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In the first place, let us notice how little influence the rate of a cyclone's motion has on the velocity of the wind. All that we know for certain about the influence of the motion of a cyclone is that a high rate increases the general intensity of the wind and weather everywhere, but that it does not prevent the centre from being calm, or the wind from being light on any side where the gradients are slight.

In squalls the independence of the velocity of wind to that of the squall as a whole is still more curious. The latter may be travelling, perhaps, only twenty miles an hour, but the first blast may come at the rate of sixty miles an hour. This fact we must consider as due to an impulse being propagated which induces wind of such a velocity, but not as due to wind, or a gust, moving solidly over the earth's surface. Such an impulse is found in the trough of a cyclone or V-depression.

We have not thought it necessary to give the general principles of the dependence of wind-circulation on the earth's rotation, as that may be found in any text-book of physical science. The modification of Halley's old theory of north-east and south-west winds, which has been proposed by Professor Ferrel, has been universally adopted all over Europe and the United States. The theory is hardly known in England, and is too mathematical for this work. No doubt the earth's rotation is the real cause of the general direction of circulation in cyclones of either hemisphere, but what we cannot explain is the inclination of wind to the isobars. Theoretically, any small difference of temperature should set up a wind from the cold to the hot area; but we have seen already,

and shall see still more in our next chapter on Heat and Cold, that differences of temperature even over large areas have wonderfully little influence on wind. The most that local differences of heat and cold do is to set up local breezes, such as land and sea, or valley winds. Then, theoretically, this cold wind should flow nearly straight toward the hot area, only a little deflected to the right or left, according to circumstances, by the earth's rotation. In like manner, any difference of pressure, from the high to the low barometer, however caused, should draw wind nearly straight. But, in our chapter on Diurnal Weather, we shall find some land and sea breezes which blow nearly parallel to the coast-line.

On the other hand, if we look at a cyclone purely as a circulating mass of air, the wind should be parallel to the isobar, perhaps even a little outcurved from centrifugal force. Now, in practice the wind is always incurved, and the depression of a cyclone is certainly not caused by centrifugal force. The fiercest wind which ever blew would only depress the barometer a few hundredths of an inch, instead of which we find depressions of two inches and more with no wind over fifty miles an hour. This, of course, is on the supposition that whirling air acts like a fluid.

The idea has been suggested that the friction of the wind on the earth's surface is the cause of the incurvature, and that without friction the wind would be parallel to the isobars, as we find it at the level of the lowest cloudlayers. It is extremely probable that this is at least partially true, for several experiments can be devised with whirling water, in which friction of small particles

on the bottom does cause them to be collected in the centre, instead of being thrown out to the edges of the vessel.

RELATION OF FORCE TO VELOCITY.

Lastly, we may say a few words about the relation of force to velocity. The velocity of wind is a real quantity, which is perhaps capable of measurement in the abstract, though we are at present far from being able to gauge it accurately. But it is quite certain that there is no such thing as an absolute force which corresponds to a given velocity. According to the theory of stream-lines, when even an inelastic fluid meets an obstacle, if the angles of the obstruction do not break the continuity of the fluid SO as to form eddies or vortices, the same amount of pressure which is imposed on the body by the first deflection of the fluid is given back again as the streamlines of the fluid close up behind the obstruction. For instance, if a ship is lying at anchor in a current, the same amount of strain which the current causes on her cable when forced asunder by the bows, is given back when the current closes in behind her; so that the total pressure which she experiences is only that due to the friction of the water on her skin. This is, of course, on the supposition that her lines are so easy that they do not break the stream-lines so as to form little eddies or vortices.

Now, the same thing holds with wind. If we put up two square plates of different sizes, face to the wind, the pressure on each is not proportional to the area, while in

light breezes neither will record anything. The reason is that, in light wind, a thin mobile fluid like air can glide round even the sharp angles of a square without forming eddies, and as there is no vacuum formed behind the plate, there is no pressure recorded. In higher winds, the stream-lines are broken, and every shape and every sized plate of the same shape form a different series of eddies round the rim of the obstacle. Then the amount of rarification behind the various plates is neither identical nor proportional, and therefore every shape and size of anemometer indicates discordantly at every different velocity.

From all this it follows that, though we might say that the pressure on a board one foot square was twenty pounds, and might compare this force with that on another board of the same size and mounting, we should not be justified in saying that the force of the wind was twenty pounds per square foot in the abstract, because a board ten feet square, even if of the same shape, would have given a different number.

CHAPTER VII.

HEAT AND COLD.

IN this chapter we purpose to go a little more into the details of the manner in which changes of temperature are produced. What are the causes of burning heats and hard frosts; why is the same day of the month hot in one year and cold in another; why at the same season do hot and cold days follow one another without any apparent sequence; and why is England sometimes warmer than France, though the latter is nearer the equator?

All these questions we propose to answer, and to point out how easily they can be explained by means of synoptic charts. The difficulties of getting rid of annual and diurnal variations have tempted many meteorologists still to adhere to the old method of averages, which can only lead to unsatisfactory, if not to delusive, results.

DIURNAL ISOTHERMS.

The question we have to solve is this. We know that the sun is the principal source of all heat, and, if nothing disturbed his rays, there would be a regular diminution

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