showed that, in a general way, the mercury fell before rain and wind, and rose for finer weather. Also that bad weather was more common when the whole level of the barometer was low, independent of its motion one way or the other, than when the level was high. But as with prognostics, so with these indications, many failures. occurred. Sometimes rain would fall with a high or rising barometer, and sometimes there would be a fine day with a very low or falling glass. No reason could be given for these apparent exceptions, and the whole science of barometric readings seemed to be shrouded in mystery.

STATISTICS.

The science of probabilities came into existence about the commencement of this century, and developed the science of statistics. By this method the average readings of meteorological instruments, such as the height of the barometer or thermometer, or the mean direction and force of the wind, at any number of places were calculated, and the results were sometimes plotted on charts so as to show the distribution of mean pressure, temperature, etc., over the world.

By this means a great advance was made. Besides giving a numerical value to many abstract quantities, the plotting of such lines as the isothermals of Dové conclusively showed that many meteorological elements hitherto considered capricious were really controlled by general causes, such as the distribution of land and sea.

Still more fruitful were these charts as the parents of the more modern methods of plotting the readings of the

barometer over large areas at a given moment, instead of the mean value for a month or year. We shall refer to the results which have been thus obtained more fully presently. Then by tabulating statistics of the relative frequency of different winds at sea, many ocean voyages -notably those across the "doldrums," or belt of calms near the equator—were materially shortened.

Statistics also of the annual amount of rainfall became of commercial value as bearing on questions of the economic supply of water for large towns, and much valuable information was acquired as to the dependence of mortality on different kinds of weather. Of more purely scientific interest were the variations of pressure, temperature, wind, etc., depending on the time of day, or what are technically known as diurnal variations, which were brought to light by these comparisons.

This branch of the subject is known as "Statistical Meteorology," and has advanced very little since it was first developed by Dové and Kaemtz.

When the attempt was made to apply statistics to weather-changes from day to day, it was found that average results were useless. The mean temperature for any particular day of the year might be 50°, if deduced from the returns of a great many years, but in any particular year it might be as low as 40°, or as high as 60°. The first application of the method was made by the great Napoleon, who requested Laplace to calculate when the cold set in severely over Russia. The latter found that on an average it did not set in hard till January. The emperor made his plans accordingly; a sharp spell of cold came in December, and the army was lost.

It has now been thoroughly recognized that statistics give a numerical representation of climate, but little or none of weather, and that large masses of figures have been accumulated, to which it is difficult to attach any physical significance. The misuse of statistics has done much to bring the science of meteorology into disrepute.

SYNOPTIC CHARTS.

But within the last twenty years a new treatment of weather problems has been introduced, known as the synoptic method, by which the whole aspect of meteorology has been changed. By this method, a chart of a large area of the earth's surface is taken, and after marking on the map the height of the barometer at each place, lines are drawn through all stations at which the barometer marks a particular height. Thus a line would be drawn through all places where the pressure was 30′0 inches, another through all where it was 29.8 inches, and so on at any intervals which were considered necessary. These lines are called "isobars," because they mark out lines of equal pressure. When these charts were first introduced, the estimation of the value of the mean pressure was so great that, instead of drawing lines where pressure was equal at the moment, they were drawn through those places where the pressure was equally distant from the mean of the day for each place. These lines were called "is-abnormals;" that is, equal from the mean. This was, however, soon abandoned, for reasons which will be explained farther on in this work. After the isobars have been put in, lines are usually

drawn through all places where the temperature is equal at the moment. These are called "isotherms," or lines of equal temperature. Then arrows to mark the velocity and direction of the wind are inserted; and finally letters, or other symbols, to denote the appearance of the sky, the amount of cloud, or the occurrence of rain or snow. Such a chart is called a "synoptic chart," because it enables the meteorologist to take a general view, as it were, over a large area. Sometimes they are called "synchronous charts," because they are compiled from observations taken at the same moment of time.

When these came to be examined, the following important generalizations were discovered :

1. That in general the configuration of the isobars assumed one of seven well-defined forms.

2. That, independent of the shape of the isobars, the wind always took a definite direction relative to the trend of those lines, and the position of the nearest area of low pressure.

3. That the velocity of the wind was always nearly proportional to the closeness of the isobars.

4. That the weather-that is to say, the kind of cloud, rain, fog, etc.—at any moment was related to the shape, and not the closeness, of the isobars, some shapes enclosing areas of fine, others of bad, weather.

5. That the regions thus mapped out by isobars were constantly shifting their position, so that changes of weather were caused by the drifting past of these areas of good or bad weather, just as on a small scale rain falls as a squall drives by. The motion of these areas was

found to follow certain laws, so that forecasting weatherchanges in advance became possible.

6. That sometimes in the temperate zone, and habitually in the tropics, rain fell without any appreciable change in the isobars, though the wind conformed to the general law of these lines.

Observation also showed that, though the same shapes of isobars appear all over the world, the details of weather within them, and the nature of their motion, are modified by numerous local, diurnal, and annual variations. Hence modern weather science consists in working out for each country the details of the character and motion of the isobars which are usually found over it; just as the geologist finds crumplings and denudation all over the world, and works out the history of the physical appearance of his own scenery by studying the local development of these agencies.

So far the science rests on pure observation—that such and such wind or weather comes with such and such a shape of isobars. But it has been found, still farther, that the seven fundamental shapes of isobars are, as it were, the product of so many various ways in which an atmosphere circulating from the equator to the poles may move. Just as the motion of a river sometimes forms descending eddies or whirlpools, sometimes backwaters in which the water is rising upwards, or yet at other times ripples in which the circulation is very complex, so it now appears that the general movement of the atmosphere from the equator to the pole sometimes breaks up into a rotating and descending movement round that configuration of isobars known as an anticyclone, some