than in any other quarter; and that the motion of the squall as a whole is much slower than that of the wind which accompanies the first blasts. If, at the same time, we watch our barometer closely, we find that if the squall is sufficiently strong, the mercury invariably rises—sometimes as much as one-tenth of an inch-and returns to its former level after the squall is over. No difference is observed in this sudden rise, whether the squall is accompanied with rain, hail, or thunder and lightning; and though we are unable exactly to explain why the wind sometimes takes this irregular method of blowing, we have still to do with a comparatively simple phenomenon.

THUNDER-SQUALLS.

The simplest kind of thunderstorm may more properly be described as a squall accompanied by thunder and lightning, instead of only with wind and rain. In Great Britain these thunder-squalls are very common on our extreme west and north-west coasts during the winter months, while they are very rare in Central or Eastern England at any season of the year. On a wild, stormy day, with common squalls, one or two of these, which are exceptionally violent, will be accompanied by one or two claps of thunder with lightning. The principal interest which attaches to this type of thunderstorm consists in the proof which is afforded that there is no essential difference between a common squall and another which may be associated with electrical discharge, except intensity. The look and motion of the clouds, and the sudden rise of the barometer, are identical in both cases.

We can readily conceive, since the formation of cumulus above the squall points unmistakably to the presence of an ascensional current, that when the uptake is only moderate, the condensation of vapour may take place so gradually that none of the electricity—which there is reason to believe is given off under these circumstances— is discharged disruptively; but when the uprush is so violent as to inject the moist air into strata which are so cold and dry that the electricity cannot pass off silently, then a disruptive discharge with thunder and lightning will be produced. In Western Europe this class of thunderstorm is much more common in winter than in summer, which is the reverse of what takes place with all other kinds of thunderstorm. So much is this the case

that in Iceland there are no summer thunderstorms, but only winter ones, of this simple squall type. In Norway both types occur; and the winter ones are there found to be the most destructive, because they are lower down, and therefore the lightning is the more likely to strike buildings. In that country, however, the summer thunderstorms are not nearly so violent as in more southern latitudes.

BAROMETER IN SQUALLS AND THUNDERSTORMS.

We have just mentioned that the barometer usually rises just as the rain of a squall or thunderstorm strikes a place, and this is as true on the Equator as on the Arctic Circle. Since this fact is of great importance in the discussion of the more complicated phenomena that are called line-squalls, we will devote a few paragraphs to

the elucidation of the details of these barometric fluctuaWe can do this best

tions.

by an actual example.

In Fig. 48 we give a photographic engraving of the barometer-trace given by the author's barograph in London, on May 18, 1878. The original was recorded on smoked paper, and is here reproduced by photography, absolutely untouched by the engraver. By this

means

the most delicate fluctuations are faithfully rendered, and those who are familiar with sensitive self-recording instruments will readily recognize that characteristic uneasiness of the whole trace, which can never be copied by hand. In the figure the vertical lines represent intervals of six hours, while the horizontal lines indicate a difference of 0.5 inch of mercury. Confining our attention to the right-hand portion only of the diagram, we have

FIG. 48.-Barometer in thunderstorms.

to note, soon after midnight of May 17, a small curious dip of the barometer, followed immediately by a sudden rise.

This is marked a, and it occurred during a thunderstorm. Just before 6 a.m., and for some time after, we find the still more remarkable fluctuations marked b. These were also associated with a series of thunderstorms, none of which were particularly violent. Still later, about 8 a.m., we see the singular dip marked c. This occurred with gloomy, threatening weather, but neither with wind, rain, nor thunder, at the place of observation in London.

The chart for the day at 8 a.m. showed that a series of small secondaries lay over Great Britain, but there were no bends in the isobars that would explain such curious barometric oscillations.

The origin of this characteristic rise of the barometer in squalls and thunderstorms is at present unknown. It has been suggested that it is due to a rush of air, carried down by the rain. That such is partially the cause is extremely probable, for we sometimes see a small rise under a heavy splash of rain without either thunder or wind. But it is equally certain that this downrush does not entirely explain the phenomenon, for sometimes a rise occurs without any rain at all, or of an amount which bears no relation to the heaviness of the fall. Still more puzzling are the small dips of the mercury which we occasionally find with thunderstorms, and of which some examples are given in our last figure. These dips are more rare than the rises, and though in most cases they are, as in this example, more or less associated with the rises, still they occasionally occur alone. In the first dip shown in our last figure, about 1 a.m., the depression was associated with a storm; while in the second case, about 8 a.m., no storm or rain occurred locally, though un

doubtedly storms were in existence not far off. We are, therefore, almost driven to the conclusion that some of these curious fluctuations of the barometer must be due to a sort of true wave-action, through which the disturbance, caused perhaps by falling rain, may be propagated by the elasticity of the air to some distance from the place of original disturbance. In connection with this idea of air being brought down by falling rain, we may notice that very striking effects are sometimes observed in avalanches of snow, which always bring down an immense amount of imprisoned air with them. It is usually found that persons caught in the blast of the avalanche have their clothes torn into ribbons. The suggestion has also been made that if rain is the product of the condensation of an ascensional current of air, then the more violent the uptake, the greater must be the reaction downwards; but, unfortunately, our knowledge of the dynamics of air in motion is not sufficiently advanced to enable us to say exactly what the nature of pressure would be under these circumstances.

But though we cannot altogether explain the origin of these barometric fluctuations, we know enough to say that they are of a totally different nature from any motion of the mercurial column due to the action of cyclones or the propagation of isobars over any station. When, then, we see on a barogram these peculiar irregularities, we can at once infer that they are the product of squalls or thunderstorms, and not of cyclones, and so far we are enabled to increase our knowledge of the method of reading barograms, to which we have already given so much attention. These dips and rises