Pagina-afbeeldingen
PDF
ePub

great dominating force" to which all natu- It became manifest also, at an early ral phenomena connected with climate are epoch, that terrestrial conditions must be due, the sun has special influence on all intimately involved in all questions of the most noteworthy variations of wea- weather, since the year in different counther. The seasons are due to solar influ- tries in the same latitudes presents diffeence; and here we have an instance of a rent features. Such differences are of two power of prediction derived from solar kinds,--those which have a tendency to study though belonging to a date so re- be constant, and those which are in their mote that we are apt to forget the fact. nature variable. For example, the annual It seems so obvious that summer will be weather in Canadian regions having the on the whole warmer than winter, that we same range of latitude as Great Britain, overlook the circumstance that at some differs always to a very marked degree, epoch or other this fact, at least in its re- though not always to the same degree, lation to the apparent motions of the sun, from that which prevails in this country: must have been recognised as a discovery. here then we have a case of a constant Men must at one time have learned, or per- difference due unquestionably to terrestrial haps we should rather say, each race of men relations. Again, when we have a hot or must at one time have noticed, that the dry summer in this country, warm or damp varying warmth on which the processes of weather may prevail in other countries in vegetation depend, correspond with the the same latitudes, and vice versa ; varying diurnal course of the sun. So differences of this kind are ordinarily* soon as this was noticed, and so soon as variable, and in the present position the periodic nature of the sun's varying of weather science are regarded as accimotions had been ascertained, men had dental. acquired in effect the power of predicting Hitherto, weather-science has depended that at particular times or seasons, the solely on the study of these terrestrial efweather on the whole would be warmer fects as they vary under varying condithan at other seasons. In other words, tions. Modern meteorological research is so soon as men had recognised the period confined to the record and study of the acwe call the year, they could predict that tual condition of the weather from day to one half of each year would be warmer day at selected stations in different counthan the other half. Simple as this fact tries. It cannot be denied that the inquiry may seem, it is important to notice it as has not been attended with success. At vast the beginning of weather prediction; for expense millions of records of heat, rainas will presently appear, it has an impor- fall, winds, clouds, barometric pressure, tant bearing on the more complex ques- and so on have been secured; but hithertions at present involved in the prognosti- to no law has been recognized in the varication of weather-changes.

ations thus recorded,---no law at least It became manifest almost as soon as from which any constant system of predicthis discovery had been made, that the weather of particular days or even of

* I use this qualifying word, because some week and longer periods could not, by differences of the kind are more or less regular. its means, be predicted. A week in Thus, when there is a dry summer in certain resummer may be cold, and a week in win- gions in the West of Europe, there is commonly ter may be warm; nor, so far as is even

a wet summer in easterly regions in the same

latitude, and vice verså, the difference simply deyet known, is there a single part of any pending on the height at which the clouds travel year the temperature of which can be cer- which are brought by the south-westerly countertainly depended upon, at least within the trade winds. When these clouds travel high, temperate zone. In certain tropical re.

they do not give up their moisture until they have

travelled far inland or towards the east; when gions there are tolerably constant wea- they travel low, their moisture is condensed so ther variations; but so far is this from soon as they reach the western landslopes. It is being the case in the temperate zones of not uncommonly the case again, that when we either hemisphere, that it is impossible the Atlantic, and our drought is simply due to

in England have dry summers, much rain falls on to affirm certainly, even that during a the fall of this rain before the clouds from the week or fortnight at any given summer south-west have reached us. More commonly, season there will be one hot day, or that however, drought in England is due to the delay during a corresponding period in the win- the south-west travelling at a greater height than

of the downfall, in consequence of the clouds from ter there will be one day of cold weather. usual.

NEW SERIES.-VOL. XVIII., No. 3

22

tion for long periods in advance can be gan thus quaintly indicates his interpretadeduced.

tion of one particular expression of Sir G. On this point I shall quote first a re- Airy's opinion :-“ In the report to the markable saying of Sir W. Herschel's, Greenwich Board of Visitors, for 1867, which appears to me, like many such the Astronomer Royal, speaking of the sayings of his, to be only too applicable to increase of meteorological observatories, the present state of science. In endea- remarks, “Whether the effect of this voring to interpret the laws of weather, movement will be that millions of useless "we are in the position,” Herschel re- observations will be added to the millions marks,“ of a man who hears at intervals a that already exist, or whether somefew fragments of a long history related in thing may be expected to result which a prosy, unmethodical manner. A host of will lead to a meteorological theory, I circumstances omitted or forgotten, and cannot hazard a conjecture.' This is a the want of connection between the parts, conjecture, and a very obvious one; if prevent the hearer from obtaining posses- Mr. Airy would have given 234d., for the sion of the entire history. Were he al- chance of a meteorological theory formed lowed to interrupt the narrator, and ask by masses of observations, he would never him to explain the apparent contradic- have said what I have quoted." tions, or to clear up doub!s at obscure The simple combination of terrestrial points, he might hope to arrive at a gene- considerations with the effects due to the ral view. The questions that we would sun's varying daily path having thus far address to nature, are the very experi- failed to afford any interpretation of the ments of which we are deprived in the sci- varying weather from year to year, it is ence of meteorology."

natural to inquire whether the variations The late Professor De Morgan, indeed, in the sun's condition from year to year selected meteorology as the subject on may not supply the required means of inwhich, above all others, systematic obser- terpreting and hence of predicting weathervations had been most completely wasted, changes. We know that the sun's condi

as a special instance of the failure of the tion does vary, because we sometimes see true Baconian method (which be it no- many large spots upon his surface, whereticed is not, as is so commonly supposed, as at others he has no spots, or few and the modern scientific method). “ There small ones. We can scarcely doubt that is an attempt at induction going on," says these variations affect the supply of heat De Morgan, “which has yielded little or and light, as well as of chemical action no fruit, the observations made in the me- and possibly of other forms of force; and teorological observatories. This attempt hence we are certainly dealing with a vera is carried on in a manner which would causa, though whether this real cause be have caused Bacon to dance for joy” an efficient cause of weather-changes re(query); “for he lived in times when mains yet to be determined. Chancellors did dance. Russia, says M. It may perhaps be as well to inquire, Biot, is covered by an army of meteoro- however, in the first place, whether any graphs, with generals, high officers, subal- peculiarities of weather can be traced to terns, and privates, with fixed and defined another circumstance which ought to be duties of observation. Other countries, at least as efficient, one would suppose, as also, have their systematic observations. any changes in the sun's action due to the And what has come of it? Nothing, says spots. I refer to his varying distance M. Biot, and nothing will ever come of it: from the earth. It is known doubtless to the veteran mathematician and experimen- all my readers that in June and July, altal philosopher declares, as does Mr. Ellis” though these are our summer months, the (Bacon's biographer), “ that no single sun is farther away than in December,branch of science has ever been fruitfully and this, not by an inconsiderable distance, explored in this way." A special interest at- but by more than three millions of miles. taches, I may remark, to the opinion of M. Accordingly, on a summer day in our heBiot, because it was given upon the propo- misphere we receive much less heat than sal of the French government to construct is received on a summer day in the soumeteorological observatories in Algeria. thern hemisphere. Or instead of compar

It is well known that our Astronomering our summer heat with summer heat in Royal holds a similar opinion. De Mor- the southern hemisphere, we may make comparison between the quantity of heat It is worthy of notice, however, that even received by the whole earth on a day in in this case, where we cannot doubt that a June and on a day in December. Either great difference must exist in the solar way of viewing the matter is instructive; action at particular seasons, we find ourand I believe many of my readers will be selves quite unable to recognise any pecusurprised when they hear what is the act- liarities of weather as certainly due to this ual amount of difference.

difference. We receive in fact, on June 30th, less I have spoken of a second way of viewheat and light than dwellers at our anti- ing the difference in question, by considerpodes receive on December 30th, by the ing it as it affects the whole earth. The amount which would be lost if an opaque result is sufficiently surprising. It has disc having a diameter equal to one-fourth been shown by the researches of Sir J. of the sun's,* came upon the sun's face as Herschel and Pouillet, that on the average seen on December 30 at our antipodes. our earth receives each day a supply of It need hardly be said that no spots whose heat competent to heat an ocean 260 effects would be comparable with those yards deep over the whole surface of the produced by such a disc of blackness have earth from the temperature of melting ice ever been seen upon the face of the sun. to the boiling point. Now, on or about Spots are not black or nearly black, even June 30, the supply is one thirtieth greater, in their very nucleus. The largest ever while on or about December 30, the supseen has not had an extent approaching ply is one thirtieth less. Accordingly, on that of our imagined black disc, even June 30, the heat received in a single day when the whole dimensions of the spot, would be competent only to raise an ocean -nucleus, umbra, and penumbra,—have 2515 yards deep from the freezing to the been taken into account. Moreover, all boiling point, whereas on December 30 round a spot there is always a region of the heat received from the sun would so increased brightness, making up to a great heat an ocean 268}yards deep. The degree, if not altogether, for the darkness mere excess of heat, therefore, on Decemof the spot itself. So that unquestionably ber 30, as compared with June 30, would the summer heat in the southern hemi- suffice to raise an ocean more than 17 sphere exceeds the summer heat in our yards deep and covering the whole earth, hemisphere to a much more marked de- from the freezing point to the temperature gree than the heat given out by the sun of boiling water! It will not be regarded when he is without spots exceeds the heat as surprising if terrestrial effects of some of a spotted sun.

importance should follow from so noteIt is, however, rather difficult to ascer- worthy an excess, not merely of light and tain what effect is to be ascribed to this heat, but of gravitating force, of magnetic peculiarity. It is certain that the Austra- influence, and of actinic or chemical aclian summer differs in several important tion, exerted upon the earth as a whole. respects from the European summer ; but Accordingly we find that there is a recogit is not easy to say how much of thé dif- nisable increase in the activity of the ference is due to the peculiarity we have earth's magnetism in December and Janubeen considering, and how much to the ary as compared with June and July. But characteristic distinction between the north- assuredly the effect produced is not of such ern and southern halves of the earth, - a character as to suggest that we should the great excess of water surface over find the means of predicting weather if it land surface in the southern hemisphere. were posssible for us now to discover any

solar law of change resulting in a corre* It is easily shown that such would be the size sponding variation of solar action upon the of the imagined black disc. For the sun's dis- earth. tance varies from about 93 millions of miles to This leads us to consider the first great about 90 millions, or in the proportion of 31 to 30. Hence the size of his disc varies in the pro; systematic variations like the sun's varying

law of solar change as distinguished from to 900. The defect of the latter number 900 change of distance and his varying daily amounts to 61, which is about a sixteenth part of path on the heavens. This law is that the larger number. But a black disc having a which regulates the increase and decrease diameter equal to a quarter of the sun's would cut off precisely a sixteenth part of his light and

of the solar spots within a period of about heat, which was the fact to be proved.

eleven years. The sun's condition does

not, indeed, admit of being certainly pre- is usual, even at the period of maximum dicted by this law, since it not unfrequently spot-frequency. happens that the sun shows few spots for From all this it will be manifest that we several weeks together, in the very height have a well-marked peculiarity to deal of the time of spot-frequency, while on with, though not one of perfect uniformity. the other hamd 'it often happens that Next to the systematic changes already many and large spots are seen at other considered, this alternate waxing and wantimes. Nevertheless, this general law ing of spot-frequency might be expected holds, that, on the whole, and taking one to be efficient in producing recognisable month with another, there is a variation weather changes. Assuredly, if this should in spot-frequency, having for its period an not appear to be the case, we should have interval of rather more than eleven years. to dismiss all idea that the sun-spots are

Now, the difference between a year of weather-rulers. maximum spot-frequency, and one of mi- Now, from the first discovery of spots, nimum frequency, is very noteworthy, not- it was recognised that they must, in all withstanding the exceptional features just probability, affect our weather to some mentioned, which show themselves but for degree. It was noticed, indeed, that our short periods. This will be manifest on auroras seemed to be in some way influthe consideration of a few typical in- enced by the condition of the sun's surstances. Thus, in the year 1837, the sun face, since they were observed to be more was observed on 168 days, during which numerous when there are many spots than he was not once seen without spots, while when there are few or none. Singularly no less than 333 new groups made their enough, the effect of the spots on tempeappearance. This was a year of maxi- rature was not only inquired into much mum spot-frequency. In 1843, the sun later (for we owe to Cassini and Mairan was observed on 312 days, and on no less the observation relating to auroras), but than 149 of these no spots could be seen, was expected to be of an opposite characwhile only 34 new groups made their ap- ter from that which is in reality produced. pearance. This was a year of minimum Sir W. Herchel formed the opinion that spot-frequency. Passing to the next maxi- when there are most spots the sun gives mum year, we find that in 1848 the sun out most heat, notwithstanding the dimiwas observed on 278 days, during which nution of light where the spots are. He he was never seen without spots, while 330 sought for evidence on this point in the new spots made their appearance. In price of corn in England, and it actually 1855 and 1856 together, he was observed appeared, though by a mere coincidence, on 634 days, on 239 of which he was that corn had been cheapest in years of without spots, while only 62 new groups spot-frequency, a result regarded by Hermade their appearance. The next maxi- schel as implying that the weather had mum was not so marked as usual, that is, been warmer on the whole in those years. there was not so definite a summit, if one It was well pointed out, however, by may so speak, to the wave of increase ; Arago, that "in these matters we must be but the excess of spot-frequency was none careful how we generalise facts before we the less decided. Thus, in the four years, have a very considerable number of obser

1858, '59, 60, 61, the sun was observed vations at our disposal.” The peculiarion 335, 343, 333, and 322 days, on not ties of weather in a single and not extenone of which he was spotless, while the sive country like England, are quite insufnumbers of new groups for these four ficient to supply an answer to the wide years were respectively, 202, 205, 211, question dealt with by Herschel. The

The minimum in 1867 was very weather statistics of many countries must marked, as 195 days out of 312 were be considered and compared. Moreover, without spots, and only 25 new groups very long periods of time must be dealt appeared. The increase after 1867 was un

with.* usually rapid, since in 1869 there were no spotless days, and 224 new groups were * When Herschel made his researches into seen, though the sun was only observed on this subject, the law of spot-frequency had not 196 days. The number of spots in 1870, in this law, as some have since done, the explana

been discovered. He would probably have found 1871, and 1872, as well as their magni

tion of the seven years of plenty and the seven tude and duration, have been above what years of famine typified by the fát kine and lean

and 204.

M. Gautier, of Geneva, and later MM. lesser depths “confusion worse confoundArago and Barratt made a series of re- ed” (this, of course, is no fault of Professearches into the tabulated temperature at sor Smyth's, who here merely records several stations, and for many successive what had actually taken place), I take the years. They arrived at the conclusion that, temperatures at a depth of 24 French on the whole, the weather is coolest in feet. Now, neglecting minor features, I years of spot-frequency.

find the waves of temperature thus arrangBut recently the matter has been more ed. They go down to a little more than closely scrutinised, and it has been found 464 degrees of the common thermometer that the effects due to the great solar spot in 1839-40; rise to about 47 degrees in period, although recognisable, are by no 1847; sink to 47} degrees in 1849; mount means so obvious as had been anticipated. nearly to 47% degrees again in 1852–53 ;

These effects may be divided into three are at 47 degrees in 1856-57; are nearly classes,-those affecting (1) temperature, at 48 degrees in 1858-59; then they (2) rainfall, and (3) terrestrial magnetism. touch 47 degrees three times (with short

As respects the first, it has been disco- periods of rising between), in 1860, 1864, vered that when underground temperatures and 1867; and rise above 47} degrees in are examined, so that local and temporary 1869. Now if we remember that there causes of change are eliminated, there is were maxima of spots in 1837, 1848, a recognisable diminution of temperature 1859–60, and 1870, while there were miin years when spots are most frequent. nima in 1843, and 1855-56, I think it will We owe this discovery to Professor C. P. be found to require a somewhat lively Smyth, Astronomer Royal for Scotland. imagination to recognise a very striking The effect is very slight; indeed, barely association between the underground temrecognisable. I have before me, as í perature and the sun's condition with write, Professor Smyth's chart of the quar- respect to spots. If many spots imply terly temperatures from 1837 to 1869, at diminution of heat, how does it come that depths of 3, 6, 12, and 24 French feet. the temperature rises to a maximum in Of course, the most remarkable feature 1859, and again in 1869 ? if the reverse, even at the depth of 24 feet, is the alter- how is it that there is a minimum in 1860 ? nate rise and fall with the seasons. But I turn, lastly, to the chart in which the it is seen that while the range of rise and sun-spot waves and the temperature waves fall remains very nearly constant, the crests are brought into actual comparison, and and troughs of the waves lie at varying I find myself utterly unable to recognise levels. After long and careful scrutiny,

the slightest association between them. I find myself compelled to admit that I Nevertheless, I would not urge this with cannot find the slightest evidence in this the desire of in any way throwing doubt chart of a connection between under- upon the opinion to which Professor Smyth ground temperatures and the eleven years has been led, knowing well that the long period of sun spots. I turn, therefore, to and careful examination he has given to the chart in which the annual means are

this subject in all its details, may have given; and noting in the means at the afforded ample though not obvious evi

dence for the conclusions at which he has

arrived. kine of Joseph's dream. For if there was a

I note also, that, as he points period of eleven years in which corn and other out, Mr. Stone, director of the Cape Town produce of the ground waxed and waned in pro- Observatory, and Mr. Cleveland Abbe, ductiveness, it would be not at all unlikely that director of the Cincinnati Observatory, whenever this waxing and waning chanced to be unusually marked, there would result two series

have since, “ but it is believed quite indeof poor and rich years apparently ranging over

pendently, published similar deductions fourteen instead of eleven years. We have seen,

touching the earth's temperature in refeabove, that the waves of spot-waxing and spot- rence to sun-spots.” All I would remark waning are not all alike in shape and extent. is, that the effect is very slight and very Whenever then a wave more marked than usual came, we should expect to find it borrowing, so

far from being obvious at a first inspection. to speak, both in trough and crest, from the Next as to rainfall and wind. waves on either side. It would require but a Here, again, we have results which can year or so either way to make the wave range

hardly be regarded as striking, except in over fourteen years; and observed facts even during the last half-century only, show this to be

the forcible evidence they convey of the no unlikely event.

insignificance of the effects which are to

« VorigeDoorgaan »