ADMIRAL FITZROY'S WEATHER-BOOK.*
IT is probable that Admiral Fitzroy knows more about the weather than any other man in England. As a scientific meteoro- ologist he may have his equals, or possibly his superiors ; but, there is no one who has devoted himself so energetically to the intelligent observation of phenomena, or has striven so success- fully and systematically to utilize the results of such observation for the purpose of forecasting probable changes of weather. Meteorology has not yet won a place in the milks of the exact sciences ; and it is not to be wondered at if the Admiral's pre- dictions turn out occasionally not to be justified by the event. We have, however, no doubt, not only that the system of meteorological observation which he has instituted has led to practical results of the highest importance, but also that he has succeeded in obtaining a glimpse of certain real meteorological laws, from the further study of which a considerable extension of our knowledge on the subject may fairly be expected to accrue. Such being our opinion of Admiral Fitzroy as an authority on meteorological matters, we need scarcely say that we were prepared to extend an eager welcome to the volume now before us, and it has been a real disappointment to us to find that, in many respects, it fails to satisfy our not unreasonable expectations. That the Admiral's knowledge of his subject is real and substantial, we do not for a moment doubt ; but we cannot avoid the conclusion that he does not possess the faculty of communicating what he knows in an effective and intelligible manner. Though lie tells us that his book is "designed for many, rather than for few," professing that, " under so plain a title neither abstruse problems nor intricate difficulties should be found," the fact is that his expositions of the leading princi- ples of meteorology are, for the most part, conveyed in language so involved and obscure, that no one who does not already possess considerable acquaintance with the subject will have much chance of extracting from them any definite or tangible in- formation. Nothing would be easier than to fill the whole space at our disposal with extracts confirmatory of our opinion on this point. We prefer, however, the far pleasanter task of directing the reader's attention to some of the items of valuable informa- tion which, however, disguised by imperfect expression, the Admiral's volume does undoubtedly contain.
Everybody is aware that the barometer is the most trustworthy indicator of weather changes that we possess ; but few, probably, know how difficult it is to interpret its indications with any- thing like accuracy. On this point Admiral Fitzroy gives us much interesting and valuable information. The words "rain," " fair," " set fair," and so on, which commonly appear on a barometer, are of but little use or importance ; for indications of
• The Weather-book. A Mutual of Premical Meteorology. By limnadmiral Fitzroy. Longman and Co. approaching change are shown much less by the actual height of the mercury than by its rise or fall. It must never be forgotten that the barometer indicates, not the weather that is now present, but that which is to come. The longer the time that elapses between the occurrence of the sign and the change which it foretells, the longer such altered weather will last; and the shorter the time between the warning and the change, the shorter will be the duration of the predicted weather. Hence it follows that rapid oscillation of the mercury always indicates unsettled weather. In order to interpret satisfactorily the indications of the barometer, those of the thermometer must be simultaneously observed, for temperature is materially influenced by the direc- tion of the prevailing or coming wind. The same causes which induce the barometer to rise generally cause the thermometer to fall. Thus, a rising barometer, with a falling thermometer, indi- cates less wind, less wet, and a polar wind.; while a falling barometer, with a rising thermometer, indicates more wind, more wet, and a tropical wind. We need hardly add that a rising barometer indicates a northerly wind in the northern, and a southerly wind in the southern, hemisphere. In our latitudes the ordinary range of the barometer is about an inch and a half, i.e., from 30.5 to 29 inches—(its average height at the sea-level being about 29.95 inches) ; but its extreme range is nearly three inches, i.e., from 30.9 to less than 28 inches. There are at least three causes which appear specially to affect the barometer. The first of these is the direction of the wind, the N.E. wind tending to raise it most, and the S.W. wind to lower it most, the effect produced by intermediate winds being proportionate to their nearness to one or other of these extreme points. N.E. and S.W. may, therefore, as Dove has suggested, be called the wind's poles. The second cause is the amount of moisture in the wind, whether in the form of vapour, rain, or snow. Each of these causes is capable of producing an extreme change amounting to about half an inch. The third and most important cause is the strength or force of the wind, which appears to be capable of producing a rise or fall ranging, in extreme cases, to more than two inches. It is, of course, but seldom that these three causes act simultaneously to their full extent in the same direc- tion ; so that extreme variations in the height of the barometer are of comparatively rare occurrence. When the barometer is verylow, as during a gale, the first rising of the mercury generally in- dicates an increase of wind, possibly a heavy squall from a northerly quarter, after which a gradual rise foretells better weather, provided it be accompanied by a falling thermometer. Should warmth continue, however, the wind will probably shift against the sun, and more southerly or south-westerly wind may be expected to follow. The barometer begins to rise consider- ably before the conclusion of a gale, sometimes even at its commencement. It generally, but not always, falls on the approach of thunder and lightning. Instances of fine weather with a low glass are of rare occurrence, but they always indicate a continuance of rain or wind, if not of both. Dew or fog is an indication of fine weather ; and, on the other hand, when the atmosphere near the horizon is unusually clear, rain, if not wind, may be expected. On the question as to the amount of change in the barometer and thermometer, which may, in these latitudes, be taken as indications of a dangerous storm, Admiral Fitzroy gives us the following information :—" Great changes or storms are usually shown by falls of barometer exceeding half an inch, and by differences of temperature exceeding about fifteen degrees. Nearly a tenth of an inch an hour is a fall presaging a storm or very heavy rain. The more rapidly such changes occur, the more risk there is of dangerous atmospheric commotion."
We scarcely feel ourselves qualified to say anything at all definite respecting the views entertained by Admiral Fitzroy as to the origin of winds, for the simple reason that it is, as we have already hinted, scarcely possible to be perfectly satisfied that we have succeeded in ascertaining correctly what those views really are. We believe, however, that we are right in stating that he admits the existence of only two primary atmospheric currents, one from the pole to the equator, the other from the equator to the pole ; and that, owing to the rotation of the earth, the polar current moves towards the west as well as towards the equator, while the equatorial current moves toward the east as well as towards the pole. This great general circulation, which affects the whole atmosphere of the globe, is greatly modified, locally, by the configuration of sea and land ; and it is this modification which gives rise to the varieties of winds, storms, and climates experienced in each hemisphere, all of which are " alike in origin, all in accordance as to general principles, and all now explicable by the same natural laws." We have no doubt what- ever that Admiral Fitzroy has in his own mind a perfectly clear idea, both of the nature and the mode of action of these laws ; but whether it be owing to the innate difficulty of the subject, or to an inability on his part to give clear expression to his views, he certainly fails to render either of these points readily intelligible to the general reader. We must, therefore, content ourselves with indicating a few questions of considerable interest on which the views held by our author differ from those advocated by some of the leading authorities on meteorological matters. He thinks, for instance, that the generally received estimate of the depth of the atmosphere is considerably too high, and expresses an opinion that the sensible atmosphere does not probably reach higher than from ten to fifteen miles, while above it there may possibly be a few miles of very light gas. Another point on which he expresses his dissent is the theory of wave-like motions or undu- lations in the atmosphere, which has been suggested to account for the wave-like form of the curve which represents on paper the oscillations of a barometer while the wind is veering round the compass. He adduces the following cogent reasons against the admission of this hypothesis :—Firstly, the curve so traced varies not only with the barometer, but with the direction of the wind. Secondly, while the wind remains in one quarter, the curve or line remains almost unvaried, except in consequence of altered strength of wind or much rain, each of which has a com- paratively small effect. Thirdly, the lowest part of the curve (called the trough of the wave) always corresponds to the lowest barometer, or lightest air ; whereas it is the lightest air that rises highest, and therefore the crest, not the trough, of an atmospheric wave ought to be over the place of lowest baro- meter. Fourthly, aeronauts always find upper ktmospherie currents differing in direction from those below ; and these super- posed and successive strata in rapid cross motion must tend to, check undulation, if not to destroy it altogether. The Admiral then proceeds to offer his own explanation of these oscillations, attributing them to the alternate influence of the polar and tropical currents, the former of which raises, and the latter de- presses, the barometer ; and, as these movements are more or less directly with the surface wind which goes round the compass, a line traced on paper co-ordinating the barometric heights will have a wave-like form, in which the crests will correspond to one current, and the hollows (or troughs) to 'the other. There are or three points on which Admiral Fitzroy holds opinions directly opposed to those advocated by Lieutenant Maury, in his well-known work on the " Physical Geography of the Sea." He agrees, for instance, with Sir John Herschel in thinking that the heaping of surface water in the Gulf of Mexico, by the continued action of the trade-winds, and its subse- quent escape by the Gulf of Florida into the North Atlantic, are quite sufficient to account for the Gulf Stream ; and he does not appear to think it necessary even to acknowledge the existence of the well-known objections which have been urged against this view by the American meteorologist. Another point relates to the red fogs which are so often met with in the neighbourhood of the Cape de Verde Islands, and to the dust which they deposit upon the decks and sails of ships in that part of the Atlantic. Many of our readers will remember that Lieutenant Maury, relying upon Ehrenberg's decided opinion that this dust consists of the remains of microscopic insects which are only found in Brazil, concludes that it must have come from that country, and regards its transport across the Atlantic as a striking proof of his theory that the trade-winds cross near the equator, and there become upper or counter currents. Admiral Fitzroy expresses no decided opinion as to the value of this theory ; but he declines to believe that the dust in question comes from Brazil, urging, not very philo- sophically, that though the insects of which it is composed have never been found in Africa, it is more likely that they should exist there than that they should be wafted across the Atlantic by an upper current. A third point of difference between the English and the American meteorologist has reference to thle theory advanced by the latter, in order to account for the low average level of the barometer in high southern latitudes—viz., that there is a permanent and considerable diminution of atmospheric pressure throughout the region immediately sur- rounding the pole. Admiral Fitzroy denies that this supposition is either probable or necessary. He urges that there is no more reason to expect the pressure to go on diminishing to the pole than there is to expect it to go on increasing to the equator ; and he is of opinion that the phenomenon may be accounted for without calling in the aid of any hypothesis at all. The prevalent winds in these latitudes are from the west; and a westerly wind
always lowers the barometer all over the world. He adds, further, that most of our observations in these regions have been made in the summer and autumn, while the few voyages that have been made in winter and spring afford frequent instances of easterly winds with a comparatively high barometer. There is one vulgar error destroyed by the Admiral, which, though not of a strictly meteorological nature, appear s to be worthy of special mention, if only on account of the universality of its adoption—we allude to the general impression that even during the heaviest storm, waves never reach a greater height than forty feet, measured from trough to crest. Our author disposes of this view at once and for ever by the simple statement that he has himself witnessed waves the vertical height of which was at least sixty feet by actual measurement. In conclusion we must direct attention to a somewhat remarkable statement of the Admiral's, to the effect that ozone "seems to be oxydized chlorine gas." Seems to him, he should have said ; for he adduces no grounds for his opinion beyond the combination of the two facts, that ozone is found to be most abundant over the open sea, and that the sea contains chlorine—(in combination with sodium). Chemists will probably require somewhat stronger evidence than this, before they admit that Admiral Fitzroy has hit upon the correct solution of this much-disputed problem.
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