than a bright day, because the rays of the sun are obstructed; but if there is little wind, a cloudy day is more oppressive to men than one with sunshine. Near the equator there is very little diurnal radiation of any kind, owing to the excessive amount of vapour in the air.

We may sum up all the effects of heat and cold briefly thus: In winter wind, cloud, and rain in temperate regions tend to raise the temperature, as they check cold radiation; calm, on the contrary, induces hard frost. In summer wind, cloud, and rain are cooling influences, as they check hot radiation; calm, on the contrary, is then hot, because it allows full play for the

sun's rays.

We may, in fact, look at the opposing forces of hot and cold radiation as in a state of constant conflict. The rotundity of the earth always weakens the power of the sun in the north. Water-vapour in some shape forms, as it were, a blanket for the earth, and saves her from being burnt up and frozen alternately. The incessant circulation of the atmosphere sometimes eddies in a cyclonic form, and develops dense cloud, which shields the earth from the radiation of the season and latitude; at other times the circulation of the air eddies downwards in an anticyclone, and the clear, dry, calm atmosphere gives full play to radiation, and some extreme of heat or cold is then developed.

The task of the meteorologist is to trace how the varying forms of atmospheric circulation modify the distribution of heat and cold over the world from day to day, by the application of the general laws we have just laid down.

!

EXAMPLES OF DAILY TEMPERATURE-CHANGES OVER

EUROPE.

For instance, let us consider the changes of temperature which occurred in Europe on the three days, February 26-28, 1865; that is, during three of the days for which we shall give synoptic charts in Figs. 68-70, when discussing the westerly type of weather. We have to explain now why European temperature varied as it did on those days.

The isotherms for the period in question are given in Figs. 45-47. In all of these there is a thermal gradient

FIGS. 45 and 46.-Isotherms in Europe for three consecutive days.

from south-west to north-east instead of a slope towards the north-west, as undisturbed natural isotherms should have at eight o'clock in the morning-the hour for which the charts are constructed. The reason for this broad feature is that in winter a continental area is always

colder than a sea-surface, and therefore, whatever smaller variations may occur from day to day, the general slope of temperature will always be from frozen Russia towards moisture-bathed Portugal. This feature belongs to the season, and is found in every chart; what we have now to explain is the fluctuation

in the position of the isotherms caused by the varying development of heat and cold locally.

Glancing at both the synoptic charts of pressure and temperature, we see that on the morning of February 26 a V-depression covered Great Britain, with warm south-west winds in front. Straight isobars lay over Scandinavia, an anticyclone stretched over

Western Europe from the Atlantic, and a calm col lay over Russia. From all this England was warm, as shown by the projection northwards of the isotherm of 41° Fahr. (5° C.) ; Continental Europe and Russia were very cold. In the latter country, -4° Fahr. (−20° C.) is reported, and local patches of cold as low as 4° Fahr. (-15° C.) are reported in different parts of France and Germany. These should be noticed, for they are most characteristic of the abrupt local variations of temperature which we often find are caused by local differences in radiation. They are identical with all the frosts which occur in Great Britain,

to which we have before alluded. Observe also that they have no influence whatever as a cause of weather; they are the product of the general circulation of the atmosphere, allowing free play for radiation, not a cause of that circulation themselves, though the influence of the general thermal slope from Russia to Portugal is an important factor in determining the path of the cyclones. By next morning the British V and Scandinavian straight isobars have formed a well-defined cyclone, some secondaries appear over various parts of Europe, while a calm wedge covers England. England is, therefore, colder than on the previous day, because of the radiation of the wedge, and the isotherm of 41° Fahr. (5° C.) has retreated southwards. Russia and Continental Europe are much warmer, because the cyclones and secondaries have destroyed radiation.

By the morning of the third day the Scandinavian cyclone has died out, but a new one lies over the north of Scotland. Secondaries still cover the greater portion of Europe, but in Russia the weather would be calmer. From this it results that England is warmer, so that the isotherm of 41° Fahr. (5° C.) projects northwards again; Continental Europe is a little colder, without many local frosts; Russia is a great deal colder, but not so cold as the first day, for the conditions are less favourable to radiation. None of these cyclonic changes reach so far south as Spain, and therefore we see the isotherm of 50° Fahr. (10° C.) scarcely alters its position during the three days.

We may also put the changes of temperature over Europe in a very striking light by looking at the isotherm

of 32° Fahr. (0° C.). On the first day it stretches from Belgium to the Black Sea; the second day it has been driven back almost to the Gulf of Bothnia and to Poland; the third day it has advanced again, but not so far as on the first day. So on the conflict would go between the frost and sun till the sun at last drove that isotherm out of Europe. In the autumn the battle would be renewed; but then the sun would be beaten, and frost remain supreme for several months in the more northern portions of that continent.

Had our limits permitted, we would have given examples of the reversal of radiation effect which occurs in summer, when an anticyclone means heat instead of cold. Then we may often find England hotter than France, for if the calm centre of an anticyclone lies over the former country, the sun's rays have more power there than in the more windy southern edge, which would cover France under these circumstances.

We may, however, refer again to Figs. 21 and 22, which relate to the same day of May-the 17th-in two different years, and in which diurnal variation is allowed for by constructing the charts at the same hour. On the first day (Fig. 21) a cyclone covers Great Britain, and the isotherm of 50° Fahr. (10° C.) reaches to the north of Scotland and Denmark, under the influence of southerly winds and a cloudy sky.

On the second day (Fig. 22) the isotherm of 50° Fahr. (10° C.) runs north and south down England, and a corner of the line of 40° Fahr. (5° C.) appears over Northern Germany. This shows that to the west of the isotherm of 50° the temperature rises towards 60° Fahr. (15° C.),