From the same example of a fine day we can readily pass to the idea of diurnal variation. In the kind of day which we have just described we find a regular sequence from fog to blue, and back again to fog, following the course of the sun-that is, the time day.

But besides the amount of mist, every other meteorological element has a complex series of diurnal changes which depend on the time of day. For instance, both the direction and velocity of the wind have a marked diurnal period, and so has the amount of cloud, the amount of vapour, and every other component of weather. These, and many others, will form the subject of a subsequent long and rather difficult chapter. All that we have to note here is the important principle of meteorology -that the primary character of all weather is given by the shape of the isobars, whether cyclonic, anticyclonic, or otherwise; that a complex series of diurnal changes are superimposed on this, which modify, but do not alter the intrinsic quality; and that the resulting weather is the sum of the two together. Thus in a cyclone the changes of weather due to its motion are so marked and so strong that diurnal changes are often entirely obliterated; while in a calm anticyclone, where there is no motion, the uniform character of the general weather allows full play to radiation, and the diurnal changes are very prominent.

Throughout this work we shall call the character and changes of weather which are due to the shapes of the isobars, the general character and general changes, because they are caused by alterations in the general distribution of pressure over a large portion of the earth's surface. On the contrary, changes which are due to the

time of day, the season of the year, or to any local peculiarity, we shall call diurnal, seasonal, or local variations of the general character. The first are really changes, the second only variations. The reason why

many prognostics which are due to radiation and diurnal causes are signs of settled fine weather, is because in a country like England they can only occur in an anticyclone. An anticyclone means settled fine weather, not only because the weather at any moment in it is fine, but because it is usually stationary, and so there is nothing to change the existing conditions. All anticyclone prognostics fail when the anticyclone breaks up suddenly, or in the not very common case when it moves onward along a definite path.

We will now give a few prognostics due to the variations of an anticyclone in some detail.

The sky being generally clear and the air calm, the temperature is high in the day and low at night. In summer brilliant sunshine prevails during the day, and at night there is a heavy dew, and, in low-lying places, mist.

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"Heavy dews in hot weather indicate a continuance of fair weather, and no dew after a hot day foretells rain." "If mists rise in low ground and soon vanish, expect fair weather."

Fine, bright, genial weather raises the spirits and exerts an enlivening influence not only on human beings, but also on animals, birds, insects, etc. Hence the sayings

"When sea-birds fly out early and far to seaward, moderate winds and fair weather may be expected."

"If rooks go far abroad, it will be fine."

"Cranes soaring aloft and quietly in the air foreshadows fair weather."

"If kites fly high, fine weather is at hand."

"Bats or field-mice coming out of their holes quickly after sunset and sporting themselves in the open air, premonstrates fair and calm weather."

"Chickweed expands its leaves boldly and fully when fine weather is to follow."

These are merely samples of innumerable similar prognostics in all parts of the world.

In winter frost is generally prevalent in the central area of an anticyclone, accompanied frequently by fog, which is most dense in the neighbourhood of large towns. This is all due to the radiation of calm weather.

"White mist in winter indicates frost."

The wind is usually very light in force.

"It is said to be a sign of continued good weather when the wind so changes during the day as to follow the sun."

This "veering with the sun," as it is called, is the ordinary diurnal variation of the wind, which in England is only very obvious with the shallow gradients of an anticyclone. At seaside places in summer very often "the wind is in by day and out by night," which is the equivalent of the land and sea breezes of the tropics. Like the preceding prognostic, it is only in anticyclones that local currents of air, probably due to unequal heating of sea and land, can override the general circulation of the atmosphere in this country.

Sometimes in winter, on the southern side of the anti

cyclone, bitter east winds with a black-looking sky will prevail for several days together, when it may truly be said

"When the wind is in the east,

It is neither good for man nor beast."

This class of anticyclone prognostics hold good as long as the anticyclone remains stationary. Occasionally the anticyclone moves on, and is replaced by some other form of isobars; but far more frequently the anticyclone breaks up that is to say, it disappears without moving on, and is replaced by a cyclone or some other type of isobars.

WEDGE-SHAPED ISOBARS.

We have already defined wedge-shaped isobars as a projecting area of high pressure moving along between two cyclones. This wedge may point in any direction, but in practice by far the most frequently to the north. We have therefore selected such a one for the diagram of the wind and weather in an ideal wedge, which we give in Fig. 7. There the highest pressure is at the bottom of the diagram, while the wedge-shaped isobars project towards the north. On the right hand we see the rear of a retreating cyclone; on the left, the front of an advancing depression. As these two cyclones move forward, the wedge goes on between them, so that there must always be a line of stations where, after the barometer has risen owing to the onward passage of the first cyclone, the mercury has just begun to fall, owing to the advance of the second depression. This line is called the crest

of the wedge, and is marked by a dotted line in the diagram.

The wind blows round the wedge in accordance with the universal law of gradients. Thus on the east side of the wedge the wind is from north-west; in the centre it is calm; and on the west side, from south-west to south

FIG. 7.-Wedge-shaped isobar prognostics.

east, as marked by the symbols on the diagram. In practice the gradients are never steep, so the force of the wind rarely rises to above that of a pleasant breeze.

The broad features of the cloud and weather in a wedge are written across the diagram (Fig. 7). In front