Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/230

Rh 216 the ellipticity was actually calculated by him, long before any measurement had conﬁrmed such a conclusion. The tendency of modern research is thus to give proba- bility to the conception that not only in our own solar system, but throughout the regions of space, there has been a common plan of evolution, and that the matter diffused through space in stars, nebulze, and systems is substantially the same as that with which we are familiar. Hence the study of the structure and probable history of the sun and the other heavenly bodies comes to possess an evident geological interest, seeing that it may yet enable us to carry back the story of our planet far beyond the domain of ordinary geological evidence, and upon data not less reliable than those furnished by the rocks of the earth’s crust. II. THE MOVEMENTS or THE EARTII IN THEIR GEOLOGICAL REL-i'r10.'s. We are here concerned only with those aspects of the earth’s motions which materially inﬂuence the progress of geological phenomena. 1. 11’ota.tion.——In obedience to the impulse communicated to it at its original separation, the earth rotates on its axis. This movement is completed in about 24 hours, and to it is due the succession of day and night. So far as observa- ti.)n has yet gone, this movement is uniform, though recent calculations of the inﬂuence of the tides in retarding rota- tion tend to show that a very slow diminution of the angular velocity is in progress. This velocity varies relatively in different places, according to their position on the surface of the planet. At each pole there can be no velocity, but from these two points towards the equator there is a continually increasing rapidity of motion, till at the equator it is equal to a rate of 507 yards in a second. ' T 0 the rotation of the earth are due certain remarkable inﬂuences upon currents of air, which circulate either towards the equator or towards the poles. Currents which move from polar latitudes travel from parts of the earth’s surface where the velocity of rotation is small to others where it is great. Hence they lag behind, and their course is bent more and more westward. An air current quitting the north polar or north temperate regions as a north wind is deﬂected out of its course and becomes a north-east wind. On the opposite side of the globe a similar current setting out straight for the equator is changed into a south-east wind. This is the reason why the well- known trade-winds have their characteristic westward de- ﬂexion. On the other hand, a current setting out northwards or southwards from the equator passes into regions having a less velocity of rotation than it possesses itself, and hence it travels on in advance and is gradually deﬂected eastward. The aerial currents blowing steadily across the surface of the ocean produce currents in its waters which have a westward tendency communicated to them indirectly from the effect of rotation. A certain deﬂexion is said to be experienced by such rivers as ﬂow in a meridional direction, like the Volga. Those which ﬂow polewards are asserted to press upon their eastern rather than their western banks, while those which run in the opposite direction are stated to be thrown mere against the western than the eastern. The reality of this action may be doubted. 2. Revolution.-—Besi(les turning on its axis the globe performs a movement round the sun, termed revolution. This movement is accomplished in rather more than 365 days. It determines for us the length of our year, which is, in fact, merely the time required for one complete revolu- tion. The path or orbit followed by the earth round the sun is not a perfect circle but an ellipse, with the sun in one of the foei, the mean distance of the earth from the sun GEOLOGY [1, COSMICAL. being 92,400,000 miles. By slow secular variations the form of the orbit alternately approaches and recedes from that of a circle. At the nearest possible approach between the two bodies, owing to change in the ellipticity of the orbit, the earth is 14,368,200 miles nearer the sun than when at its greatest possible distance. These maxima and minima of distance occur at vast intervals of time. The last considerable eccentricity took place about 200,000 years ago, and the previous one more than half a million of years earlier. Since the amount of heat received by the earth from the sun is inversely as the square of the distance, eccentricity must have had in past time much effect upon the climate of the earth, as will be pointed out further on (section 7, p. 218). 3. Precession of the Eqm'noxcs.—If the axis of the earth were perpendicular to the plane of its orbit, there would be equal day and night all the year round. But it is really inclined to that plane at an angle of 23.‘_.°. Hence our hemisphere is alternately presented to and turned away from the sun, and in this way brings us the familiar alter- nation of the seasons—the long days of summer and the short days of winter. Again, were the earth a perfect sphere of uniform density throughout, the position of its axis of rotation would not change. hit owing to the pro- tuberance along the cqua tori-al regions, the attraction ehiclly of the sun and moon tends to pull the axis aside, or to make it describe a conical movement like that of the axis of a top round the vertical. Hence each pole points succes- sively to different stars. This movement, called the preces- sion of the cquinoxes, in combination with other planetary movements, completes its cycle in 21,000 years. At present the winter in our northern hemisphere coincides with the earth’s approach to the sun, or pm-i/zelz'o2z. In 10,500 years hence it will take place when the earth is at the farthest part of its orbit from the sun, or in up/u-_/ion. This movement acquires great importance when considered in connexion with the secular variations in the eccentricity of the orbit (see section 7). 4. C’/range in the 0blI'qm'ty of the Ecliptic.——The angle at which the axis of the earth is inclined to the plane of its orbit does not remain strictly constant. It oscillates through long periods of time to the extent of about a degree and a half, or perhaps a little more, on either side of the mean. According to Dr Croll,1 this oscillation must l1ave consider- ably affected former conditions of climate on the earth, since, when the obliquity is at its maximum, the polar regions receive about eight and a half days more of heat than they do at present—tl1at is, about as much heat as lat. 76° enjoys at this day. This movement must have augmented the geological eﬂfects of precession, to which reference has just been made, and which are described in section 7. 5. Stubilz't_2/ of the 1i'a.rth’s A.u's.—That the axis of the earth's rotation has successively shifted, and consequently that the poles have wandered to diﬂ'erent points on the surface of the globe, has been maintained by geologists as the only possible explanation of certain remarkable condi- tions of climate, which can be proved to have formerly obtained within the Arctic Circle. Even as far north as lat. 81° 45' abundant remains of a vegetation indicative of a warm climate, and including a bed of coal 25 to 30 feet thick, have been found in situ. It is contended that where these plants lived the ground could not have been per- manently frozen or covered for most of the year with thick snow. In explanation of the difliculty, it .l1as been sug- gested that the north pole did not occupy its present posi- tion, and that the locality where the plants occur lay in more southerly latitudes. Without at present entering on I Croll, Trans. (Veal. Soc. f-'Ias_(/010, ii. 177.