caused by the drifting past of a cyclone, neither wind nor rain would follow in England. The centre of these great depressions, which are not true cyclones, is usually associated with cool, bright weather and cumulus cloud, and therefore weather of that description would probably be experienced. From a case of this sort, we learn how to avoid the popular errors that the violence of a gale is always proportional to the fall of the barometer, and that a very low barometer is necessarily associated with very bad weather.

COMPLICATIONS ON BOARD SHIP.

All the examples which we have now given in this chapter will sufficiently explain the nature of forecasting by means of a single barometer and observations on the appearance of the sky, as also the true nature of the apparent exceptions to the ordinary relationship between weather and the movements of the mercury in a barometer tube.

Our space, unfortunately, does not permit us to describe the still greater difficulties which occur when the observations are taken on board a moving ship; then, of course, we have not only the motion of cyclones, but also that of the ship to take into account, and it is manifest that many of the rules which we have laid down for land-stations would require considerable modification. The same limitation of space also compels us to omit the notice of the theory of handling ships in the small cyclones which occur in tropical countries under the names of hurricanes, typhoons, etc., but the author hopes to make this branch of meteorology the subject of another work.

CHAPTER XV.

FORECASTING BY SYNOPTIC CHARTS.

STATEMENT OF THE PROBLEM.

By synoptic forecasting we mean that branch of weatherprevision which is carried on by means of synoptic charts. The forecaster in a central bureau is in telegraphic communication with observers for many hundred miles round. From their reports he constructs synoptic charts at such intervals as seem necessary. To the indications which he derives from the appearance of these maps, he adds all his own accumulated experience of the nature of the meteorology, and the motion of depressions in his own country; and also such knowledge of the recurrent periods of different kinds of weather as he may be acquainted with. From all that he forms his own judgment as to what changes are likely to take place, and issues his forecast accordingly.

From the nature of things there can never be many forecasters. The rapid nature of meteorological changes makes the employment of the electric telegraph absolutely necessary, and the great expense which is thereby in

curred, compared with the uncommercial nature of the results, practically relegates forecasting to the functions of a Government office.

From the preceding chapters we now know what weather is. Instead of dealing with abstractions called wind, rain, cloud, heat, etc., we have gradually been led up to the idea that all meteorological phenomena are the products of the motion and circulation of a moist atmosphere. Now we know that when we talk about forecasting weather, we mean that we are going to say how or where certain aerial eddies will move, or when new ones are likely to form; also whether any cyclone will be violent or gentle.

AIDS TO FORECASTING.

In this chapter we propose to make some additional remarks on the whole aspect of the subject. We shall enumerate several aids to forecasting which can be obtained from various sources, and point out both the present difficulties and the future possibilities of weatherprevision. Finally, we shall give some examples of successful and unsuccessful forecasts in different countries, and an account of the various percentages of success which the different offices have achieved. In an international work we shall better illustrate the general principles of the subject by exemplifying forecasts in different countries than by trying to give any one in detail. A tolerably full account of the nature of forecasting, and of the details of the methods and machinery for issuing storm-warnings in Great Britain, will be found in

the author's work, "Principles of Forecasting by Means of Weather Charts," issued by the authority of the Council of the Meteorological Office.

UNEQUAL BAROMETRIC CHANGES.

We have already fully explained the use of the recognition of weather-types in every country, during which sequence the motion of depressions follow either a certain general direction or maintain a certain general position; but in variable climates we often find tracts of weather which can be assigned to no particular type. The forecaster is then at a great disadvantage, for he has little to guide him as to the future.

The very idea of weather-type involves the knowledge that the sequence of changes will follow in a certain groove, so that when no type is obvious, there is little basis on which to frame a forecast. In most cases the forecaster has to rely on the difference of barometric rate in various districts. If he sees that the barometer is falling much more rapidly in one district than in any other-even if no definite depression is formed-he knows that steeper gradients must thereby be formed, so that the wind must increase, and whatever weather is due to the existing shape of isobars will get worse.

Conversely, if he finds pressure increasing in a district of low barometer, he knows that gradients will decrease, and that both wind and weather will moderate. The details vary indefinitely, and no rule can be laid down even for a single country; everything must be left to the judgment and experience of the forecaster.

CYCLONE-PATHS.

The paths of cyclones, and the nature of the influences which deflect or otherwise alter them, are so important that we propose to devote some paragraphs to their consideration, of course with a special reference to the bearing which they have on forecasting.

When the paths of the rare but violent cyclones of the tropics, which are known as hurricanes, typhoons, or cyclones, are plotted on a chart, we find that, though there is a general similarity in their tracks, there is still so much difference that we cannot attempt to lay down any absolute law of their motion.

For instance, the West India hurricanes usually begin with a westward course, and then gradually bend round till they end by moving towards the east or northeast. But in some instances they continue in a westerly direction, and traverse the southern portion of the American Union, instead of curving round across the Atlantic.

For this reason, if a ship was handled on the supposition that the hurricane would always go the same course, she would be exposed to very great danger.

In the temperate zone, where cyclone-paths are still more irregular, any attempt to lay down any hard and fast rule for the tracks of depressions could only lead to disastrous failure of any forecasts which were based on that system; but though the numerous causes which have been found to modify the paths of cyclones cannot be allowed for in estimating the probable future path of