frequently as ever, but they cease giving lightning flashes, whenever they reach the town, and they begin again to do so as soon as they have passed over it.'

Sometimes a luminous brush is seen round the point of a lightning-rod. This is the St Elmo's fire of the sailors, frequently seen at the points of masts and spars. The fall of rain and hail aids in discharging a thundercloud; so much so, that rain-drops often give a spark just before touching the ground. When this occurs at night, we have the phenomenon of 'luminous rain.'

279. Earth Currents.-The earth is a great magnet, and its surface is pervaded by lines of magnetic force; and we know (see par. 74) that it is sufficient for a conductor to cross such lines to have an electric current produced in it. Again, any two conductors differing in construction, with a fluid between them for which they have any affinity, become oppositely electrified, and are ready to maintain a current (see par. 65). Thermo-electric combinations also (see par. 75), of a more or less effective kind, must occur in almost every body, or system of bodies, in nature. We are thus surrounded on all hands by electric currents. They are for the most part feeble, and unprovided with wires or other regular channels to flow in, so that their presence is not easily detected; but they must be there. The telephone and microphone are making us alive to their existence in cases where it had not been suspected.

But, besides these local and circumscribed manifestations, more powerful currents are often observed flowing through the earth for hundreds of miles in the same direction. The cause of these has not yet been ascertained with certainty. Sudden disturbances in these currents cause what are called magnetic storms, producing rapid variations in the earth's magnetism, and

deranging the working of electric telegraphs. These storms are found to be intimately connected with the sun-spots, and to vary in frequency and intensity with them, having the same cycle of about eleven years.

280. The Aurora or Polar Lights.-The appearance of auroras is familiar to every one. That they are electrical phenomena is undoubted, but how they are caused is only matter of conjecture. Nothing is known with certainty as to the height of auroras, or even as to what it is that is rendered luminous. The geographical extent must often be vast, the same aurora being occasionally seen over not only a whole hemisphere, but nearly the whole globe. One theory ascribes the phenomenon to inductive electrical effects of changes in the earth's magnetism. This opinion is favoured by the fact that they are usually coincident with magnetic storms.

281. The Mariner's Compass consists essentially of a magnetised bar of steel, called the needle, supported horizontally on a central pivot, round which it is free to move and point in any direction. A circular card, like the dial of a clock, is fixed to the needle so as always to move along with it, and round this card are marked thirty-two equidistant points. The four cardinal points, north, south, east, and west, are indicated by their initials respectively; while the subordinate ones are marked by various letters, as NbE for north-by-east, NNE for north-north-east; and so on. To know from the compass where the true north lies, the mariner must know the declination of the magnet (see par. 54) at the particular part of the earth where he is, and this information is furnished by his sailing charts. An iron vessel becomes magnetic through the induction of the earth's magnetism, and various arrangements have to be made to guard against this disturbing influence on the compass.

CHANGES UNDERGONE BY THE EARTH'S

SURFACE IN PAST AGES.

282. Changes in the Conformation of Land and Sea.The forces still at work altering the surface of the earth have been described in several of the preceding sections; and numerous facts have been adduced, showing the vast effects thus produced in the course of geological time (see pars. 151, 152, 155-168, 177, 260-273). Wherever we see stratification we have ocular proof that that portion of the present land surface had been for ages under water, and had then been upheaved into its present position. The several strata have been arranged by geologists according to the order of their super-position and the character of the fossils they contain, into regular chronological series-Formations as they are called; and from these, as from the leaves of a record, they have learned to spell out to a considerable extent the history of the earth's crust. They can affirm, for example, with confidence that 'some of the most marked features of the globe's surface are of comparatively modern date; for neither the Alps nor the Himalayas had any existence before Eocene times. At the time the nummulitic limestone was accumulating, the sea flowed over the sites of these mountain ranges, and over the areas where in subsequent ages the Pyrenees and the Carpathians made their appearance. In short, it becomes evident that, during the deposition of the Eocene strata, Europe existed as an archipelagothe sea then covering large areas which are now dry land.' -Historical Geology, by James Geikie, F.R.S., author of The Great Ice Age. (W. & R. Chambers.)

283. Alternations of Climate.-Not only have the conformations of land and sea undergone vast changes; there are also evidences of great variations in climate. In the British Isles, and even in the Arctic regions, there are found fossil remains of kinds that now live only in tropical or sub-tropical countries. More than this, these tropical conditions seem to have repeatedly alternated with ages of glacial cold-ages when Scandinavia, like Britain, was invested with an ice-sheet which filled up the Baltic, and extended into Northern Germany. The Swiss and other glaciers of Europe greatly exceeded in size their present puny successors. All North America was covered with ice down to the latitude of New York' (J. Geikie). The cause of these alternations of climate is believed to be found in the periodic increase and diminution to which the eccentricity of the earth's orbit is subject, combined with the precession of the equinoxes. How these movements co-operate to produce the effects in question, falls to be explained in the Advanced Course of our subject.

284. Changes of Flora and Fauna.-While inorganic nature was undergoing these changes, the face of the organic world was not more fixed. When life first makes its appearance on the geological record, the forms. of the organisms were altogether different from any that now exist; and in the lapse of ages the flora and fauna of our globe have been again and again completely changed. Myriads of species of plants and animals have successively appeared and then vanished for ever. But throughout this gradual extinction of old forms of life and substitution of new, there is everywhere manifest a progress from lowly types up to more highly organised structures.

285. Earth-worms as Agents in changing the Earth's

Surface. Among the causes of change in the earth's surface must be classed the agency of worms.

In

a recent work (Vegetable Mould and Earth-worms, 1881), Mr C. Darwin has shown that the mould which covers the greater part of the dry land is the work of these apparently insignificant creatures. In making their burrows they swallow the finer mineral particles of the soil, together with the fragments of decaying organic matter which form their food. These materials are triturated in their powerful gizzards, in which small stones and other hard objects are retained to act as millstones; and the whole being reduced to a fine paste, is voided at the mouth of the burrows as worm-castings. Mr Darwin calculates that in many parts of England a weight of more than ten tons of dry earth annually passes through their bodies, and is brought to the surface on each acre of land (forming a layer one-fifth of an inch in thickness); so that the whole superficial bed of vegetable mould passes through their bodies in the course of every few years.' It is in this way that objects left on the surface of the earth work themselves downwards,' as the phrase is. When a field is laid down in pasture, however stony the surface may be at first, the stones after a few years disappear; worms excavate the earth from below them, and lay it on the surface. It is thus that many ancient ruins have been buried and have lain hid for ages.

286. How Worms promote Levelling and Denudation. Where the surface of the land is clothed with vegetation, the denuding action of the weather (see pars. 260-263) seems in a manner to be shut out. But worms are constantly bringing loose mould to the surface; this, being acted upon by gravity, by rain and wind, is constantly tending to a lower level, while the finer particles