means, whatever it may be, by which the distribution of moisture on the surface of the earth takes place, is most skilfully adapted to the purposes of organic life.

Another property which has been said to belong to the principle of electricity, is the assistance which it affords to the processes of vegetation. While these processes proceed, it is ascertained, by some late experiments, that there is a constant circulation of this fluid, if it deserves that name, between plants and the atmosphere, and there is, therefore, reason to believe, that this circulation is essential to the growth and health of the former. Various contradictory experiments, however, have been made on this subject, producing conflicting theories, and not ending in any very satisfactory result. The Abbé Nollet and the Abbé Bertholon, both made experiments, which seemed to prove that the artificial application of electricity considerably accelerated the vegetable process, and rendered it more vigorous; and the latter took so strong a view of this subject, that he seriously proposed the erection of what he named electro-vegetometers, or thunder rods, for bringing down the electricity of the atmosphere to the earth, for the purpose of fertilizing the soil. After describing his plans, he thus expresses himself:- "By these means we shall have an excellent vegetable manure, or nourishment, brought down, as it were, from heaven, and that, too, at so easy an expense; for, after the construction of this instrument, it will cost nothing to maintain it it will be, moreover, the most efficacious you can employ; no other substance being so active, penetrating, or conducive to the germination, growth, multiplication, or reproduction of vegetables." Abbé's views, however, appear to be visionary; and the experience of several other philosophers is far from confirming the effects above-mentioned, so that, nothing more can be positively affirmed, from actual experiment, than the existence of a circulation of electricity in plants during vegetation. Further experiments are required to elucidate this very interesting subject.

The

It is now generally understood, that the phenomena of electricity depend on the existence of two fluids, of

different but corresponding qualities. Into this subject it scarcely lies within my province to enter; and all that is needful to be noticed is, that there is a negative and a positive electricity, each possessing peculiar laws, by whatever theory these laws may be accounted for. These agents, or properties, give rise to the remarkable natural phenomenon of thunder. This has been ascertained abundantly by direct experiment; and it may be held as proved, that the explosion takes place in consequence of a communication between two substances, of which the one is positively and the other negatively electrified. This chiefly happens in certain conditions of the clouds, when the equilibrium of the atmosphere has been disturbed. When different currents of air bring into contact two clouds differently electrified, the consequence is, that a sudden interchange of the electric principles takes place, which gives rise at once to the thunderclap and the flash: a similar effect is produced by the electric or non-electric state of the earth, when the atmosphere is in an opposite state. The lightning, in this case, does not flash from cloud to cloud, but descends in a thunderbolt to the earth, or issues from it in the form of a brilliant flame. Under any of these circumstances thunder is produced; but it is only when the interchange is with the ground, that there is danger of fatal consequences to organized existences. There is another form which the electric matter frequently assumes in the fine evenings of summer. It is called sheet-lightning, and bursts generally from behind a dark cloud near the horizon, in a broad and vivid flash, never accompanied with the sound of an explosion. In all these instances, the intention is, that the atmosphere shall be restored to a salubrious state, by carrying the fluid from places which are overcharged, to others which are deficient.

Although we do not know the precise relations between electricity and the other elements which constitute the atmosphere, yet, so far as we can penetrate, these relations are highly salutary, and those who best know the properties of each, will be most ready to acknowledge, that there must be some wise and wonderful adjustment,

some mysterious, but most necessary regulating principle, by which the one has been rendered compatible with the useful operation of all the rest. With regard to thunder, in particular, the effects of which are sometimes so tremendous, it is useful to remember, that this striking phenomenon is the result of an effort of Nature to recover itself, and restore the equilibrium of its constitution, and may therefore be compared to the crisis of a fever, and various other indications of the vis medicatrix [healing power] in the animal frame. It is awful but beneficial.

The deep lowering gloom of the thundercloud," says Mr. Whewell, "the overwhelming burst of the explosion, the flash from which the steadiest eye shrinks, and the irresistible arrow of the lightning, which no earthly substance can withstand, speak of something fearful, even independently of the personal danger which they may whisper. They convey, far more than any other appearance does, the idea of a superior and mighty power, manifesting displeasure and threatening punishment. Yet we find that this is not the language that they speak to the physical inquirer: he sees these formidable symptoms only as the means or the consequences of good. What office the thunderbolt and the whirlwind may have in the moral world, we cannot here discuss; but certainly he must speculate as far beyond the limits of philosophy as of true piety, who pretends to have learnt that there their work has more of evil than of good. In the natural world, these apparently destructive agents are, like all the other movements and appearances of the atmosphere, parts of a great scheme, of which every discoverable purpose is marked with beneficence as well as wisdom."*

FIRST WEEK-FRIDAY.

CLOUDS.

An important part of the machinery of the atmosphere is the formation of clouds, and their condensation into * Whewell's Bridgewater Treatise, p. 112.

rain. We have elsewhere noticed the immense and constant evaporation which takes place from the land and sea, and the great quantity of moisture which is, in consequence, at all times floating around and above us. This moisture is sometimes held in solution in the atmosphere, and then it remains invisible, or can only be observed by an undulating motion, apparent in warm and clear weather, near the surface of the earth, occasioned by the constant change in the density of the medium, as it rises in steam from the soil. But when the water, mingled with the atmosphere, comes in contact with a stream of cold air, which reduces its temperature below the point of deposition, it is immediately condensed, and passes into the form of mist; and this mist, when produced at some distance above the ground, takes the appearance of a cloud.

As soon as a cloud is formed, its increased specific gravity will cause in it a tendency to descend; but, as it falls, it will probably pass into a warmer region, where it will again be evaporated, and pour its mixture upward till it once more reach a temperature sufficiently low to recondense it, when it will again begin to sink. This oscillation, however, will cease to take place in circumstances where the temperature and moisture are so balanced, that the evaporating process is on the point of commencing, or rather has actually begun; and the situation where this condition occurs, is generally found to be between two or three miles above the surface of the earth. Here, if a further reduction of temperature suddenly or extensively take place, assisted by the influence of electricity, more copious drops are produced, and the cloud, condensed into rain, falls in that form to the ground.

The changes I have mentioned are doubtless greatly promoted by opposite currents of wind, which, coming in contact with each other, when of different temperatures, and therefore differently charged with humidity, destroy the equilibrium, and produce rain. These currents, again, are produced chiefly by an unequal distribution of heat, which causes one column of air to ascend, on account of its superior lightness, and another to take

a downward course, on account of its gravity. It is thus that an interchange of air is constantly taking place between the equator and the poles. This disturbance has an extensive influence; because, when any considerable portion of the air is removed, the surrounding atmosphere must rush in to fill the vacant place, and thus currents are formed in all directions; while these, again, in their passage, producing condensation or evaporation of moisture, as has been said, by that very process, alter the equilibrium of the air, and increase or diminish the force of the wind. There is thus a very complicated operation continually going on in the atmosphere, which forms the machinery of the weather.

The circumstance to which we have at present to attend, is the formation of clouds; and I have to state, with regard to this subject, that there is here, as in every other atmospherical arrangement, abundant proof of providential superintendence. There are, indeed, so many and such powerful agents in operation, in constant opposition or combination, that it may be difficult, if not impossible, to trace the law by which the effect is produced; but that the alternations of clouds and sunshine, of moisture and drought, taken at an average, are salutary to vegetation, no person need be informed.*

* "In the midst of all this apparent confusion, however," says Mr. Whewell, after enumerating the effects produced by the different laws regulating air and evaporated moisture, "we can see much that we can understand; and, among other things, we may notice some of the consequences of the difference of the laws of temperature, followed by steam and by air in going upward. One important result is, that the atmosphere is much drier, near the surface, than it would have been, if the laws of density and temperature had been the same for both gases. If this had been so, the air would always have been saturated with vapor. It would have contained as much as the existing temperature could support, and the slightest cooling of any object would have covered it with a watery film like dew. As it is, the air contains much less than its full quantity of vapor. We may often cool an object ten, twenty, or thirty degrees, without obtaining a deposition of water upon it, or reaching the dew-point, as it is called. To have had such a dripping state of the atmosphere as the former arrangement would have produced, would have been inconvenient, and, so far as we can judge, unsuited to vegetables, as well as animals. No evaporation from the surface of either, could have taken place under such conditions."Whewell's Bridgewater Treatise, pp. 104, 105.