/// that if the whole body of air contained between the latitudes D D, and GG, be set in motion from the south towards the north, an observer in the latitude D D, will receive the wind from E E, nearly as S.; that from F FI more as SW.; and that from G G11, nearly as W. Similarly, if we take points dd, d d ee, e, em f f f f 9 9 9 9 /// /// in the southern hemisphere, such that those in the latitude g g, are the most northerly, and those in d d, the most southerly, we shall see that if the air lying between d d, and gg, be set in motion from the north towards the south, an observer at d d11, will receive the wind from e e,,, nearly as N.; that from f f,,, more as NW.; and that from g g.,, nearly as W. /// A west wind, in either hemisphere, will interfere with fresh equatorial currents, and bring them to a state of relative rest. If the tendency of the air to flow towards the pole continue, the phenomena above described will be repeated until new polar currents change the west wind in the northern hemisphere, through NW. towards N., and in the southern through SW. towards S. This gives For the northern hemisphere the change S., SW., W., NW.; N. For the southern hemisphere the change N., NW., W., SW., S. From the above considerations we derive the following laws: A. In the northern hemisphere, when polar and equatorial currents succeed each other, the wind veers in general in the direction S., W., N., E., S., round the com pass. Exceptions to this rule are more common between S. and W. and between N. and E., than between W. and N. or between E. and S. B. In the southern hemisphere, when polar and equatorial currents succeed each other, the wind veers in general in the direction S., E., N., W., S., round the compass. Exceptions to this rule are more common between N. and W. and between S. and E., than between W. and S. or between E. and N. This is the phenomenon which I have termed THE LAW OF GYRATION. The Trade-winds and the Monsoons may be considered as special cases of this law. In the torrid zone, the only atmospherical current which is felt at the surface of the ground is the polar current; and consequently a complete shift of the wind round the compass can never be observed. The deviation of the vane which is observed is proportional to the distance of the point of observation from the initial point of the current, and is only modified at the different seasons, to a certain extent, by the variation of the position of this initial point. The most obvious instance in proof of this statement is the NE. Tradewind of the northern hemisphere. We find, however, that, owing to the preponderance of land in the northern hemisphere, the zone of maximum temperature of the globe, at which the heated air rises, does not coincide with the equator, but falls in the northern hemisphere. This zone is the district towards which the polar currents of both hemispheres flow, between the tropics; and consequently the SE. Trade-wind crosses the line into the northern hemisphere. This wind, in the earlier portion of its course, moves over points whose velocity of rotation from W. to E. is continually increasing: on its entrance into the northern hemisphere it moves over points whose velocity of rotation is continually decreasing, and consequently the polar current is converted into an equatorial current. The wind, which had previously assumed a more easterly direction, shifts through SE. to S., and may finally become south-westerly. The configuration of the land about the Indian Ocean produces this phenomenon on the most extensive scale. In this district, the SE. Trade-wind enters the northern hemisphere as the SW. Monsoon during our summer, and the NE. Tradewind enters the southern hemisphere as the NW. Monsoon during our winter. The conversion of an equatorial into a polar current can only take place in consequence of the equatorial current passing over the pole, so that its deviation, which in the earlier part of its course was westerly, becomes subsequently easterly. We are as yet without observations to control our conclusions relative to this phenomenon. If the air, which rests on any parallel of latitude, could be instantaneously transferred to another parallel, we should see the results of the difference in the velocity of rotation between this portion of the earth's surface, and the air which has been placed in contact with it, developed in their full extent. This is, however, never realised, as, in the passage of the air from one parallel to another, the surface of the ground with which it is in contact has a tendency to impart to the air its own velocity of rotation by means of friction. In the neighbourhood of the equator, the parallels of latitude increase in diameter less rapidly than in higher latitudes; and inasmuch as in the neighbourhood of the heated belt, from which the air rises, the velocity of the wind is diminished, the tendency of the earth's surface to impart its motion to the air with which it is in contact has a greater effect than elsewhere. Hence we 1. can easily see why the NE. Trade-wind has a less easterly direction at its interior boundary, than in the centre of the Trade-wind district. In the region of the Calms, the velocity of rotation of the air is the same as that of the surface of the ground; and as the change in the velocity of the air which produces the calm cannot be a sudden one, it must take place according to the principles just explained. This reasoning does not hold for the upper counter Trade-wind (the return Trade), which preserves its original equatorial velocity of rotation unchanged, up to the time that it descends to the surface of the ground. In this case, as it is not in contact with the earth, its velocity cannot be modified by the earth, and it is easy to see that the mutual friction of two atmospherical currents is less than the friction of air against water, and much less than its friction against dry land. From considerations like these, we see why the Trade-winds should assume a more definite form on the smooth and open surface of the sea than in the interior of continents, and why the counter Trade-wind should reach the earth with a considerable westerly deflection, while the region of Calms is bounded by currents which blow in directions nearly diametrically opposite to each other. The preceding consideration is totally independent of the manner in which we conceive the motion of the air between the latitudes in question to have arisen, whether simultaneously at all points on the same meridian, or successively by suction or impulsion. It is also immaterial whether the currents which arise in the north and the south are directly opposed to each other, or more or less inclined towards each other and towards the meridian. The influence of the velocity of the wind on these appearances will be easily understood from the preceding explanation. If the air move slowly, the surface over which it is passing will impart to it more of its own velocity of rotation than if it moved more rapidly. Hence a greater velocity of the wind will produce a greater deviation of the vane than a smaller velocity. This shows us that a change in velocity of the wind produces a deviation of the vane. In the foregoing investigation, it has been shown that in our latitudes a northerly current becomes more easterly the longer it lasts, and therefore that a NE. wind is a N. wind which has come from higher latitudes than the wind which reaches us as a N. wind; and similarly, that a SW. wind is a S. wind which has come from lower latitudes than our S. wind. Hence we shall be prepared to admit that a rotation of the vane can be an indication of the existence of a permanent current. The perpetual disregard of this fact is the chief cause of the confusion which exists, and continually appears afresh in the consideration of questions connected with the theory of the wind. The essential difference between the rotation of the vane produced by an ordinary atmospherical current, and that produced either by a centripetal motion or by a whirling motion with advancing centre, is, that in the first case, the rotation is always in the same direction, while in the second, it is opposite on the opposite sides of the path of the current. If, in the northern hemisphere, we term the rotation S. W. N. E., as is usual, 'with the sun,' or direct, and the rotation in the direction S. E. N. W., 'against the sun,' or retrograde, and in the southern hemisphere the rotation S. E. N. W., 'with the sun,' and that S. W. N. E., 'against the sun,' * we find that * Contrary to nautical usage. The expression 'with the sun' originated in the northern hemisphere, beyond the tropic, where one looks at the sun rising in the east, and going from left to right across the south meridian to the west. This, of course, should be reversed in southern latitudes generally; |