meter sometimes appears to fail, and also how much the older knowledge can be increased by a knowledge of synoptic charts. The space at our disposal will not, however, permit us to explain the modern developments of the principles of handling ships in hurricanes, which would naturally come in this chapter. 2. To show what a meteorologist can do, seated in a central bureau, with telegraphic communication in all directions, and who, after making a synoptic chart, and combining it with every other modern aid, issues telegraphic forecasts to all parts of the country. This is the highest problem of meteorology. CHAPTER II. WEATHER-PROGNOSTICS. INTRODUCTION. THE second stage in the history of meteorology, after the mythic phase has been passed, is the collection of numerous observations on the appearance of the sky, the movements of animals, etc., before rain or fine weather into the form of short sayings, which are usually known as popular prognostics. For instance, halos round the sun, or swallows flying low, are known all over the world very frequently to precede rain. On the other hand, a copious deposition of dew, or a white silvery moon, are equally widely known as precursors of fine weather. One of the earliest collections of prognostics is found in the "Diosemeia" of Aratus, a Greek who flourished in Macedonia and Asia Minor about 270 B.C. The principal interest attached to his work is that many of his prognostics were incorporated by Virgil in his Georgics, and that from them-through the medium of the Latin monks, during the revival of learning in the Middle Ages -a very considerable number have been translated into modern European languages, and are in current use at the present time. EARLY EXPLANATIONS. From classic times, down to the commencement of this century, it can hardly be said that this branch of meteorology made any advance. Few, if any, new prognostics had been discovered, and neither their physical explanation nor their meteorological significance had been found out. But about eighty years ago, some physical explanations were given. It was found that the air always contained a certain quantity of uncondensed vapour, and means were invented for measuring this amount accurately. From this, the nature and conditions of the formation of dew were discovered, and also that before many cases of rain the air became more charged with vapour. This latter fact gave the explanation of several rain-prognostics. For instance, when walls sweat, stones grow black, and clouds form on hilltops, rain may be expected almost all the world over. But even when these reasons had been discovered, the science flagged. A large number of rain-prognostics could not be shown by any means to depend on an increase of moisture, and, as vapour cannot grow in the air, some explanation was needed to account for its variable quantity. And even when, in a general way, the prognostic had been explained, no clue whatever had been found for what we may call the meteorological significance. What was the relation of the damp to the rain? Why did the prognostic sometimes fail? Why are there many ratnprognostics associated with a tolerably dry air? Why is C not all rain preceded by the same set of prognostics? To all these questions no answer could be given. Prognostics had almost fallen into disrepute; they were considered no part of science, and had been supposed to be only suitable for rustics and sailors. MODERN DEVELOPMENTS. So the subject remained till the introduction of synoptic charts. Then it was soon seen that in Temperate regions the broad features of weather depend on the shape of the isobaric lines, and later on it was shown— the author believes, mainly by himself—that nearly all prognostics have a definite place in some shape of isobars, and that all the above questions, formerly insoluble, receive a ready explanation. It has also been demonstrated that prognostics can never be superseded for use on board ship, and that even in the highest developments of weather-forecasting by means of electric telegraph, prognostics often afford most valuable information. But before we attempt to explain how this is done, we must introduce the reader into the elements of synoptic meteorology. SYNOPTIC CHARTS. Synoptic meteorology is that part of the science which deals with the results obtained by constructing synoptic charts. Formerly, all meteorology was deduced from the changes which took place in the instrumental readings at any one place during any interval of time, say one day. For instance, a great deal bad been discovered as to the connection between a falling or rising barometer and the accompanying rain or wind. Synoptic charts, on the contrary, are constructed by taking the readings of any instrument (say the barometer), or any observations on the sky or the weather (say where rain is falling, or cloud or blue sky is seen), at a large number of places at the same moment (say 8 a.m. at Greenwich). A map of the area or district from which the observations have been received is then taken, the barometer-readings are marked down over their respective places, and then lines are drawn through all the stations where the pressure is equal. For instance, through all the places where the pressure is 29.9 inches (760 mm.), and again at convenient intervals, generally of about two-tenths of an inch, say 29-7 ins. (755 mm.), 29-5 ins. (750 mm.), and so on. These lines are called isobaric lines, or more shortly isobars— that is, lines of equal atmospheric weight or pressure. This method of showing the distribution of pressure by isobars is exactly analogous to that of marking out hills and valleys by means of contour lines of equal altitude. Similarly, the places which report rain, cloud, blue sky, etc., are marked with convenient symbols to denote these phenomena. In Great Britain, a system known as Beaufort's weather-notation is exclusively used. It is as follows This will be useful, as it is employed in all our charts. SYMBOL. BEAUFORT'S NOTATION OF WEATHER. b Blue sky, whether with clear or hazy atmosphere. d Drizzling rain. |