There are other forms of electrical discharges not distinctly connected with the then existing condensation of moisture. What the sailors call St. Elmo's fire—a brush of electric light from the mast tops and other projections of the ship—indicates the passage of electrical energy between the vessel and the atmosphere. Similar lights are said sometimes to be seen rising from the surface of the water. Such phenomena are at present not satisfactorily explained. Perhaps in the same group of actions comes the so-called "Jack-o'-lantern" or "Will-o'-the-wisp" fires flashing from the earth in marshy places, which are often described by the common people, but have never been observed by a naturalist. If this class of illuminations really exists, we have to afford them some other explanation than that they are emanations of self-inflamed phosphoretted hydrogen, a method of accounting for them which illogically finds a place in many treatises on atmospheric phenomena. A gas of any kind would disperse itself in the air; it could not dance about as these lights are said to do, and there is no chemical means known whereby it could be produced in sufficient purity and quantity from the earth to produce the effects which are described.[3]
In the upper air, or perhaps even beyond the limits of the field which deserves the name, in the regions extending from the poles to near the tropics, there occur electric glowings commonly known as the aurora borealis. This phenomenon occurs in both hemispheres. These illuminations, though in some way akin to those of lightning, and though doubtless due to some form of electrical action, are peculiar in that they are often attended by glows as if from clouds, and by pulsations which indicate movements not at electric speed. As yet but little is known as to the precise nature of these curious storms. It has been claimed, however, that they are related to the sun spots; those periods when the solar spots are plenty, at intervals of about eleven years, are the times of auroral discharges. Still further, it seems probable that the magnetic currents of the earth, that circling energy which encompasses the sphere, moving round in a general way parallel to the equator, are intensified during these illuminations of the circumpolar skies.
Geological Work of Water.
We turn now to the geological work which is performed by falling water. Where the rain or snow returns from the clouds to the sea, the energy of position given to the water by its elevation above the earth through the heat which it acquired from the sun is returned to the air through which it falls or to the ocean surface on which it strikes. In this case the circuit of the rain is short and without geological consequence which it is worth while to consider, except to note that the heat thus returned is likely to be delivered in another realm than that in which the falling water acquired the store, thus in a small way modifying the climate. When, however, the precipitation occurs on the surface of the land, the drops of frozen or fluid water apply a part of their energy in important geological work, the like of which is not done where they return at once to the sea.
Fig. 10.—Showing the diverse action of rain on wooded and cleared fields, a, wooded area; b, tilled ground.
We shall first consider what takes place when the water in the form of drops of rain comes to the surface of the land. Descending as they do with a considerable speed, these raindrops apply a certain amount of energy to the surface on which they fall. Although the beat of a raindrop is proverbially light, the stroke is not ineffective. Observing what happens where the action takes place on the surface of bare rock, we may notice that the grains of sand or small pebbles which generally abound on such surfaces, if they be not too steeply inclined, dance about under the blows which they receive. If we could cover hard plate glass, a much firmer material than ordinary stone, with such bits, we should soon find that its surface would become scratched all over by the friction. Moreover, the raindrops perceptibly urge the small detached bits of stone down the slopes toward the streams.
If all the earth's surface were bare rocks, the blow of the raindrops would deserve to be reckoned among the important influences which lead to the wearing of land. As it is, when a country is in a state of Nature, only a small part of its surface is exposed to this kind of wearing. Where there is rain enough to effect any damage, there is sure to be sufficient vegetation to interpose a living and self-renewed covering between the rocks and the rain. Even the lichens which coat what at first sight often seems to be bare rock afford an ample covering for this purpose. It is only where man bares the field by stripping away and overturning this protecting vegetation that the raindrops cut away the earth. The effect of their action can often be noted by observing how on ploughed ground a flat stone or a potsherd comes after a rain to cap a little column. The geologist sometimes finds in soft sandstones that the same action is repeated in a larger way where a thin fragment of hard rock has protected a column many feet in height against the rain work which has shorn down the surrounding rock.
When water strikes the moistened surface it at once loses the droplike form which all fluids assume when they fall through the air.[4]