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significance of the drop of rain. We can well understand how it is that “the clouds drop fatness on the earth,” when we estimate the powers expended in their genesis. All the coal which could be raised by man from the earth in thousands of years, would not give out heat enough to produce by evaporation the earth's rain-supply for one single year! The sunwhose influence is often contrasted with that of the rainshower—is the agent in producing that shower as well as in pouring out his direct heat on the soil with such apparently contrasted effect.
The actual process of the production of rain has not yet been completely explained. We are, in fact, doubtful as to the true nature of clouds, fogs, and mist, and, therefore, it is intelligible that some difficulty should surround the explanation of a phenomenon of which these meteors are, so to speak, the parents.
It is generally supposed that clouds consist mainly of hollow vesicles of water, and not of minute drops. Yet meteorologists are far from being agreed on this point. On the one hand we have the evidence of De Saussure and Kratzenstein, who actually saw, or supposed they saw, the constituent vesicles of clouds and fogs. De Saussure, indeed, tells us how we may see the vesicles for ourselves. If a cup of coffee, or of water tinctured with Indian ink be placed in the sun, minute vesicles of various thickness will be seen to ascend from the surface of the liquid. He adds that those vesicles which rise differ so much in appearance from those which fall, that it is impossible to doubt that the former are hollow. Kämtz, also, made measurements of the vesicles of fogs in Central Germany and in Switzerland; and in his valuable work on Meteorology, gives a table and a figure, showing the law according to which the dimensions of the vesicles vary in the course of the year.
Despite this evidence, Sir John Herschel holds a contrary opinion. He points out that the observations of De Saussure and Kratzenstein may be readily referred to the effects of optical illusion. The strongest argument put forward by Kratzenstein is founded on the fact that rainbows are never formed on clouds or fogs, as they would be (according to the undulatory theory of light) if these meteors were composed of globules of water. Sir John Herschel, a higher authority on optical questions than either De Saussure or Kratzenstein, is of opinion, on the contrary, that it is possible a re-examination of the very difficult point in question would give a different account than that usually accepted.
Herschel points out the difficulty of understanding in what manner the condensation of true vapour should result in the formation of a hollow vesicle. Tyndall points out, on the
other hand, a difficulty depending on the state into which waterparticles at high elevations sometimes pass. “It is certain," he says, “that they possess, on or after precipitation, the power of building themselves into crystalline forms; they thus bring forces into play which we have hitherto been accustomed to regard as molecular, and which could not be ascribed to the aggregates necessary to form vesicles.”
In whatever state the particles of a cloud really exist, it is certain that the fall of rain depends on a process of increased condensation. The causes producing such condensation have been thus summed up by Professor Nichol:-
(1.) The cooling of clouds through the effect of radiation from them;
(2.) The mingling of vapours at different temperaturesmingling effected by the agency of the winds;
(3.) The rising of vapours towards colder strata of the atmosphere;
(4.) The increase of atmospheric density or pressure ;
And (5.) The accumulation and impinging of masses of vapour against some obstacle.
Singularly enough he omits the most important of all known agencies in the production of rain, viz. :
(6.) The transfer from the equator towards the poles of large masses of moisture-laden air by means of the upper S.I., or counter trade-winds.
I must note also that cause (4) is more than doubtful. Tyndall has shown that rarefaction is an efficient agent in producing the precipitation of vapour. By increase of pressure a larger quantity of moisture is, indeed, compressed within any given space; but, on the other hand, there is an increase of heat within the space which more than counterbalances the former in effect. “The heat developed,” says Tyndall, speaking of an experiment illustrating the effects of increased pres. sure, “is more than sufficient to preserve it" (the moisture added to a given space) “ in the state of vapour.”
It will be seen at once from the above imperfect enumeration of causes affecting the production of rain, that the phenomenon is no simple one. When we add the variety of circumstances affecting the action of different causes-as the latitude of the place, the elevation above the sea-level; the proximity of the sea ; the laws affecting the seasonal variations at the place; the prevailing winds; and the configuration of the surrounding surface, it will become evident that meteorologists may well be perplexed by the very complex set of agencies acting in the production of rain ; and so fail-as they have hitherto done-in interpreting any save the most general laws influencing the phenomenon.
Some of these general laws I now proceed to consider.
In the first place, it may be accepted as generally true that the amount of moisture present in the atmosphere is greatest near the equator, and diminishes towards the poles. With the sun's change of declination the zone of greatest moisture passes to the north or to the south of the equator, following the sun. The mean region, it is to be noted, is not absolutely coincident with the equator, but some four or five degrees north of that circle. It is easily intelligible that the hottest regions shouldbe, cæteris paribus, those over which the amount of moisture present in the atmosphere is greatest, since the heat vapourises the water over these regions. It may not seem, at first sight, quite so obvious that the same regions of greatest heat should also be those in which the rainfall should be in general heaviest. For it might appear that the same heat which produced the evaporation should maintain the water in the state of vapour. The fact, however, that aqueous vapour is lighter than air, operates to produce ascending currents over the region of evaporation, currents strengthened by the expansive effects of the heat. Accordingly, the vapour rises rapidly, and when it has thus risen, many circumstances operate to produce precipitation. First, the upper regions are rarer; secondly, they are colder; thirdly, radiation of heat takes place rapidly from the upper surface of clouds, brought here, as Tyndall expresses it, into the presence of pure space (dry air having scarcely any appreciable effect in checking radiation). The result is, that the uplifting of clouds under the sun's influence is followed regularly over the equatorial regions by the precipitation of heavy rainshowers. And coeteris paribus, the fall of rain decreases with distance from the equator of heat, though not so regularly as the amount of moisture decreases.
The next great law which presents itself to our consideration is this, that winds blowing towards the equator are, in general, dry winds, and winds blowing from the equator rainy. This law is the direct consequence of the former, but it is necessary, for several reasons, to present it as a separate law. There is an erroneous method of accounting for this law which is very commonly met with in works on meteorology. It is argued that as winds blowing towards the equator are carrying masses of air from colder to warmer regions, they are necessarily dry winds, since, if the air is saturated, or nearly so, at starting, it cannot be saturated when it has become warmer. And vice verså, winds blowing towards the poles are carrying masses of air to colder regions. The air accordingly grows colder, and if not far from being saturated at starting, cannot fail to become unable to keep its whole burden of moisture in the state of vapour, and must accordingly precipitate a portion as rain. This explanation is insufficient. It would, indeed, be just as reasonable to reverse the argument thus : & wind blowing towards the equator must bring rain; for as it brings cold air into warm regions, if the air in these regions is nearly saturated, the introduction of cold air must lead to the precipi. tation of a part of the moisture, and vice versa, a wind blowing towards the poles must be a dry, because it is a heat-bearing wind. The simple explanation of the law is, that winds blowing towards the equator are dry, becaase they are blowing from regions over which moisture is less, to regions over which moisture is more abundant, and vice versa. Of course we must superadd to this the facts mentioned above, because a moist wind blowing towards a heated region would not bring rain with it, and a comparatively dry wind, blowing towards a cold region, might bring rain. But it must not be forgotten that the main question to be considered is the relative moistness of the transported masses of air.
We meet with corresponding laws affecting the rain-producing powers of winds travelling over continents and oceans. A wind blowing over an ocean towards a continent brings rain to the continent, unless the heat over the latter exceeds slightly, or at the least, does not fall short of the heat over the neighbouring ocean. Such a wind is certain to bring rain to an elevated continental region not protected by a mountain barrier yet more elevated. On the other hand, a wind blowing over a continent towards the ocean in general brings no rain.
Lakes, marshes, and rivers, act in a small way a similar part towards the adjoining lands as oceans towards neighbouring continents.
There are circumstances also to be considered as affecting the rainfall in a different manner, viz., not by supplying a greater or less amount of moisture to the atmosphere, but by affecting the power of the atmosphere to keep the moisture it supports in the vaporous state. Such are the contour and elevation of a country, the nature of its soil, the quantity of forest land, or, wanting this, the relative abundance or paucity of trees, and so on.
A moist and warm current of air impinging on a mountain range, or even on any well-defined rising slope, so as to be carried with sufficient suddenness to colder and rarer regions, is compelled to part with a large portion of its moisture in the form of rain; and conversely a wind which has passed over a mountain range or an elevated plateau, and descends' to a lower region, appears as a dry wind, unless that region is one over which a continual process of evaporation sufficient to maintain the air nearly in a state of saturation is going on. In this latter case the effects of the descending wind will vary with circumstances. It will in general appear as a dry wind, but may produce local showers, since it may act, through the sudden addition of cold air, the part of a condenser.
Forests are great generators of rain. This is mainly due to the peculiar radiative power of trees and vegetables. The soil, where it is covered with vegetation, receives no heat directly from the sun, and but little through contact with the heated air. It may seem like a confusion of cause and effect to speak of vegetation-covered countries as rain-generators, since abundant rain is so important a requisite for the abundant growth of vegetables. This is, however, a case in which cause and effect are interchangeable. Rain encourages vegetation, and vegetation in turn aids in producing a state of the superincumbent atmosphere, which encourages the precipitation of rain. The result is that, apart from external agencies, regions covered with abundant vegetation, and especially with high trees, present year after year, and century after century, a ranker and yet ranker luxuriance of vegetable growth.
On the contrary, arid regions prevent, by their very aridity, and consequently by the intense heat of the soil and superincumbent air, the downfall of the showers which would nourish vegetation. The result is, that even when the soil itself is favourable, it is exceedingly difficult to convert an arid into a vegetation-covered district, the want of moisture being destructive to trees planted in such soils with the object of encouraging rain-falls. The process of change must be a gradual one. On the other hand, the improvement of a region over which rain falls too heavily through overabundant vegetation is a comparatively simple process, a judicious system of clearing invariably leading to the desired result.
The influence of the seasons remains yet to be mentioned among the circumstances affecting the distribution of rain over the earth's surface. The influence of the seasons is different in different zones of the earth's surface. Under the tropics the laws affecting the fall of rain are much more regular than elsewhere. On the ocean we have clear skies where the tradewinds are blowing steadily, and heavy rain falls by day over the intermediate zone of calms; but on the land we have regular dry and wet seasons within the tropics. There is, properly speaking, no winter or summer; but applying these terms to the periods at which winter or summer prevails in the temperate zones of either hemisphere, we may say that the sky is serene in the winter, becomes moist in spring, and the rainy season sets in when the sun is near the zenith. Where there is a considerable interval between the sun's passages of the zenith, as in places not very far from the equator, there are two wet seasons, both occurring in summer. In countries in which monsoons
VOL. XII.-NO. V.