maximum and minimum each day. Saussure and Schübler have shown that the first maximum occurs from 7 to 8 a.m. in summer, and from 10 A.M. to noon in winter; it then falls slightly to the first minimum between 5 and 6 P.M. in summer, and between 2 and 3 P.M. in winter, or shortly after the period of the maximum temperature; it rises to a second maximum a little after sunset, and then decreases to a second minimum, which occurs about daybreak. The daily march of the electric tension is best marked in clear settled weather.

585. Sources of Atmospheric Electricity.-1. Evaporation. -Electricity is produced when impure water is evaporating, or water in which some degree of chemical decomposition takes place, none whatever being produced by the evaporation of pure water. Evaporation from water containing an alkali or a salt gives off negative electricity to the air, and leaves positive electricity behind; but when the water contains acid, positive electricity is given off, and negative is left behind. Hence it is supposed that seas, lakes, and rivers are abundant sources of electricity, particularly of the positive sort. 2. Vegetation. The vegetable kingdom is also a source of electricity, (1) from the evaporation going on by which water is separated from the sap of the plants, and (2) from the giving off of oxygen gas during the day, and carbonic gas during the night. In these cases positive electricity arises from the plants, and negative is left behind. 3. Combustion. -During the process of burning, bodies give off positive electricity, and become themselves negatively electrified. This is frequently seen on a grand scale during volcanic eruptions. 4. Friction.-Wind, by the friction it produces upon terrestrial objects, the particles of dust, and the watery particles in the vesicular state which it carries with it, contributes to the electricity of the air. Electricity is not generated, if the moisture be in the form of pure vapour.

586. Effect of the Condensation of Vapour.-The following are Sir John Herschel's views on the relations subsisting between the condensation of vapour and atmospheric electricity. When a great multitude of the ultimate molecules of

vapour are condensed by cold into a drop or snow-spangle, however minute, that drop collects and retains on its surface the whole electricity of the molecules from which it is formed. Suppose a number, say 1000, of such globules to coalesce into one, the electric contents, being the sum of those of the elementary globules, will be increased one thousandfold, and, being spread entirely over the surface, will have a tenfold density or tension. This view appears to explain the amount of electricity observed in the lower stratum of air when dew is being deposited, and the highly electrical state of fogs and clouds. It also explains the annual fluctuation; for since in winter the condensation of vapour is greater and occurs with greater frequency than in summer, the average quantity of electricity, as observed, will be greater in winter than in summer. At this season, also, the south-west winds acquire their greatest frequency. The daily fluctuation is similarly explained. When the sun has risen, and from the increasing temperature evaporation is most active, the first maximum period of electricity occurs; but when the air becoming still warmer is able to hold its vapour in solution, and evaporation has become feebler, the intensity decreases. Again, after sunset, when dew is most copiously deposited, the second maximum period is attained; and lastly, toward sunrise, when little dew is deposited, and evaporation is also small, the second and chief minimum period occurs. But great differences of opinion exist in regard to the electricity of the atmosphere, and many of the phenomena, especially those observed during storms, have not yet been explained. A sufficient number of trustworthy observations are the great desideratum; and not till some instrument has been devised of such a description that the observations made with it in different places may be comparable with each other, and the price be at the same time no barrier to its general use as a meteorological instrument, can we hope to be in a position adequately to investigate the subject.

THUNDERSTORMS.

587. The thunderstorm is probably originated in the same manner as cloud and rain-viz., by the condensation of vapour; but it differs in the condensation being more copious and more rapid, so as to bring about an accumulation of a sufficient quantity of electricity. If the condensation be not copious, the electricity will be too weak; and if not sudden, it escapes before collecting in sufficient quantity for a discharge. Hence each flash of lightning is almost always immediately followed by a heavy fall of rain or hail, which seems to begin to fall simultaneously with the electric discharge. On the other hand, when the centre of a great thunderstorm which occurred at Edinburgh a little before one A.M. of the 4th September 1867, was passing over the Calton Hill, Alexander Wallace, Royal Observatory, informs me that simultaneously with a vivid discharge of lightning and a loud peal of thunder, the rain, which had been previously falling in torrents, suddenly ceased.

588. Thunderstorms occur most frequently within the tropics, and diminish in frequency towards the poles. They are also more frequent in summer than in winter; during day than during night; after mid-day than before it; and in mountainous countries than in plains. Within the tropics they prevail most in the region of calms and during the rainy season; and least in arid deserts and during the dry season. Before the storm bursts, the air is felt to be exceptionally warm and stifling, and peculiarly so when the storm occurs in winter or during night; and after it has passed, the temperature falls generally to a very marked extent. Thunder and lightning often accompany whirlwinds. During thunderstorms sudden changes from positive to negative electricity, and vice versa, take place. The negative state is probably induced by the friction of the rain-drops on the air as they fall through it, Faraday having shown that the friction of drops of pure water develops negative electricity in the sub

stance rubbed; and the positive state is a return to the normal condition after the action of the rain-drops has ceased.

589. Lightning.-Arago has divided lightning into three kinds-viz., zigzag-lightning, sheet-lightning, and ball-lightning. When the electric flash darts through the air, it takes the path of least resistance; and since the conducting power of different portions of the atmosphere is unequal, the lightning frequently appears zigzag. When branches are given off at different points of its course, the lightning is said to be forked. Sheet-lightning is the most common, appearing as a glow of light illuminating the sky. The flashes often follow each other in quick succession, and the thunder which accompanies them is low and at a considerable distance. Analogous to this is silent-lightning, frequently termed heatlightning, which generally occurs during serene summer evenings, lighting up the sky fitfully for hours, with repeated faint flashes; it is not attended with thunder. It is probable that this kind of lightning is almost always the reflection of the lightning of distant storms from the vapour of the upper regions of the atmosphere, the storms themselves being so far off that their thunder cannot be heard. In Scotland, silentlightning is a prognostic of unsettled weather, it being considered as heralding a storm which is coming up. lightning is the least common. It appears as a globular mass, moving slowly or sometimes remaining stationary, and in a short time explodes with violence. It has not yet been satisfactorily explained. When Nelson's Monument, on the Calton Hill, Edinburgh, was struck with lightning on the 4th of February 1863, the lightning was observed by Mr Wallace to approach the Monument in a manner as if wafted rapidly forward by air-currents. Professor Wheatstone has shown that the duration of a flash of lightning is less than the thousandth part of a second. A wheel was made to rotate so rapidly that the spokes were invisible; on being lighted up with the electric flash, the duration of the flash was so brief that the wheel appeared quite stationary, even though rotating with the utmost speed possible, there

Ball

being no appreciable displacement of the spokes of the wheel during the time the flash lasted.

For

590. Thunder is probably the noise produced by the instantaneous rushing of the air in filling up the vacuum left by the lightning along the path of the discharge. The sound emitted by flames is a familiar illustration of a similar phenomenon. Flashes of lightning frequently extend two or three miles in length; and since the thunder is produced at every point along its course nearly at the same instant, the prolonged rolling noise of thunder arises from the different intervals of time it takes the sound to reach the ear. since sound travels at the rate of 1118 feet per second, it is first heard from the nearest point of the flash, later and later from points more distant, so that the combined effect is a continued peal of thunder. The direction and character of the peal will depend on the length of the flash, and the greater or less obliquity of its course in relation to the observer. Reverberations from clouds and from mountains frequently heighten the effect and prolong the peal. From the rate at which sound travels, if the thunder is not heard till five seconds after the flash, the distance is about a mile. Thunder has not been heard at a greater distance than 14 miles from the flash.

Sometimes,

591. Thunderstorms are generally very local. however, they extend over a wide district. On the 11th January 1815 a thunderstorm stretched from Antwerp to Minden, or about 200 miles, and from Bonn' to Nimeguen, or nearly 75 miles. When the May monsoon of 1848 burst upon India, it was accompanied with a thunderstorm which covered a district 600 miles from north to south, and 50 miles in breadth.

592. The great proportion of electrical discharges pass into the air, or into other clouds less highly electrified; a very few only take place between the cloud and the earth. The destructive effects of this latter class are known to all. By the electric discharge innumerable lives have been destroyed, the strongest trees rent to pieces, heavy bodies displaced,