Page:Physical Geography of the Sea and its Meteorology.djvu/446

420 mastered by opposing forces, always require the wind, when not blowing round in spirals and a whirl, to haul in the southern hemisphere. Now, paradoxical as it may at first seem, it is also the forces of diurnal rotation that give that same wind, when it is blowing round in spirals, its first impulse to march round in the contrary direction, or (§ 786) with the hands of a watch; but this is as it should be—it hauls one way, and marches the other. After passing a, and each of the other stations, a a, the rush of wind is sufficient, let us suppose, to create a whirl. The wind at a a a, continuing on with a circular motion, is represented thenceforward in its course by the curved arrows a e, a e'. The wind coming from the east and the west has no direct impulse from diurnal rotation, but the wind on either side of it has, and hence the prime vertical wind is carried around with the rest. If, now, we imagine the disc C to be put in motion, and the storm to become a travelling one, we shall have to consider the composition and resolution of other forces also, such as those of traction, aberration, and the like, before we can resolve the whirlwind.

789. Bernouilli's formula.—But the cyclonologists do not locate their storms in such high latitudes as the parallels of Cape Horn. Hence we might safely infer, one would suppose, that in high southern latitudes a north wind has a tendency to incline to the ^westward and a south wind to the eastward; and the cause of this tendency is in operation, whether the place of low barometer be a disc or an oblong, for it is in obedience to the trade-wind law, as expounded by Halley, that it so operates; and it will also be the case whether the wind be caused by an influx into the place of low, or the efflux from the place of high barometer; or, as is generally the case, by both together. If the distance between the place of high and low barometer were always the same, then a given difference of barometric pressure would always be followed by a wind of the same force of velocity. By expanding Bernouilli's formula for the velocity of gas jets under given pressures, Sir John F. W. Herschel has computed the velocity and the force with which currents of air or winds would issue under certain differences of barometric pressure. Under the most favourable conditions, i. e., when the places of high and of low barometer are in immediate juxtaposition, as on the inside and outside of an air-pump, an effective difference of 0.006 inch