and therefore that part of England and Ireland is warmer than on the same day of another year, when no place recorded anything as high as 50°. This is the product of the calm blue sky of an anticyclone; while the diminished temperature over Germany is due to the general thermal slope of the season, for Continental Europe does not get warm till the month of June.

If we combine all these with the other examples we have already given of temperature ranging from 100° Fahr. (40° C.) to -30° Fahr. (-35° C.) in Europe, Asia, and America, the reader will have a very fair idea of the nature of temperature-changes.

FORECASTING TEMPERATURE.

From all this it will be very evident that, though we can lay down some general laws of temperature-changes, still the modifications which occur in practice are endless. The forecaster in every country has to learn by experience the qualities of the different winds, and the power of the different radiations at each season of the year.

For instance, in Great Britain he soon learns to distinguish between the uniformly warm, close heat of winter cyclones; the oppressive, sultry heat of a summer thunderstorm; and the clear, cold air, with a hot sun, of a spring anticyclone. Any doubt which can arise as to the future course of temperature-changes depends on the same points which always make any forecasting uncertain -viz. the difficulty of knowing what the future path of the cyclones will be, or whether any new distribution of pressure is likely to set in suddenly. If the forecaster

judges rightly as to the future movements of pressuredistribution, he rarely makes a mistake as to the nature of temperature-changes which accompany them.

PRIMARY AND SECONDARY EFFECTS OF HEAT.

We will conclude with one important reflection. We know that heat is the prime mover of all atmospheric circulation; why, then, do the great local differences of temperature have so little influence on the sequence of weather? The greatest diurnal ranges are found in anticyclones, which are also associated with the steadiest weather; and in wedges, where we find strong contrasts of heat and cold, these local differences of temperature are certainly not the cause of the cyclone and rain which follow soon. At the same time, it is certain that the persistent anticyclone over Siberia during the winter months is caused by the radiation cold of that country. That is to say, we may conceive that in the general circulation of the hot air of the equator towards the pole, the direction of the currents will be profoundly modified by the surface-temperature of the earth, and that it is perhaps easier to flow over a cold surface at one season and a warm one at another.

However that may be, we are met by the apparent contradiction that, though the daily variations of temperature are undoubtedly the product of the motion of cyclones, etc., the broad situations of the areas of cyclone activity are themselves due to radiation.

The truth probably is that both inferences are correct in a modified degree, and that in this, as in every other

meteorological problem, we have to deal with a balance of influences which act and react on one another in a very complicated manner.

We have already explained the stability of a circulatory system such as a cyclone or anticyclone, and the idea that diurnal variations may merely affect the rapidity, but not the form, of the vortex system; but one observation may perhaps be noted here which probably has some bearing on the question. Our synoptic charts give surface-temperature only, but we have taken no notice of the heat of upper currents. Now, it has been discovered that over cyclones temperature diminishes from the surface upwards at the rate of about 3° Fahr. (1.5° C.) in one thousand feet; in anticyclones, on the contrary, when radiation produces frost, the air gets warmer as we ascend to a short distance, after which the temperature begins to fall as we go higher up.

What the precise significance of this may be we cannot tell, but it is interesting in connection with the manner in which pressure decreases at a slower rate over cyclones as compared with anticyclones. Further research can alone solve these problems, to which we have merely alluded to carry out our purpose of giving a picture of the state of meteorology at the present day.

CHAPTER VIII.

SQUALLS, THUNDERSTORMS, AND NON-ISOBARIC RAINS.

IN this chapter we propose to introduce the reader to details of weather totally different from any that we have hitherto described. So far we have dealt with the phenomena of wind and rain which are associated with cyclones and rapid changes of barometric pressure; now we intend to discuss changes of weather which are connected but indirectly with the distribution of surrounding pressure, and in which, if the mercury moves at all, the direction is upwards. Isobars which have been our unerring guide through the most complicated cyclonic weather will now totally fail us; and, under the heading of non-isobaric rains, we shall discuss certain rainfalls, to the origin of which we have at present but little clue. In addition to the interest which attaches to such striking manifestations of nature as squalls, thunderstorms, tornadoes, and whirlwinds, a great deal of research has been bestowed of recent years on these subjects which has not yet found its way into popular literature, and which at present is scarcely known beyond the limited circle of

professional meteorologists. We now propose to explain some of the most remarkable results which have thus been obtained.

SIMPLE SQUALLS.

If we watch the stages of gradually increasing wind, we find that as the strength rises the tendency is more and more to blow in gusts. Gradually these gusts get still more violent, and in their highest development come with a boom like the discharge of a piece of heavy ordnance. This is what sailors call "blowing in great guns," and these are the gusts which blow sails into ribbons, and dismast ships more than any amount of steady wind. These gusts only last a few minutes, but they seem to be very closely allied to the simplest form of squalls. In a true, simple squall the wind generally need not be of the exceptional violence which causes guns;" but after it has rather fallen a little, the blast comes on suddenly with a burst, and rain or hail, according to intensity, or other circumstances, while the whole rarely lasts more than five or ten minutes. At sea one often sees two or three squalls flying about at a time. Then we readily observe that over the squall there is firm, hard, cumulus cloud; that the disturbance only reaches a short distance above the earth's surface; that the squall moves nearly in the same direction as the wind; and that there is little or no shift of the wind before or during the squall. We also see that the shape of the squall is merely that of an irregular patch, with a tendency rather to be longer in the direction of the wind