marked on it, he can put in very approximately the direction and force of the wind all over the globe.

When we have explained the relation of weather to the shapes of isobars, we shall see that he could also write down very nearly the kind of weather which would be experienced everywhere.

THE SEVEN FUNDAMENTAL SHAPES OF ISOBARS.

Then as to the shapes themselves.

In Fig. 1 we give in a diagrammatic form the broad

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FIG. 1.-The seven fundamental shapes of isobars.

features only of the distribution of pressure over the North Atlantic, Europe, and the eastern portions of the United States on February 27, 1865. omitted so as not to confuse the eye, so

Coast-lines are also are lines of latitude and longitude; but the foot-note at the bottom of the figure represents the equator, and the top of the diagram would be on the Arctic Circle. All pressures

of and under 29.9 ins. (760 mm.) are shown with dotted lines, so that the eye sees at a glance the broad distribution of high or low pressure. The whole seven fundamental shapes of isobars will be found there.

Looking at the top of the diagram, we see two nearly circular areas of low pressure, round which the isobars are rather closely packed. Such areas, or rather the configurations of isobars which enclose them, are called "cyclones," from a Greek word meaning a circle, because they are nearly circular, and, as we shall see presently, the wind blows nearly in a circle round their centre.

Just south of one of the cyclones, the isobar of 29.9 ins. (760 mm.) forms a small sort of nearly circular loop, enclosing lower pressure; this is called a "secondary cyclone," because it is usually secondary or subsidiary to the primary cyclones above described.

Further to the left the same isobar of 29.9 ins. bends itself into the shape of the letter V, also enclosing low pressure; this is called a "V-shaped depression," or, shortly, a "V."

Between the two cyclones the isobar of 29.9 ins. projects upwards, like a wedge or an inverted letter V., but this time encloses high pressure; this shape of lines is called a "wedge."

Below all these we see an oblong area of high pressure, round which the isobars are very far apart; this is called an "anticyclone," because it is the opposite to a cyclone in everything-wind, weather, pressure, etc.

Between every two anticyclones we find a furrow, neck, or "col" of low pressure analogous to the col which forms a pass between two adjacent mountain-peaks.

Lastly, as marked in the lower edge of the diagram, isobars sometimes run straight, so that they do not include any kind of area, but represent a barometric slope analogous to the sloping sides of a long hill.

We may forestall succeeding chapters so far as to say that the cyclones, secondaries, V's, and wedges are usually moving towards the east at the rate of about twenty miles an hour; but that the anticyclones, on the contrary, are usually stationary for days, and sometimes for months together.

We should also note that, though the general principles of prognostics and the broad features of the weather in each of these shapes of isobars are the same all over the world, the minute details which we intend to give now apply to Great Britain and the temperate zones only.

We will now take the five more important shapes separately, and detail the kind of wind and weather which is experienced in different parts of each of them. From this we shall be led to the explanation of the nature of popular prognostics. The account of "V's" and "cols" will be reserved for our chapter on isobars, as no special prognostics are grouped round these two forms.

CYCLONE-PROGNOSTICS.

We will begin with these as they are by far the most important. In Fig. 2 we give a diagram on which we have written in words the kind of weather which would be found in every portion of a typical cyclone; arrows also show the direction of the wind relative to the isobars and to the centre.

First let us look at the isobars. We find that they are oval, and that they are not quite concentric, but the centre of the inner one we will call the centre of the cyclone. Now observe the numbers attached to the isobars; the outer one is 300 ins. (762 mm.), the inner one 29.0 ins. (737 mm.). But suppose the outer one was the same,

but the inner one was 29.5 (755 mm.) We should then have two cyclones, differing in nothing but depth; that is, in the closeness of the isobars, or the steepness of the barometric slope. Observation has shown that under these circumstances the general character of the weather and the direction of the wind everywhere would be the same; the only difference would be that the wind would

blow a hard gale in the first, and only a moderate breeze in the second case; and that what was a sharp squall in the one would be a quiet shower in the other. This is one of the fundamental principles of synoptic meteorology -that the character of the weather and direction of the wind depend entirely on the shape of the isobars, while the force of the wind and intensity of the character of the weather depend only on the closeness of the isobars.

The difference in the details of the weather in a cyclone, or any other isobaric shape which are due to difference in the steepness of the isobars, is called a difference in the intensity of the weather. Hence, when we speak of a cyclone as being intense, we mean that it has steep isobars somewhere. The word "intensity" will occur very often in these pages, for when we come to talk about the general sequence of weather from day to day, we shall find that there is no difference between the cyclones which cause storms and those which cause ordinary weather except intensity. This is another of the fundamental principles of meteorology.

Returning now to our cyclone, the whole of the portion in front of the centre facing the direction towards which it moves is called its front, and the whole of this portion may obviously be divided into a right and left front. The other side of the centre is, of course, the rear of the cyclone. Then, as the whole cyclone moves along its course, it is evident that the barometer will be falling more or less at every portion of the front, and rising more or less everywhere in rear, so that there must be a line of places somewhere across the cyclone, where the barometer has touched its lowest point and is just going to rise.