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One, which has perhaps received the most support, is that the sudden condensation of vapour in some one locality causes a diminution of barometric pressure, and a consequent inrush of air from adjacent localities; that the air so rushing in tends to form a whirl such as-on a small scale-is often seen in a fixed washhand basin with a plug hole in the bottom; that the direction of this whirl is determined by the rotation of the earth, which in northern latitudes deflects all moving bodies to the right; that as this air reaches the centre it is forced up, losing its vapour, and with it much of its elastic force, thus continuing the low pressure in the centre, the influx of fresh air, and the heavy rain caused by the condensation of vapour. The advocates of this hypothesis used to suppose further that the main influx in these latitudes was from the east, and that thus by reason of the partial vacuum on the eastern side the whirl had a continual tendency to crawl eastward. More recently this view has been modified, and Mr. Ley, who has always strenuously supported this condensation hypothesis, now conceives that the direction of a cyclone's advance is in great measure dependent upon the direction of the great upper currents.

Against this hypothesis may be placed the utter want of any reasonable explanation of the first sudden and excessive condensation which is assumed as giving rise to the influx. On the other hand, the statement as to the direction of the whirl is more than an hypothesis; it is a theory, based on legitimate mathematical calculations, and is formally enunciated thus: If a body moves in any direction upon the ' earth's surface, there is a deflecting force arising from the ' earth's rotation which deflects it to the right in the northern ' hemisphere, but to the left in the southern.' That this theorem is legitimately proved on the data used by Ferrel, there can be no doubt; so also is another theorem established by the same mathematician: that there is little or no air at all within the Arctic circle; the whole of the air being thrown off by the centrifugal tendency.* But we know that in this latter case the mathematical proof is faulty from the omission of some of the conditions of the problem; and we are not prepared to give any frank adherence to the former statement, holding that the truth of the analysis can be established only by observation and experiment.

Mr. Ley seems to appeal to these to support or illustrate

*William Ferrel's 'Meteorological Researches,' part i. page 41.

it. There is, he says, a lateral pressure tending to throw a railway train off the line to the right, which may be estimated, in this country, at about one four-thousandth part of its weight. If this were so, the right-hand rail of the permanent way of any great railway would bear witness to a greater pressure or more intense friction. We are assured by the superintendent of one of our greatest railway systems that no such difference is observed. Mr. Ley says again that the surface of a water current will not be level, but that in the northern hemisphere the water will be 'higher on the right bank than on the left '-that is, that there must be a greater pressure on the right bank of a river than on the left; that the right bank will be more subject to erosion; that, in fact, every river, great or small, in the northern hemisphere has a tendency to eat its way towards the right. Has this tendency ever been observed? We have the historical and geological record of the Nile, of the Volga, the Lena, the Yenisei, the Danube, the Rhone, the Thames, the St. Lawrence, the Mississippi, and many others for thousands, and perhaps hundreds of thousands, of years. Have they been steadily wearing away their right banks-crawling to the right? Neither history, nor geography, nor geology, tells any such tale. We refuse, therefore, to accept the mathematical demonstration as having any bearing on the meteorological problem, or as anything more than a mathematical exercise. The deflecting force which is mathematically proved to exist is absorbed in other work, probably molecular, which Ferrel neglected to examine.

Another hypothesis, to which many incline who feel the force of these objections to this, is that cyclones are formed in the great atmospheric currents like whirls in a mill race; and, more definitely, that the cyclones of the North Atlantic which reach our shores are formed off the banks of Newfoundland, where the cold wind from the north drives into the warm moist wind from the south-west, overlying the Gulf Stream. That such a clashing of currents will form a series of whirls is open to observation wherever one stream discharges itself into another; and that they will turn in the direction in which this other stream is going. It is equally open to observation that in such whirls there is a deep hole in the centre, and that the water is heaped up round the outer circumference. So, also, if a whirl is caused mechanically, as by means of a teaspoon in a teacup; when,

if the cup is nearly full, it is no difficult matter to make the tea heap itself over the edge of the cup into the saucer.

The motion in its several details may be experimentally shown in a glass cylinder partly filled with water in which are particles of a slightly greater specific weight. A cylindrical tumbler, with a little grated nutmeg, will do very well. When this water is stirred briskly, and sinks in the middle to heap itself up at the outside, we can follow the motion of the particles, and see that in the centre they go up; at the sides of the glass they go down; at the bottom they seek the centre; at the top they fly from it. As the motion stops, the particles, settling to the bottom, will be seen to range themselves in a sharp-pointed conical heap in the centre. And thus, when a whirl is formed by an intrusion of a cold stream of air from Arctic seas into the warm moist air of the Gulf Stream, an immediate condensation must take place; this whirl gives rise to an ascending current in the centre, which increases the condensation and all the effect thereof, as described by Mr. Ley and other supporters of the former hypothesis. It may thus be possible that the cyclones are due primarily to the clashing of two currents, and secondarily to the profuse condensation and precipitation so occasioned; each cause assisting and intensifying the effect of the other. We can, at any rate, see that several, if not all, of the concomitants of a cyclone could be produced in this manner: the low barometer, the ascending current, the heavy rain in the centre, and the high barometer outside, more noticeable in front, where the air is heaped up by the advancing meteor in a manner that may be compared to the bow-wave in front of one of our older ironclads. We say that this may possibly be; we certainly do not say that it is so satisfactory proof is wanting. During the winter, at any rate, cold northerly winds have a continual tendency to clash with the warm westerly winds; yet cyclones, though very frequent, are only occasional. If they are caused as has been suggested, it is not easy to understand why they are not much more frequent than they are; why they differ so enormously in their intensity, or why observation shows nothing to enable us to forecast their occurrence. As these questions cannot be answered, it is probable-perhaps we should say it is certain that there are other agencies at work of which as yet we are ignorant.

But it is not only of the determining causes of cyclones that we are ignorant. We have no real knowledge of any

one fact connected with them. Our normal winter weather is dependent on a cyclonic wind-system round an area of low barometer, stationary in the neighbourhood of Iceland; but we do not know what causes this low barometer in that particular locality. The wind-system of the North Atlantic corresponds to that of an anti-cyclone round an area of high pressure near the Azores; but we do not know what causes this high barometer in that particular locality. No doubt writers on physical geography or physiography glibly expound the phenomenon, and convince many of their readers; it may be questioned whether they really convince themselves. No physiographer or meteorologist has yet offered any satisfactory explanation of the formation or the persistence of an anti-cyclone, such as, with its easterly winds and intense cold, brought a Canadian climate, without Canadian resources, to this country during last February; or such as gave us the long drought and heat in the summer of 1893.

A direction in which many minds have searched, not perhaps so much for a cause as for an indication, is in a cycle of recurring years. Nineteen years, the lunar cycle; twenty-eight years, the solar cycle; and other periods, have been suggested and examined, but absolutely without result. Years of sun-spot intensity, the conjunction, opposition, or quadrature of planets-more particularly of Venus, Mars, and Jupiter-have been considered, but in vain; and it is almost needless to say that the phases of the moon, her erossing the equator, her turning from extreme north or extreme south declination, have furnished grounds for much futile speculation. The established opinion amongst meteorologists at the present day is that English weather has no cycle, and has no appointed relation to the position of the moon or of any of the planets. As to the sun-spot intensity, they would probably speak more doubtfully; but if there is any relation it has not yet been discovered. Mr. Abercromby believes that he detects signs of some real relation between the extent of spots on the sun's surface and the rainfall curve at Rothesay,' but thinks that any attempt to forecast rainfall by means of the sun-spot curve would be most unsatisfactory. That changes of weather are in some way dependent on magnetism is possible: magnetism and its sister, electricity, have still many secrets which future ages may reveal. The perfect uniformity of the direction of the whirl in north and south latitudes respectively may, for aught we know to the contrary, be

guided by magnetical influences, but, for aught we know to the contrary, it may be entirely independent of them; at present we only know that no relation has been traced between aërial and magnetic storms.

For anything further we are still groping in the dark; our knowledge is limited to the few hours over which our telegraphic vision can extend; our deductions are, for the most part, empirical: not entirely so, however, and no writers of the present day have done more to raise them beyond pure empiricism than Mr. Abercromby and Mr. Ley. That certain cloud or sky effects indicate coming weather is familiarly known, and has been familiarly acted on for thousands of years. Hunters, shepherds, and field labourers have always been known as capable exponents of weather signs, without, however, any understanding of the meaning of them. This meaning Mr. Abercromby has expressed in a singularly interesting manner, and Mr. Ley has done much towards establishing the interpretation of clouds on a scientific basis. Mr. Ley has been known for many years as a careful observer of the phenomena of clouds as a key to the weather of the immediate future. His opportunities have been those of a rector of a country parish, not greater than of many other dwellers in the country, and it would be an exaggeration to say that he has attained a closer knowledge than many of his humble forerunners; the difference is that alone among them, in this country, he has not only learned the message which the clouds tell as to the proximate weather-tell, as he puts it, whether to take an umbrella or a stick when starting for an afternoon walk-but has also learned the significance of the message: why, in fact, clouds of different appearances or behaviour forebode different types of weather. In this lies the exceptional charm of Mr. Ley's book. As an exponent of the meaning of clouds, it will well repay a careful and repeated study, carried out not only in the closet, but in the open country, on the road or field, moor or mountain, on land or sea it not only tells what the clouds indicate, but shows in a manner generally convincing why they indicate it.

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It is very easy to understand what clouds are; they are things which we all have seen and know. Aggregates of particles floating in the air, generally, but not necessarily, 'particles of water or ice,' is Mr. Ley's definition; it seems to require the limitation' floating in the air at some consider'able distance above us,' for no meteorologist wants to give the name 'cloud' to the volume of black smoke emerging VOL. CLXXXI. NO. CCCLXXII.

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