Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/234

Rh 0-90 with all those causes which we have just been considering, would place Europe under a glacial condition, while at the same time the temperature of the Southern (leean would, in consequence of the eiiomious quantity of warm water received, have its temperature (already high from other causes) raised enormously. And what holds true in regard to the currents of the Atlantic holds also true, though perhaps not to the same extent, of the currents of the Paciﬁc. “ If the breadth of the Gulf-stream be taken at 50 miles, its depth at 1000 feet, its mean velocity at 2 statute miles an hour, the temperature of the water when it leaves the Gulf at 65°, and the return currrnt at 40° F.,' then, as has been shown in Climate and T-inw, chapter ii., the quantity of heat conveyed into the Atlantic by this stream is equal to oiie-fourtli of all the heat received from the sun by that ocean from the Tiopie of Cancer to the Arctic Circle.’ From principles discussed at considerable length in the chapter referred to, it is shown that, but for the Gulf-st.rcani and other currents, London would have a mean annual temperature 40° lower than at present. “ But there is still another cause which must be noticed :—a strong undercurrent of air from the north implies an equally strong upper eiurent to the north. Now if the effect of the undercurrent would be to impel the warm water at the equator to the south, the effect of the upper current would be to carry the aqueous vapour formed at the equator to the north; the upper current, on reaching the snow and ice of t.emperate regions, would deposit. its moisture in the form of snow; so that it is probable that, nowithstandiiig the great cold of the glacial epoch, the quantity of snow falling in the northern region would be enormous. This would be particularly the case during summer, when the earth would be in the perihelion and the heat at the equator great. The equator would be the fur- nace where evaporation would take place, and the snow and ice of temperate regions would act as a condenser. “ The foregoing eonsidciations, as well as many others which might be stated, lead to the conclusion that, in order to raise the mean temperature of the globe, water should be placed along the equator, and not land, as was eoiiteiide(l by Sir Charles Lyell and others. For if land be placed at the equator, the possibility of conveying the sun's heat from the equatorial regions by means of ocean currents is prevented. " Inter-Glacial Pe7'iocls.—Alli1sion has already been made to the fact that there is accumulating evidence to show that changes of climate have been recurrent, and that this alternation or periodicity goes far to prove them to be due to some general or cosmical cause. Dr Croll has iii- geniously shown that every long cold period in each hemisphere must have been interrupted by several sliorte.r warm periods, and “when the one hemisphere,” he says, “ is under glaciation, the other is enjoying a warm and equable climate. But, owing to the precession of the equinoxes, the condition of things on the two hemispheres must be reversed every 10,000 years or so. When the solstice passes the aphelion, a contrary process commences ; the snow and ice gradually begin to diminish on the cold hemisphere and to make their appearance on the other hemisphere. The glaciated hemisphere turns by degrees warmer, and the warm hemisphere colder, and this continues to go on for a period of ten or twelve thousand years, until the winter solstice reaches the perihelion. By this time the conditions of the two hemi- spheres have been reversed; the formerly glaciated hemi- sphere has now become the warm one, and the warm hemisphere the glaciated. The transference of the ice from the one hemisphere to the other continues as long as the eccentricity remains at a high value. It is probable that, during the warm inter-glacial periods, Greenland and the Arctic regions would be comparatively free from snow a.I,1,d ice, and enjoying a temperate and equable climate. 1 Sir Wyville Thomson states that in May 1873 the Challenger expedition found the Gulf-stream, at the point where it was crossed, to be about 60 miles in width, 100 fathoms deep, and ﬂowing at the rate of 3 knots per hour. This makes the volume of the stream one- fifth greater than the above estimate. .- The quantity of heat conveyed by the Gulf-stream for dis- tribution is equal to i7,479,650,000,000,000,000 foot-pounds per day. The quantity received from the sun by the North Jttlantic is 310,923,000,000,000,000,000 foot-pounds, GEOLOGY [ii. oi-:oc:.'osv. PART II.—GEOGNOSY : AN I.'vEs'rio.irio.' or THE .iAri:iii.-iLs or Tin-3 i:.ii1‘n’s SUBST.-LVCE. Before we enter upon any discussion of the geological changes which our planet has undergone, it is needful first of all to study the materials of which the planet consists. It is from the evidence furnished by the nature and arrange- ment of these materials that geological history must be compiled. Viewed in a broad way then, the earth may be coii- sidered as consistiiig of (1) two cnvelopes,—an outer one of gas completely surrounding the planet, and an inner one of water covering about three-foiirths of the globe; and (2) a globe cool and solid on its surface but possessing a high internal temperature. I. THE EXVELOPES. 1. The Atmosp/tere.—Tlie gaseous envelope to which the name of atmosphere is given extends at least to a distance of 40 or 45 miles from the earth’s surface, perhaps in a state of extreme teiiiiity to a much greater height. But its thickness must necessarily vary with latitude and changes in atmospheric pressure; the layer of air lying over the poles is not so deep as that which surrounds the equator. Geologically considered, the at11iosphci‘e presents itself as an agent of change by virtue of its composition and the chemical reactions which it effects, its varying temperature and consequent influence in expandiiig and contracting rocks, and its movements. Maiiy speculations have been made regarding the chemical composition of the atmosphere during foriiier geological periods. There can indeed be no doubt that it must originally have ditfered very greatly from its present condition. The oxygen which new forms fully a half of the outer crust of the earth was originally doubtless part of the atmosphere. So, too, the vast beds of coal found all over the world, in geological formations of many diﬂ'erent ages, represent so much carbonic acid once present in the air. The chlorides in the sea likewise were probably carried down out of the atmosphere in the primitive eon- densation of the aqueous vapour. It has often been sug- gested that diiring the Carboniferous period the atmosphere must have been warmer and with more aqueous vapour and carbonic acid in its composition than at the present day, to admit of so luxuriant a ﬂora as that from which the coal seams were formed. There seems, however, to be at present no method of arriving at any certainty on this subject. As now existing, the atmosphere is considered to be normally a mechanical mixture of nearly 4 volumes of nitrogen and 1 of oxygen, with a minute proportion of car- bonic acid, and still smaller quantities of other substances. Expressed in a tabular form this composition is as follows :— Nitrogen ............................ 79'00 Oxygen... 20116 Carbonic acid ............................ .. 0'04 These quantities are liable to some variation aecordin g to locality. On the sea, for example, the proportion of carbonic acid is said to average about 003. In the air of streets and houses the proportion of oxygen diminishes, while that of carbonic acid increases. According to the minute researches of Dr Angus Smith, very pure air should contain not less than 2099 of oxygen, with 0030 of carbonic acid ; but he found impure air in Manchester to have only 2021 of oxygen, while the proportion of carbonic acid in that city during fog was ascertained to rise sometimes to 00679, and in the pit of the theatre to the very large amount of 0'273-1. Small as the percentage of carbonic acid in ordi- nary air may seciii, yet the total amount of this gas in the