Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/233

Rh MOVEMENTS OF THE EARTIL] “Vith the eccentricity at its superior limit and the winter occur- ring in the apliclion, the earth would, as we have seen, be 8,736,420 miles farther from the sun during that season than at present. The reduction in the amount of heat received from the sun, owing to his in.-rcased distance, would lower the midwintei‘ temperature to an enormous extent. In temperate regions the greater portion of the moisture of the air is at present precipitated in the form of rain, and the very small portion which falls as snow disappears in the course of a few weeks at most. But in the circumstances under consider- ation, tlie inean winter temperature would be lowered so much below the freezing-point that what now falls as rain during that season would then fall as snow. ’l‘liis is not all; the winters would then not only be colder than now, but they would also be much longer. At present the winters are nearly eight days shorter than the summers; but with the eccentricity at its superior limit and the winter solstice in aphclion, the length of the winters would exceed that of the summers by no fewer than thirty-six days. The lower- ing of the temperature and the lengthening of the winter would both tend to the same effect, viz., to increase the amount of snow accuinu- lated during the winter; for, other things being equal, the larger the snow-accuiniilating period the greater the accumulation. It may be reinai'k«_-d, however, that the absolute quantity of heat received during winter is not affected by the decrease in the sun's heat,‘ for the addition.il length of the season eoinpciisatcs for this decrease. As regards the absolute amount of heat reccivcd, increase of the sun's distance and lengthening of the winter are compensatory, but not so in regard to the ainouut of snow accumulated. The consequence of this state of things would be that, at the commencement of the short summer, the ground would be covered with the m'ntcr's accuiiiii- lation of snow. Again, the presence of so much snow would lower the summer temperature, and prevent to a great extent the melting of the snow. “There are three separate ways whereby accumulated masses of snow and ice tend to lower the summer temperature, viz. :— “First, By means of direct radiation. No matter what the intensity of the suii’s rays may be, the temperature of snow and ice can never rise above 32°. Hence the presence of snow and ice tends by direct radiation to lower the temperature of all surrounding bodies to 32’. In Greenland, a country covered with snow and ice, the pitch has been seen to melt on the side of a ship exposed to the direct rays of the sun, while at the same time the surrounding air was far below the freezing-point; a thermometer exposed to the direct radiation of the sun has been observed to stand above 100°, while the air surrounding the instrument was actually 12° below the f reczing-point. A similar experience has been recorded by travellers on the siiow-fields of the Alps. These results, surprising as they no doubt appear, are what we ought to expect under the circumstances. Perfectly dry air seems to be nearly incapable of absorbing radiant heat. The entire radiation passes through it almost without any sensible absorption. Consequently the pitch on the side of the ship may be melted or the bulb of the thermometer raised to a high temperature by the direct rays of the sun, while the surrounding air remains intensely cold. The air is cooled by contact with the snow—eovcrcd ground, but is not heated by the radiation from the sun. “When the air is charged with aqueous vapour, a similar cooling effect also takes place, but in a slightly different way. Air charged with aqueous vapour is a good absorber of radiant heat, but it can only absorb those rays which agree with it in period. It so happens that rays from snow and ice are, of all others, those which it absorbs best. The lmniid air will absorb the total radiation from the snow and ice, but it will allow the greater part of, if not nearly all, the sun's rays to pass unabsorbed. But during the day, when the sun is shining, the radiation from the snow and ice to the air is negative ; that is, the snow and ice cool the air by radiation. The result is, the air is cooled by radiation from the snow and ice (or rather, we should say, to the snow and ice) more rapidly than it is heated by the sim; and, as a consequence, in a country like Green- land, covered with an icy_mantle, the temperature of the air, even during summer, seldom rises above the freezing-point. Snow is a good reflector, but as simple reflection does not change the character of the rays they would not be absorbed by the air, but would pass into stellar space. Vcre it not for the ice, the summers of North Greenland, owing to the continuance of the sun above the horizon, would be as warm as those of England; but, instead of this, the Greenland summers are colder than our winters. (‘over India with an ice sheet, and its summers would be colder than those of England. ‘ “ 6'cco2z(l, Anot_hc.r cause of the cooling effect is that the rays which fall on snow and_icc arc to a great extent reflected back into space. iut those that are not reflected, but absorbed, do not raise the tem- perature, for they disappear in the mechanical work of melting the ice. For whatsoever may be the intensity of the s11n’s heat the I When the eccentricity is at its superior limit, the absolute quantity of peat received by the earth during the year is, however, about one three- iiglqéircdth part greater than at present. But this does not affect the question at GEOLOGY 219 surface of the ground will be kept at 32° so long as the snow and ice remain unmelted. “ Third, Snow and ice lower the temperature by chilling the air and condensing the vapour into thick fogs. The great strength of the sun's rays during summer, due to his ncarness at that season, would, in the first place, tend to produce an increased amount of evaporation. But the presence of snow-clad mountains and an icy sea would chill the atmosphere and condense the vapour into thick fogs. The thick fogs and cloudy sky would effectually prevent the sun’s rays from reaching the earth, and the snow, in consequence, would remain unmelted during the entire summer. In fact, we have this very condition of things cxem iliﬁed in some of the islands of the Southern Ocean at the present ay. Sandwich Land, which is in the same parallel of latitude as the north of Scotland, is covered with ice and snow the entire summer; and in the island of South Georgia, which is in the same parallel as the centre of England, the perpetual snow descends to the very sea-beach. Captain Sir James Ross found the perpetual snow at the sea-level at Admiralty Inlet, South Shetland, in lat. 64°; and while near this place the thermo- meter in the very middle of summer fell at night to 23° F. The re- duction of the si1n’s heat and lengthening of the winter, which would take place when the eccentricity is near to its superior limit and the winter in aphclion, would in this country produce a state of things perhaps as bad as, if not worse than, that which at present exists in South Georgia and South Shetland. “The cause which above all others must tend to produce great changes of climate, is thedcﬂexion of great ocean currents. A high condition of eccentricity tends, we have seen, to produce an accumu- lation of snow and ice on the hemisphere whose winters occur in apliclion. The accumulation of snow in turn tends to lower the summer temperature, cut off the sun's rays, and retard the melting of the snow. In short, it tends to produce on that hemisphere a state of glaciation. Exactly opposite effects take place on the other hcinisphere, which has its winter in perihelion. There the short- ness of the winters, combined with the high temperature arising from the nearness of the sun, tends to prevent the accumulation of snow. The general result is that the one hemisphere is cooled and the other heated. This state of things now brings into play the agencies which lead to the dcﬂcxion of the Gulf-stream and other great ocean currents. “Owing to the great difference between the temperature of the equator and the poles, there is a constant flow of air from the poles to the equator. It is to this that the trade-winds owe their exist- ence. Now, as the strength of these winds will, ‘-as a general rule, depend upon the difference of temperature that may exist between the equator and higher latitudcs, it follows that the trades on the cold hemisphere will be stronger than those on the warm. When the polar and temperate regions of the one hemisphere are covered to a large extent with snow and ice, the air, as we have just seen, is kept almost at the freezing-point during both suiimier and winter. The trades on that hemisphere will, of necessity, be exceedingly powerful; while on the other hemisphere, where there is compara- tivcly little snow or ice, and the air is warm, the trades will consequently be weak. Suppose now the northern hemisphere to be the cold one. The north-cast trade winds of this hemisphere will far exceed in strength the soutli-cast trade winds of the southern hemisphere. The median line between the trades will consequently lie to a very considerable distance to the south of the equator. We have a good example of this at the present day. The difference of temperature between the two hcmisphercs at present is but triﬂing to what it would be in the case under consideration; 'et we ﬁnd that the south-east trades of the Atlantic blow with greater force than the north-east trades, sometimes extending to 10° or 15° N. lat. , whereas the north-east trades seldom blow south of the equator. The effect of the northern trades blowing across the equator to a great distance will be to impel thc warm water of the tropics over into the Southern Ocean. But this is not all; not only would the median line of the trades be shifted southwards, but the great equatorial currents of the globe would also be shifted south- wards. “Let us now consider how this would affect the Gulf-stream. The South American continent is shaped somewhat in the form of a tri- angle, with one of its angular corners, called Cape St Roque, pointing eastwards. The equatorial current of the Atlantic impinges against this corner ; but as the greater portion of the current lies a little to the north of the corner, it flows westwards into the Gulf of Mexico and forms the Gulf-stream. A considerable portion of the water, howcvcr, strikes the land to the south of the cape, and is deflected along the shore of Brazil into the Southern Ocean, forming what is known as the Brazilian current. Now, it is obvious that the shift- ing of the equatorial current of the Atlantic only a few degrees to the south of its present positioii—a thing which would ce_rt_amly take place under the conditions which we have been detailing- would turn the entire current into the Brazilian branch, and instead of ﬂowing chieﬂy into the Gulf of Mexico, as at present, it would all flow into the Southern Ocean, and the Gulf-stream would con- sequently be stopped. The stoppage of the Gulf-strcain, combined