Of the Nature of the Cirrus. It was necessary to defer the consideration of the nature Nature of the of this cloud, until we had developed, in a considerable de- cirrus. gree, the principles on which our theory proceeds, The reader will have seen, that we assume the fact of the slow transmission of the electric fluid through clouds: which in this, as in a former instance, we apply rather analogically than by induction; the modification in question being usually so high in the atmosphere, that the electric state of the latter, above and below it, cannot easily be found by actual experiment. Proceeding, however, on this assumption, we suppose, that the cirrus resembles in its state a lock of hair or a feather, insulated and charged; or rather, that its ar rangements result from the same cause with those of the coloured powders, which electricians project on a cake of wax, after having touched, it with the knob of a charged phial, and which fall into a variety of configurations on the sur face. Thus the cirrus, may be formed in the air out of such floating particles of water as are present, and may serve the purpose of collecting and transmitting the electric fluid. It is during the prevalence of variable winds, that the cirrus most abounds; and it is reasonable, to conclude, that the portions of air, which at these seasons are transported from place, to place, gliding over or intersecting each other, usually differ sufficiently in temperature to occasion a slight decomposition of the vapour of one of the currents, and in their electric charge sufficiently to induce a communication by means of the conducting medium so formed. Again, in the gradual cooling of a perfectly calm plate of air, situate at a great elevation, and consequently free from the occasional causes of disturbance which prevail below, it is not improbable that the separation of the caloric from the vapour, and the collection of the electrised water from the air, may go on together, by a process similar to the crystallization of salts, in which, much caloric is liberated into the medium, This opinion, at least, seems to be advanced by Kirwan, in his " Essay on the Variations of the Atmosphere," and, we may consider the vegetating cirrus as the proper example, of it. Another Nature of the Another conjecture might yet be started as to the cirrus. Cirrus. It might be regarded as a cloud wholly formed of minute piculæ of ice; since the air, at a certain elevation, is sufficiently cold throughout the year for this effect. But if it should be found, that the particles of clouds are susceptible of a rectilinear arrangement in any case at a temperature exceeding 32°, there would be no necessity for this supposition. Nature of the nimbus. If the appearances of the cirrus are as frequent and various at sea as on land, it cannot be doubted, that intelligent mariners would find their account in keeping a register of them, as connected with the changes of wind, &c., making due allowance for the change of station in different observations when under sail. The buoyancy of the cirrus seems to be most perfect during its first increase. It always follows, at length, the common course of gravity; and the change to the cirrocumulus, or cirro-stratus, which certainly depends on the state of the medium it falls into, may be ascribed to the retention or loss of the electricity. Of the Nature of the Nimbus. This phenomenon may be thought to be improperly denominated a modification of cloud, since it consists usually of a column of descending rain, snow, or hail, seen in connection with the cloud affording it. As the concluding link in the chain of atmospherical precipitation, it seems, pevertheless, most advantageously placed here; and its his tory, though far from including all that we may observe, and could wish to have explained, on the subject of rain, is more decidedly illustrative of the nature of clouds in general than that of any other modification. Moreover it is sometimes observed to be formed before the rain begins, which affords sufficient ground for considering it as a distinct modification of cloud. We owe to the bold and penetrat ing conjecture of Franklin, on the identity of lightning and the electric spark, the invention of a method of investi gating the electricity of clouds: which, in the hands of experimentalists, has since brought out a mass of facts abundantly sufficient to establish that proposition; and which also also throws considerable light on the theory of rain, and other depositions from the atmosphere. By this method the structure of the nimbus may at any time, when it passes over us, be demonstrated to be that of a natural conductor, by which the positive charge of the higher atmosphere is brought down to the Earth. For this purpose, there is 1ts electrical provided a rod of iron, or other metal, well insulated on a state shown. pillar of varnished glass, the latter being defended from rain by an inverted funnel, soldered or cemented to the part of the rod next above it. The rod should be furnished with several points of wire, a few inches long; and it need not be an elevated one for this purpose, provided the extremity is clear of other objects capable of drawing off the fluid. The charge is ascertamed by pith balls of a larger or smaller dia meter, to suit the occasion, suspended by flaxen threads, on a wire fixed into the lower part of the rod, and terminating in a ball. Near the latter it is proper to have another ball fixed on a stout wire, passing into the ground, to which the fluid, when abundant, may escape in sparks. This instrument exhibits a charge of the same kind with that of the air in which it is immersed; or, in case of rain, &c., the charge of the latter, as compared with that of the air. We Phenomena will give, in the first place, the appearance which we have described. recently observed during the passage over the rod of a nimbus of the most simple structure, having neither a cumulus nor a cirro-stratus attached to it; which moved along with the lower current through the clear atmosphere, and dis charged a shower of large opaque hail, the air below being very dry. During the approach of the cloud from the north-east, the pith-balls remained close until the spreading crown, which characterizes this modification, had arrived in the zenith. At this time, and while the shower itself was still three or four miles distant, they opened negative. As the cloud came nearer, their divergence increased, until it amounted to full two inches, at which time sparks of considerable strength might be drawn from the rod. After this the negative charge gradually went off, and the balls touched again. In a few moments the edge of the shower, mixed with a few drops of rain, arrived at the conductor, and the balls instantly opened positive, the charge gradually increasing Whence the electricity? increasing until sparks were emitted more freely than before. This charge continued during the passage of the hail, and went off gradually as soon as it was clear of the ins strument. After having closed, the balls opened again negative, and this charge increased to a considerable inten•sity, as the shower receded towards the south and south-west, after which it gradually went off: the balls closed, and finally were left slightly positive. From these facts, the reader, who is conversant in electricity, will deduce the structure of the lower part at least of the shower. He will see, that the descending hail formed a column positively electrified. This, which might be six or seven miles in diameter, was surrounded with a cylinder of negative electricity, probably extending in every direction three miles farther, and result, ing from the action of the positive centre on the dry atmos sphere, in which it was moving. Now the amount of the hail, when melted, was considerably less than ath of an inch in the rain gauge; and could the descent of the electric fluid through the whole space, have been rendered as ob vious to our senses as that of the hail, we should probably have said, that the shower consisted of fire more truly than of ice. The question that naturally presents itself is, Whence came this flood of electricity which accompanied the bail? It was not from the circumstance of the water being frozen, since a hurd shower of rain equally exhibits a charge, but with this remarkable difference, that whereas snow, sleet, and hail, are always positive, rain is found sometimes positive, *sometimes negative. The reader may consult, on this head, an extensive collection of facts in Read's. Journal of Atmospherical Electricity, "Phil. Trans." Vol. LXXXI. The probable sources of negative rain will be presently mentioned; but to return to the question of the origin of the positive charge; if we attentively consider the structure of the nimbus, it is precisely that which, from the known properties of the electric fluid, we should propose for a conductor formed to acquire the latter. If we detach from it the falling column, and extraneous clouds which usually attend its progress, it will be found to consist of a close collection of fibres, diverging from the region of the cumulus, (where, (where, it appears, the rapid union of the particles into nonelectric rain. 'Negative, as well as nonelectric rain (which sometimes Negative or falls, though strong positive and negative signs precede or follow it in the clear air) must necessarily result from the action of a central mass of cloud, in which a strong positive charge exists, on the clouds of less extent which fall in its way; and it is to be considered also, that rain, at the eleva tion in which it is formed, may be perfectly nonelectric, (i. e. it may result from the union of clouds differing in electricity, and hence uniting in rain,) yet at the moment of arriving at the Earth it may differ so much in its charge from the atmosphere below, the only standard of comparison, as to be strongly negative or positive with respect to the latter. But these considerations belong more properly to the subject of atmospheric electricity. modifications We shall conclude with a brief review of the modifications, Review of the ascending from the stratus, formed by the condensation of of cloud. vapour, on its escape from the surface, to the cumulus, collecting the water arrested in the second stage of the ascent; both probably subsisting by virtue of a positive electricity. From these proceeding, through the partially conducting |
