result from the primary.
As we have found that this depression follows the apparent diurnal revolution of the sun from east to west, we naturally turn our eyes to that luminary as the exciting cause, and to the rotation of the earth as originating its westward motion. This being so, we are led to ask, How does the sun cause the barometer to fall ?
The sun is the acknowledged centre of the two great forces, heat and attraction; to which of these therefore must we look as the agent in effecting this depression, and first of all let us examine the agency of heat.
The action of heat as an agent in altering the pressure of the atmosphere has been proved to be almost, if not altogether, inappreciable by Sir John Herschel in his work on Meteorology. One or two remarks may, however, not be out of place. It is well known that, even within the tropics where the heat is most intense, the temperature of the higher regions of the atmosphere is below the freezing point. The heat must, therefore, chiefly affect the lower strata of the air. The particles of air in immediate contact with the earth becoming heated, give place to colder ones which descend, and are heated in their turn, and this operation goes on without disturbing the equilibrium of the atmosphere, or altering its pressure, very much in the same manner as the particles of water are observed to do in the common experiment of heating water containing oak sawdust in mechanical mixture. To produce the necessary dynamical effect, the air would require to expand in very large volumes, and that might be the case if it were heated directly by the rays of the sun. No doubt air, when confined in a vessel, expands with considerable force, but in the atmosphere the particles are perfectly free to move in any direction. Indeed, the extreme mobility of the air is one of its most striking characteristics. A simple experiment may serve to illustrate the action of heat. If a flask (containing air) be fastened to the open end of the tube of a siphon barometer, so that the outer air is completely excluded, it will be found that the barometrical pressure remains very nearly constant for all ordinary temperatures. If the heat applied be considerable, the mercury may rise, but no ordinary
degree of cold will have the least effect. That is to say, we may increase the original pressure by adding heat, but cannot diminish it by withdrawing heat. The action of heat then, if appreciable, must cause a rise in the barometer—an effect contrary to that observed.
As the intensity of gravity varies at different places on the earth's surface, this apparatus suggests itself as a measurer of that intensity.
Having now seen that heat is ineffectual in producing the depression, we may turn to consider the effect of attraction. Sir John Herschel, in the work before referred to, points out vapour tension as the probable effective cause, but I venture to express the opinion that seems to be wanting in these elements of regularity in its action required by the conditions, and I am the more convinced of its inadequacy, as I hope presently to be able to show that attraction explains clearly and fully all the phenomena.
Attraction may be defined as that mutual force which bodies exert, by reason of which they have a tendency to move towards each other. As all bodies near the surface of the earth tend to move or gravitate, as it as been called, towards the centre, the attractions must meet and neutralize each other there. The tendency towards the centre must, therefore, be greatest at the surface and diminish towards the centre, and the sum of the tendencies being all towards the centre, the particles there must be in a high state of compression, so that the centre of the earth must be the densest part, gradually diminishing in density towards the surface. This agrees with observed facts, as the average density of the bodies near the surface is found to be much less than the mean density of the earth.
These considerations and others may be proved in the following manner.