CHAPTER XII.
THE "HEAT WAVE" OF 1892.
URING the course of the summer of 1892 the papers frequently described in sufficiently striking paragraphs the abnormally high temperature which was experienced in many parts of the globe. The first tidings of this nature reached us from America. Thus we read that on the 29th of July the thermometer in the streets of New York had risen to as much as 101° and 102° in the shade. At the meteorological station in that city, where, no doubt, every precaution was adopted to insure accuracy in the record, we find that a temperature of 99° was indicated. The next day July 30 the ascent of the mercury still continued, and we hear that an observation in the Fifth Avenue showed as much as 107° in the shade. This, however, seems to have been the culmination of what had been somewhat absurdly designated "the great heat-wave." On July 31 the warmth had begun perceptibly to decline, though it was still terribly oppressive.
The descriptions received from various parts of the North American continent show that the heat was almost, if not quite, as great in many other places as it was in New York. From north and south, from east and west, we heard of abnormally high thermometers; we were told that in many localities the work in factories had to be discontinued, as the hands could not stand the heat. In some towns business seems to have been temporarily suspended, and the traffic in the streets ceased during the hottest part of the day. It was also reported from many places that heavy losses were experienced by the death of sheep and cattle. Nor was the great heat-wave without a tragic aspect. We read of a large number of cases of sunstroke occurring in various parts of America, many of which terminated fatally.
So far as we are able to form a picture of what actually happened, it would seem to have been one of the most protracted and calamitous spells of heat that have ever been recorded in America. It has been remarked as a somewhat peculiar feature, that there was an almost total absence of wind at the time when the heat was greatest; and it may also be recorded that the air was at the time largely charged with humidity. Every one who has had any experience of tropical heat knows that the suffering caused by an excessively high temperature is greatly enhanced if the air be saturated with moisture. Evaporation is then almost at a standstill, and one of the means by which the temperature of the body is kept down is so far rendered inoperative. I recollect being told by an officer who was in the Ashantee Expedition many years ago, that notwithstanding the excessive heat of the coast off which their ships lay at anchor, there was practically no evaporation, owing to the air being saturated with moisture. The towels which were hung up to dry in the morning remained wet till evening, even though the tropical sun beat on them all the day long. Heat of a somewhat similar character appears to have been experienced in America at the end of last July.
It was about a fortnight or three weeks after the New "World had its scorching that the Old World was visited by the great heat-wave. Up to the beginning of August there does not seem to have been anything unusual in European temperatures; thus, for instance, at Berlin, on August 1, the highest thermometric reading was 72°, and the lowest 61°. Even on the 7th of August, the greatest and least temperatures at Vienna were no more than 70° and 61° respectively, but towards the middle of the month the ascent of the mercury in the thermometer became marked and rapid all over Europe.
By the 17th of August, a temperature had been reached at Vienna which seems to have rivalled that attained at New York nineteen days previously. We read that on the following day (18th of August) the thermometers at Vienna showed 107° in the shade; the telegrams declare that the streets are deserted, and considering what the feelings of the reporter must have been who described it, we excuse his exaggeration that the Ringstrass was "like a furnace."
On the 19th, Berlin is reported to be almost unbearable, and, on the same day, we read that the heat is tropical at Paris, where there have been many fatal cases of sunstroke. It is further stated that 100 oxen and 300 pigs were found dead from the heat in the railway trucks as they arrived in the meat-market at Villette.
On August 22, the phase described in the papers as "almost unbearable" is recorded at Vienna, and that this language is justified will be obvious from one fact which is mentioned in the same connection. It appears that a body of troops which were out for manoeuvres in the neighbourhood of Vienna during this terrible weather, suffered so severely that there were 200 cases of sunstroke among them, and many of those so attacked did not recover. About the fourth week in August, England experienced in some small measure the effects of the great heat-wave. But only in small measure, because we happen to lie on the margin of the globe area which was the seat of the high temperature. However, it may be remarked that for two or three days, an unusually high thermometer prevailed in South-Eastern England. On August 24, 80° is recorded in the shade at Dover, and on August 23 and 24, the highest and lowest indications of the thermometer at London were 80° and 59° respectively. It follows that the temperatures attained in this country fell far short of what was experienced in so many places on the Continent, nor did the unusual heat which was reached last long in Great Britain. We find that by the 24th and 25th of August, the range at London had so far declined that the highest and lowest points were 75° and 62° respectively. It was not till some days later that the decline really set in on the Continent; for on the 25th of August, there was still a temperature of 89° in the shade at Vienna. On the 26th of August, which is the last record of the great wave that we shall here set down, the thermometer shows 84° at St. Petersburg, the report accompanying it with the emphatic word "scorching."
From the various facts we have set forth it appears that towards the end of July an extraordinarily high temperature, even for that period of the year, prevailed over a very large part of the North American continent. The so-called heat-wave then seems to have travelled eastward, and crossed the Atlantic Ocean, but, owing to the absence of information, except in such casual records as may be found by an inspection of ships' logs, we know little or nothing of the actual progress of the heated region across the Atlantic. However it may have come about, it is, at all events, certain that a fortnight after the occurrence of unusually great heat in the New World there was a similar experience in the Old World. Our knowledge of the distribution of temperatures over the whole globe is too incomplete to enable us to follow the movements of the great wave as fully as we might desire. No doubt our own Meteorological Office does most admirable work, and of course many other countries have more or less complete organisations for the study of meteorological phenomena, yet our information as to the thermal condition of the globe still falls short of what we would like to have. Certain materials are, however, available, and we shall endeavour to throw what light we can on the matter.
We often hear the question asked as to what was the cause of this exceptional heat. Let me hasten to say that neither in these pages nor anywhere else could I attempt to answer this question in the sense in which it has usually been proposed. It is very doubtful whether it would be possible to assign a single cause for such a phenomenon, even if we knew many things of which we are now completely ignorant. Indeed the most difficult problem of astronomy becomes simplicity itself when compared with the extraordinarily complex agents that are in operation even in the simplest meteorological phenomenon. Let me illustrate this contrast between the two sciences by an example.
The movement of the moon is one of the most profound dynamical problems. It depends principally on the attraction of the earth, and in a lesser degree on the disturbance caused by the sun. The forces thus arising can be submitted to calculation, and though the work involved is extremely abstruse, and though it implies a prodigious amount of numerical labour, yet it can be completely solved for all practical purposes. The consequence is that the motions of the moon have become so well known that we can foretell not only the hour but even the minute at which eclipses will occur next year or in a hundred years to come. Contrast the certainty of this knowledge with the vagueness of our knowledge of meteorological phenomena. "We can tell you precisely where the moon will be at noon next Christmas Day, or, for that matter, where the moon will be at noon on Christmas Day in the year 1994. But who can tell what the temperature will be at noon next Christmas Day on London Bridge? No scientific man could venture on such a prophecy. He knows that he has no data to go by. The number of causes which are in operation is so great that the problem becomes of a highly complex nature. There is, however, a certain mathematical principle which applies in this case. It does not, indeed, enable us to predict the actual amount of any meteorological element, but it appears to demonstrate with all desirable fulness that there must be definite laws governing the changes of the different meteorological elements if only we were able to discover them.
The argument on which we are about to enter is perhaps a somewhat difficult one, but it will be worth while to face it. The method indicated seems to offer the only hope of our ever attaining such a knowledge of meteorlogical phenomena as will enable us to rise to the supreme position of being able to predict the facts of climate with assured accuracy, and for a long time in advance. Let us first enumerate some of the particular phenomena which are necessarily more or less connected together. The most fundamental of all the elements concerned is the pressure of the air as indicated by the barometer; then there is the temperature of the air and the degree of its saturation, the amount and character of the clouds, the rainfall, together with comparatively exceptional incidents such as hailstorms and thunderstorms. At present, no doubt, we are enabled, by the careful collection of observations all over the world, to predict in some degree the recurrence of these phenomena. Our newspapers give us each morning a forecast of the kind of weather that may be expected. But every one knows that, though these forecasts are often useful, they yet have a very inferior degree of accuracy to the kind of prediction which we find in the "Nautical Almanac," where the occurrence of an eclipse of the moon, or of an occultation of a star, or a transit of Venus, or any similar astronomical event, is foretold with definiteness and with perfect certainty of fulfilment. Yet no one can really doubt that the temperature at London Bridge next Christmas Day, or the height of the barometer at noon on January 1, 1900, are each of them quite as certainly decided by law as the time of high water or any other astronomical element.
We know that there will be a transit of Venus in the year A.D. 2004, and that there will be no such phenomenon until then, while there will be a repetition of the occurrence in A.D. 2012. It is certain that these predictions will be fulfilled, yet why is it that we can make no assertion of a similar character with regard to the meteorological phenomena? The one is just as amenable to law as the other, but the difference is that the extreme intricacy of the causes which affect the meteorological phenomena have hitherto prevented us from discovering the laws by which they are regulated. Perhaps the differences between the state of our knowledge of the astronomical and of the meteorological phenomena will be more conveniently explained by choosing a branch of astronomical science with which we are at present only imperfectly acquainted. Let us take, for instance, the showers of shooting stars, which are wont to occur on November 14-16. Every one knows that there was a superb display from this shower in 1866, and there were some reasons to expect that there might also be a superb display in 1899. As will be well remembered, this expectation was not realised, so we only cite the illustration to point out that there are certain kinds of expectation which stand on a very different plane from astronomical predictions of a more legitimate character.
Nor is it hard to see the reason why this is so; we know in a general way the orbit of that swarm of bodies whose incursions into our atmosphere give us shooting-star showers. There are, however, many circumstances in connection with the movements of these little objects, which as yet are only imperfectly disclosed to us. We have no very accurate knowledge as to the manner in which the shoal of meteors is disposed around the vast ellipse which constitutes its path, and consequently our predictions must necessarily be put forth with some feeling of insecurity. It is quite certain no doubt that the earth crosses the track of the meteors every November 12—14, and it may also be regarded as tolerably certain that when the earth is in this position in the year 1899 the shoal of little bodies will be in our vicinity. We believe that the earth will actually pass through the shoal, in which case a great meteoric display will be the result—if the weather permits! It may, however, happen that we shall only traverse a sparsely occupied portion of the great host, in which case the shower will fall much short of others which have been recorded. An enormous volume of quite unattainable knowledge would have to be at our disposal were we to be able to predict with certainty all the circumstances of such phenomena; we should have, to know exactly what meteors there were in the shoal, and the dimensions and other features of the orbit which every single meteoroid followed. If such knowledge as this were possible, then the future circumstances of the shower might be predicted with almost as much accuracy as the announcement of the next eclipse or the next opposition of Mars.
This illustration will suffice to explain the reasons why our knowledge of meteorological phenomena is at present in such an imperfect state as compared with those of astronomy. The supreme test of the completeness of any physical theory is the successful prediction of results— we are not yet able to predict great heat-waves or great storms with any assured confidence, not because such phenomena do not observe definite physical laws, but because the knowledge that we should require before we could exactly specify these laws is in a great measure wanting. We are, however, not without grounds for encouragement in the belief that the time may yet come when the definite prediction of meteorological phenomena may become possible. An instructive illustration of the direction in which we may look for success is afforded by the study of the tides. Of late years the problem of tidal prediction has occupied a great deal of attention, and by the labours of Lord Kelvin, Sir George H. Darwin, and others, the investigation has received a completeness which renders it a typical example of how the solution of a problem of this kind is to be attained.
If we are ever to progress in meteorological prediction we can only do so by following the same lines which have already been pursued with striking results in the case of the tides. Of course the tides primarily depend on the attraction of the moon, but to a secondary extent the great undulations of the ocean are affected by the influence of the sun. As the movements of both these heavenly bodies may be regarded as sufficiently known, the matter of tidal prediction would be indeed a simple affair were there no other element to be taken into consideration. But the time of high water at any port as well as the actual height which the water attains are by no means regulated solely by the positions of the sun and moon; it is the configuration of the surrounding coasts, the depths of the seas in the neighbourhood, the proximity or the remoteness of the open ocean, and other purely local circumstances which affect the result all these have to be taken into account.
The most instructive method of exhibiting the present state of tidal theory is given by Lord Kelvin's tide-predicting machine. In this arrangement the difference between what we may call the astronomical factors and the terrestrial factors of the tides, is clearly brought out. A cord passes over a number of pulleys and the centres of each of these pulleys are made to revolve in periods which are determined by the movements of the sun and moon. When the machine is to be employed for predicting the tides in any particular port, the positions of all these pulleys must be set so to speak in conformity with certain individual circumstances connected with the particular port—thus, though the tides at Madras are totally different from the tides at London Bridge, yet the same machine may be used to calculate both. The fundamental movements of the machine are constant for all ports, but the various pulleys will in the one case have to be set in conformity with the local conditions of Madras, and in the other case they would have to be set in conformity with the local conditions of London Bridge. Two totally distinct tide-tables, appropriate however to the two ports named, could thus be generated by the revolutions of this useful machine.
It would perhaps be too much to anticipate that the time will ever come when meteorological phenomena shall admit of being worked out by a machine on the principle of the tide-predicting engine. But yet it does not seem altogether vain to strive for such a result. We can, in fact, give some reasons for indulging a hope that something of this kind may yet be accomplished. In the first place it is perfectly clear that the radiation of heat from the sun must be the chief factor in the variations of all meteorological quantities. The fluctuations of temperature with the changing seasons are among the most obvious instances of the connection between the sun and the climate, but it may be shown that the changes of every other meteorological element are also primarily dependent on the sun.
Let us take, for instance, the pressure of the air as indicated by the height of the barometer, and show that the oscillations of the mercury must be due to the sun. Imagine for a moment that the sun were to be extinguished, one consequence of the cessation of the arrival of heat at the earth would be that winds would blow no longer. There could be hardly any movements whatever in the air except such as might arise from atmospheric tides. Perhaps also I ought to add that the internal heat of the earth as manifested by earthquakes or by occasional volcanic outbreaks might produce some local and temporary disturbance of the air. It is, however, quite certain that such influences would have very slender effects on atmospheric pressure. The argument will at all events suffice to show that the fluctuations of the barometer to which we are accustomed are almost entirely attributable in one way or another to the action of the sun. It can similarly be shown that the changes in every meteorological element will depend primarily upon the great luminary. In some cases, of course, the rotation of the earth on its axis is also an important element, and to some minute degree the moon must be reckoned with. But when these influences have been i considered, we have no further concern with the heavens; it is the topographical features of the earth which complete the determination of all meteorological quantities.
I cannot here go into the discussion of a celebrated mathematical theorem which bears the name of Fourier. It seems, however, to demonstrate that any meteorological element, such as the height of the barometer or the temperature, must admit of being expressed in a somewhat similar fashion to the height of the tide. No doubt the arrangement of pulleys would have to be extremely complex, so as to enable the elements to be determined which were dependent upon so many considerations. It is, however, quite plain that if we are ever to succeed in subjecting meteorological phenomena to numerical precision it must be in some such direction as I have indicated. To put the matter a little more plainly; we have reason to believe that a system of pulleys could be so arranged, and the relative movements of them could be so actuated, that a cord passing over those pulleys and adjusting a pencil could be made to show the height of the barometer for every day in the year at a given place. A similar machine might also be conceived which should show the temperature at any stated locality for every hour in the year. I do not for a moment assert that the information at present at our disposal would enable us to construct such machines. All I am now contending for is that mathematical theory seems to declare the possible realisation of such contrivances. The fact that an engine has already been constructed for the comparatively simple case of the tides leads us to hope that the time may arrive when meteorological engines shall have been designed by which meteorological prediction shall become as determined as the prediction of high water.
This discussion will at all events enable us to make some reply to the question which has been often asked, as to what was the cause of the great heat-wave. I do not indeed think that the question admits of any off-hand answer of the kind that is frequently expected by those who ask it—the only kind of answer that seems possible is of a somewhat indirect character. We may here again revert to our illustration of the tides. It sometimes happens that an unusually high tide occurs. In the port of Dublin, for instance, we have had from time to time exceptionally high water in the Liffey, which has flooded the basement stories of low-lying dwellings. The cause of such extraordinary phenomena is not to be attributed to any unusual development of the strength of the moon's tide-producing capacity, it is rather to be explained in a manner which the tide- predicting engine renders easy to understand. Besides the main lunar tide and the main solar tide, there are several minor tides, so to speak, arising from the different combinations of the movements of the sun and moon. Each pulley in the tide-predicting engine is, in fact, allocated to each particular tide—the consequence is, that the height of the water at any moment is the net result of one or two large tides, and of a number of small ones; thus, for instance, every one knows that the spring tides, as they are called, are exceptionally high because the sun and moon conspire; while the rise and fall at the time of neap tide are comparatively small, because then the solar tides act oppositely to the lunar tides, and what is actually perceived is only the difference between the two. There are also the numerous minor tides to be considered; of course, it will not generally happen that these are all consentaneous: some of them are high and some of them are low, and others may be at intermediate phases at the time of high water, as determined by the great predominating tide. But it is easy to imagine that every now and then, under exceptional circumstances, there will happen to be a concurrence between the time of high water in the small tides and in the great ones. Then, of course, there will be the exceptional flooding that is occasionally experienced.
It is somewhat in this manner that we must seek to explain what is called the great heat-wave. The temperature at any place has of course the main annual period corresponding to the variation with the seasons, but there are many other periodic fluctuations in the temperature analogous to those minor tides to which we have already referred. Generally speaking, of course, these will not all conspire; some will be tending to elevate the temperature and others to depress it at any time. It is the net result of all that we actually perceive. Sometimes, however, it will happen that several of these, or at all events some of the more important ones, move in the same direction; then of course we have great exaltation of temperature such as that which the newspapers have called the great heat-wave.
It is, however, quite possible that certain changes in progress on the sun may act in a specific manner on our climate. I do not indeed say that there is much reason for thinking that the great heat-wave has really been connected with any intrinsic changes in our luminary, but it is just possible that something of the kind may have occurred. I would consequently like to devote some little space to the consideration of this interesting subject.
In the discussion of such a question there is a fundamental point which must always be borne in mind. We must remember the full extent of the earth's indebtedness to sunbeams. We have spoken of a temperature of 100° during the continuance of the great heat-wave, and it is necessary to understand all that this implies. Of course, on our thermometric scale a temperature such as we have mentioned merely means 100° above a certain arbitrary zero, but the sun has sent us more heat than those hundred degrees express. If the sunbeams were totally intercepted, so that the earth derived no heat whatever from this source, the temperature of our globe would fall not merely to zero, but it must sink down to a point far below zero, even to the temperature of space itself; what this may be is a matter of some uncertainty, but from all the evidence attainable it seems plain that we may put it at not less than 300° below zero. It therefore follows that at the time of the heat-wave, when the thermometer indicated 100°, the sun's beams actually maintained the affected region of the earth at 400° above what it would have been if the sun were absent. This will show us that the heat- wave was not after all such a very exceptional matter so far as the sun was concerned. Had the temperature been only 80° at New York we should never have heard of the sun-strokes and all the other troubles; it was the extra 20° which made all the difference—in other words, so long as the sun merely kept the earth about 380° above the temperature of space no one thought anything about it, but the moment it rose to 400° it was expected that something tremendous must have happened. This way of looking at the matter places the great heat-wave in its proper cosmical perspective; it was no such great affair after all; it merely meant a trifling addition of 5 per cent, to the temperature usual at that season—that is to say, when the temperature is measured in its proper way. This shows us that a very trifling proportional variation in the intensity of the sun's radiation might be competent to produce great climatic changes. It seems hardly possible to doubt that if, from any cause, the sun shed a small percentage of heat more than it was wont to do, quite disproportionate climatic disturbances would be the result.
It cannot be denied that local if not general changes in the sun's temperature must be the accompaniment of the violent disturbances by which our luminary is now and then agitated. It is, indeed, well known that there are occasional outbreaks of solar activity, and that these recur in a periodic manner; it is accordingly not without interest to notice that the present year has been one of these periods of activity. We are certainly not going so far as to say that any connection has been definitely established between a season of exuberant sun-spots and a season remarkable for excessive warmth, but, as we know that there is a connection between the magnetic condition of the earth and the state of solar activity, it is by no means impossible that climate and sun-spots may also stand in some relationship to each other.
As to the activity of the sun during the hot summer, a very striking communication was made at the time by one of the most distinguished American astronomers, Professor George E. Hale. He has invented an ingenious apparatus for photographing on the same plate at one exposure both the bright spots and the protuberances of the sun. Professor Hale delivered an interesting lecture at a meeting of the American Association for the Advancement of Science at Rochester. A report of this lecture appeared in Nature, in which we are told of a remarkable application of Professor Hale's apparatus. On the 15th of July a photograph of the sun showed a large spot. Another photograph taken in a few minutes exhibited a bright band; twenty-seven minutes later a further exposure displayed an outburst of brilliant faculæ all over the spot. At the end of an hour the faculæ had all vanished, and the spot was restored to its original condition. It was not a mere coincidence that our magnetic observatories exhibited considerable disturbance the next day, and that brilliant auroras were noted. The whole communication was of such an interesting description that we are not surprised to hear that a vote of thanks was passed to Professor Hale amid much enthusiasm. It is quite plain that we have yet much to learn concerning the effect of the sun on things terrestrial. This new method, in which Professor Hale has extended and improved the processes of his predecessors, is full of hope for the future.