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of vapour so introduced is constantly undergoing precipitation, and is thus continually being withdrawn from the total mass, leaving behind it, however, to accumulate, the dry air which accompanied it. Thus, if we regard the total barometric pressure as sub-divided into that of the dry air, and of the aqueous vapour, and denote the former by P, the latter by V, we see that the dry pressure is diminished in the hot, and increased in the cold hemisphere, without any countervailing action, while V is in process of increase from below by evaporation, and of diminution from above by overflow, in the former and vice versa in the latter. If, then, the observed barometric pressure at every point in either hemisphere be analysed by calculation into its two constituents, by taking account of the hygrometric state of the atmosphere, and subtracting from the total pressure P+V the portion V due to the amount of vapour present, the remainders ought to exhibit, as a general result, an excess of dry pressure P in the winter hemisphere over that in the summer.
"(166.) So far as observation has hitherto gone, this result is perfectly corroborated, though, unfortunately, there are not yet accumulated sufficiently numerous and extensive series of observations in which the effects of the aqueous pressure can be duly separated from the dry. As examples, we shall select the series for the Indian stations, Calcutta, Benares, Seringapatam, and Poonah, calculated by Dove from the observations of Prinsep, Sparmann, and Colonel Sykes, as compared with that at Apenrade from those of Neuber, and with the results obtained in the meteorological observatories of Prague, Toronto, and Hobart.
P.-Pressure of Dry Air.
V.-Pressure of Vapour.
Max. in. Min. in. Difference. Max. in. Min. in. Difference.
These differences are large quantities; but we see that as the maxima of P correspond in point of time with the minima of V, it is only their differences which constitute the total or observed annual fluctuation of barometric pressure.
(170.) The great length of time in which the efficient causes are acting in one direction, to produce the annual oscillation in question, admits of a very considerable fraction of the atmosphere to be transferred from hemisphere to hemisphere, and to allow a range in the values of P, for instance, to the large extent (as we have seen in the case of Benares), of nearly an inch and a quarter of mercury, partially neutralised by a fluctuation of more than half-an-inch of aqueous vapour. Thus the effects are brought out into prominence, in both elements, by the long-continued action of the causes; and thus, by the study in the first instance of the annual oscillation, we are led to any easy understanding of the perfectly analogous phenomena in the diurnal oscillations (or, as they have sometimes, though, in fact, improperly, been called, atmospheric tides '), which have a good deal perplexed meteorologists, but whose analysis into what we have for convenience called wet and dry pressure, has happily been suggested by M. Dove as affording a rational explanation.
(171.) To simplify our conception of the diurnal oscillation, we will suppose the sun to have no declination, but to remain constantly vertical over the equator. The surface of the globe will then be divided into a day and a night hemisphere, separated by a great circle passing through the poles, coincident with the momentary horizon, and revolving with the sun from east to west in twenty-four hours. The contrast of the two hemispheres, both in respect of heat and evaporation, in this case will evidently be much greater than in that of Art. 165, and therefore the dynamical cause, the motive force, transferring both air and vapour from the one to the other, will be much greater. But, on the other hand, much less time is afforded for this power to work out its full effect, and long before this can be accomplished for any locality, the circumstances are reversed, and a contrary action commences. The causes, then, and the mode of their agency, are perfectly analogous, in the production whether of the annual or diurnal oscillation; but in the former the feebler acting cause is aided by the very much greater length of the period; in the latter its superior intensity is in great measure neutralised by the frequency of its reversal.
"(172.) It ought to be observed, that the oscillations in question are only in appearance analogous to those of the oceanic tides. In the latter the tide wave is merely a circulating form without any bodily transfer to any great distance. The sun's heating action is not one which, destroying a portion of its gravity, tends to alter its form of equilibrium, but one which, leaving its gravity unaltered, tends to throw its strata by their dilatation, and by the introduction of vapour from below, into forms incompatible with equilibrium, and therefore necessarily productive of lateral movements. When anemometry is further perfected, we may expect to trace the influence of this chain of
causation into a morning and evening tendency of the wind (on a long average of observation), to draw towards the time of sunrise and sunset, to compensate the overflow from off the heated hemisphere which takes place aloft in a contrary direction."
I have quoted these passages in full, because they stand for what, 40 years ago, was regarded as proved. Two assumptionswhich, however, were not then thought to be assumptions-underlay the theory one, that the air expanded by the heat of the sun flows upwards and outwards from the light hemisphere to the dark; the other, that what is called the " pressure of dry air" can be obtained by subtracting the vapour pressure, as determined by the hygrometer, from the total barometric pressure.
Now the diurnal variation of wind-direction is not quite in accordance with Sir John Herschel's idea. Taking Kimberley as an example, since we have here no very decisive prevailing winddirection, we ought to be in a very good position to detect any drawing of the wind if it exist. But what really happens is this: Though there is no decisive prevailing direction when the whole year is considered, there is some tendency to a seasonal prevailing direction, which, in the course of the year, backs completely round the compass. Thus in the spring the prevailing direction is S. W., in the summer nearly N., and in the winter about S. E. But superimposed on this is a strong diurnal rotational veering of the vane evident at all seasons. (Fig. 5.) In this diurnal scheme the normal direction is N.E. at sunrise, N.W. at noon, S. W. at sunset, and S. E. at midnight. Now, all over the world this same diurnal rotation may be with more or less trouble separated from the prevailing directions. It has been demonstrated for several stations in India. I have shewn that it is quite obvious, even at East London, where the prevailing directions (with high velocities) are almost exclusively N.E. or S.W.-i.e., up or down the coast. (Fig. 6.) If we separate this rotational veering into its mechanical components at right-angles N. and E., we find that the E. component varies directly with the temperature of the air, and therefore indicates some such relation to the prevailing direction, as the diurnal temperature variation does to the annual. Also, if we combine velocity and direction, there is some sort of obscure agreement between the E. component wind movement and the barometric phases, which is chiefly noticeable in the second of the periodic terms in the sine series representing each. More than this cannot be said. Now this rotational veering of the vane implies, as F. Chambers has pointed out, not a movement inwards of the air to replace that which is supposed to have flown off above, but an outward movement, that is, not a convection current, but an anticonvection current. Also the diurnal velocity of the wind is not greatest at 4 p.m., when the barometer is lowest, as it perhaps might be if there were a partial vacuum to fill, but is greatest some hours earlier -earlier, indeed, in some months than the epochs of maximum air-temperature. It cannot be said, therefore, that the supposition
Coming now to the P-V theory, we raise the dust of ancient polemics. Is the spring of the air the same as its gravity? Is the total pressure of a mechanical mixture of gases equal to the sum
of diurnal air movements analogous to the annual ones is sufficiently substantiated by facts to explain the diurnal variations of the barometer.
COMPONENT WIND DIRECTIONS AT KIMBERLEY.
The Times are for the Hours ending -.
whether the remaining pressure of dry air" may be shewn to vary throughout the day in some manner depending upon the temperature? The answer to the first part of the question is that it is not legitimate. It has been shewn more than once that the pressure on the barometer of the total quantity of aqueous vapour in the air is only perhaps at quarter of what is called the vapour tension. Moreover, it varies considerably from place to place over areas throughout which the total barometric pressure may not be much affected. And even if we
of the pressures of the separate gases? Is the pressure of the aqueous vapour in the air the same as it would be in an atmosphere of aqueous vapour independently existing? And, if all these things are provable say under the receiver of an air pump, are they necessarily true at large? To all these relatively important side issues it is not necessary to-day to volunteer a direct answer. We may confine ourselves to the simple question (1) whether it is legitimate to subtract the cbserved vapour tension from the barometric pressure; and (2), if so,