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rence throws doubt on the supposition that the whole of the Atlantic was at this time under a high barometric pressure. The facts of the storms of the 1st and 2d January 1855 do not, so far as I can see, offer any support to the theory of the simple mutual interference of currents generating storms.
576. An examination of weather-changes over large portions of the surface of the globe from day to day, leaves a deep and lasting conviction on the mind of the essential unity of the earth's atmosphere, and, a fortiori, the oneness of comparatively so small a portion as that of Europe, in respect of the intimate relations of its different parts, and their absolute dependence on each other. We have seen waves of temperature creeping over that continent, apparently so vast that only a mere fragment of one of them could be exhibited by the whole continent at one time ; and the same remark applies with equal force to the waves of barometric pressure which pass across it. When the pressure in the north of Europe is generally low, then the winds over the whole continent become southerly, accompanied with a general rise in the temperature ; when the pressure in the north is high, and in the south low, northerly currents and low temperatures prevail ; when the pressure in the west is generally low, but high in the east, then easterly winds prevail, and the temperature rises or falls according to the season ;* when the pressure in the east is diminished, then westerly winds prevail ; and finally, when the pressure is low over some contracted space such as the British Islands, then winds flow in upon it from all sides, giving rise to an extensive whirl in the atmosphere over that region, as the winds turn round and in upon the centre of least pressure.t
577. But of the causes of these vast atmospheric changes we are very ignorant. The general prevalence of the polar and equatorial currents may be, perhaps, satisfactorily rea
* See fig. 20, page 134, representing the pressure during the great frost of Christinas 1860.
+ See Plate VII.
soned about; but why on any particular day the polar current descends from the frozen regions, and spreads itself over Europe, and why at another time the equatorial current flows wholly or partially over that continent, the area of our observations is too contracted to show. Meteorology is eminently the science of observation and averages, and before those inquiries now raised regarding the general movements of the atmosphere can be satisfactorily and adequately discussed, it is indispensable that the field of observation be extended, so as to embrace nearly the whole of the northern hemisphere.
578. The present state of our knowledge of the science may be thus put: Given in any locality an excess or diminution of atmospheric pressure; an excess or diminution of atmospheric temperature; and an excess or diminution of atmospheric moisture ; we know the atmospheric changes which will take place in restoring the equilibrium thus disturbed, and can to a considerable extent turn this knowledge to account in predicting the weather. But as regards the specific conditions out of which those great atmospheric disturbances take their origin, we know little or nothing; and it is to acquire this all-important knowledge that we urge the extension of the field of observation, so that synchronous charts of the northern hemisphere might be constructed, which would supply the information desiderated—viz., trustworthy facts, in place of vague and unsatisfactory theorisings. CHAPTER XIII.
579. The identity of lightning and electricity was first suspected by Wall in 1708, but it was reserved to Franklin to prove it. In 1749, he suggested, as the mode of proof, the erection of pointed metallic conductors properly insulated. Acting on this suggestion, Dalibard erected near Paris a pointed iron rod, 40 feet in length, and insulated ; and on the 10th of May 1752 obtained electrical sparks from it. In June of the same year, Franklin, impatient at the delay in erecting the spire on which to place his pointed conductor, conceived the happy idea of obtaining electricity from the clouds by flying a kite. The kite was flown with a hempen string, to the lower end of which a key was attached ; and the whole was insulated by tying a silk ribbon to the key, the other end of the ribbon being attached to a post. On the approach of the thunder-cloud, he raised the kite, ana soon the fibres of the hempen string began to erect themselves and repel each other; and at last, when the rain had moistened the string, he had the intense satisfaction of drawing electrical sparks from the key. The experiment was repeated by Romas, during a thunderstorm in France, in June 1753. Instead of a string, he used fine wire (550 feet long), and obtained flashes of electrical fire, 9 or 10 feet long, and an inch in thickness, which were accompanied with a loud report. Thirty of these were obtained in one hour. In August of the same year, Professor Richmann of St Petersburg lost his life when engaged in similar experiments. In observing the electricity of clouds when the tension is great, a metallic ball must be placed near the bar, and be well connected with the ground; and care must be taken to keep at a greater distance from the bar than the ball, so that if a discharge should take place it will strike the ball and not the observer.
580. Electrometers are instruments used for indicating the electricity present in the atmosphere. A pole is erected in an open situation on a rising ground, having an insulated pointed metallic wire on the top, to which an insulated wire is attached for conveying the electricity to the electrometer in the place of observation. Fig. 47 represents Bennet's gold-leaf electrometer, which consists of a glass jar with a metallic cap, in the centre of which a wooden wedge is inserted. On each side of it a thin strip of gold-leaf, two inches long, is attached, and opposite each, tinfoil is pasted within the jar, rising a little above the lower edge of the gold-leaf, and connected below with the brass stand of the instrument. A pointed wire rests on the cap in connection with the gold-leaf. When this pointed wire receives electricity, the gold-leaves diverge, and by the degree of divergence, measured on a graduated arc, the intensity of the electricity is ascertained. A condenser is used when the electric tension is too feeble to cause the goldleaves to diverge. In Volta's electrometer, two thin blades of straw are used instead of the gold-leaf; and in Cavallo's, two pith-balls. In Henley's quadrant electrometer, fig. 48, a semicircle of ivory is fixed upon a rod rising from a stand, from the centre of which a pithball is hung by a piece of slender cane; and Fig. 50. the degree of elevation of this ball indicates the quantity of the electricity.
581. Electroscopes show whether the electricity is positive or negative. In Bohnenberger's electroscope, a gold-leaf is suspended between the poles of two “dry piles," charged with the opposite electricities; when, therefore, an electric body is brought into contact with the knob, the kind of electricity is known by the gold-leaf being attached to the opposite electricity.
582. It has been found that the atmosphere always contains free electricity, which is almost invariably positive. At Kew Observatory, during the years 1845-6-7, of the 10,500 observations, 10,176 were positive, and only 364 negative. When the sky is cloudless, the electricity is always positive; but the intensity varies with the height, being greatest in the highest and most isolated situations. Positive electricity is only found at a certain height above the ground. On flat ground it becomes manifest at a height of 5 feet. It is not found in houses, in streets, or under trees. The negative observations almost all occurred during heavy rain. When the sky is clouded, the electricity is sometimes positive and sometimes negative, according to the electrified condition of the clouds. In relation to the air the earth's surface is always negative.
583. The electricity of the air increases in intensity with the height. This was shown by an ingenious experiment made at the Great St Bernard, by Becquerel and Breschet. A silk cord, with a fine wire twisted into it, was attached to an electrometer at one end, and an iron arrow tied to the other, and was shot from a bow to the height of 250 feet. As the arrow ascended, the thin straws of the electrometer separated more and more, and at last struck against the sides of the jar. The arrow was then shot horizontally, but no increase of the electric tension was observed. This conclusion has been confirmed by flying paper kites, and sending up captive balloons into the air.
584. The electricity of the atmosphere is stronger in winter than in summer, increasing from June to January, and decreasing from January to June. It is subject to a double