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Show CAUSES OF 122 . ~ that without an entire revolu-seems to authorize us to ·1 0h1etr 'c ause t 11 e mean tempera· tudre t·o tion of the surface, w~ rnlg o 20o or even soo of latltu e,- to an extent eqmvalcnt t the temperature of the tor- ;:r~ther words, we might t\~nlsf~nd of the latter, to the arctic r1' d zo ne ' to the m. .e an 1p t aarnas peo s'.i tw. ns, therefore ' ofland and sea, 1 h regw. n s · By additwna r ill greater var i'ati'on ' so that, t noug ~ might bring about a st l ld disappear from the earth. we 1 . f frost s lOU l . ll t the year, al signs o . ld ·ise in the atmosp Jere m a ou "fhe plane of congelatiOn wou. I u1d place all the highest latitudes ; and as our h. yhothesis wo l d . h r ne the would be clot Je wit ntains in the tornd zo '. y We must recollect that ~ohu vegetation to their summits. d to the height of 15,000 nc . ·y to ascen n now it IS necessar . d in the Himalaya moun~ ~vet in the Andes under the hne ; ~n to 17 000 feet before we lee 'th ut the tropic, ' . f t ins which are Wl 0 a , . . f perpetua1 snow. When the absorptiOn of reach the hmit o . d d ven in winter' by a coat o the solar rays was ummp~ e ' eth's crust would augment to snow the mean heat of t le ~ar which we know to be an ' l d ths and sprmgs, . b considerab e ep ' f the climate would e warmer h temperature o ' d . 1• ndex of t e mean f l kes therefore, an rivers, in all latitudes. Tl waters o a ' 'll d 1e . . t nd would be never chi e would be much hotte~ m wfm er, a A remarkable uniformity in summer b y the meltm. <T 0 snow. · 1 f the o . d the numerous archlpe agos o of climate would prevail a~l 1 the tepid waters of equatorial Polar ocean, amongst :V ulc lt The general humidity of the currents wou ld freely cucu a de . tl t of the present pen·o d' for atmosphere would far excee la oration in nll parts of the increased heat w?uld pro~~t~eev;Yst heated in their passage globe. The. wmds . wou nd would then gather moisture from over the tropical p1ams, ~11 bar ed with vapour, they would the surface of the dee'R, ti ' c d ~ncountering a cooler atm~sarrive at northern regiOns~ an ' in warm rain. If, durmg phere would discharge thei~ burd;~ snows should whiten the the lo~g night of a po~~r w;:t:~d i:e collect in the bays of the summit of some arctic IS an ' d' ssolved as rapidly by thereremotest Thule, they would be lf Etna by the blasts of the turn.m g sun, as a re the snows o . sirocco. 1 have studied the geographical We learn from those w l? er low latitudes, at present, distribution of plants, that m v y CHANGES OF TEMPERATURE. 123 the vegetation of small islands remote from continents has a peculiar character, and the ferns and allied families, in particular, bear a great proportion to the total number of other vegetables. Other circumstances being the same, the more remote the isles are from the continents, the greater does this proportion become. Thus, in the continent of India, and the tropical parts of New Holland, the proportion of ferns to the phanerogamic plants is only as one to twenty-six ; whereas, in th-e South Sea Islands, it is as one to four, or even as one to three*. We might expect, therefore, in the summer of the "great year," which we are now considering, that there would be a great predominance of tree-ferns and plants allied to palms and arborescent grasses in the isles of the wide ocean, while the dicotyledonous plants and other forms now most common in temperate regions would almost disappear from the earth. 'l'hen might those genera of animals return, of which the memorials are preserved in the ancient rocks of our continents. The huge iguanodon might reappear in the woods, and the ichthyosaur in the sea, while the pterodactyle might flit again through umbrageous groves of tree-ferns. Coral reefs might be prolonged beyond tl1e arctic circle, where the whale and the narwal now abound. 'l'urtles might deposit their eggs in the sand of the sea beach, where now the walrus sleeps, and where the seal is drifted on the ice-floe. But, not to indulge these speculations farther, we may observe, in conclusion, that however great, in the lapse of ages, may be the vicissitudes of temperature in every zone, it accords with our theory that the general climate should not experience any sensible change in the course of a few thousand years, because that period is insufficient to affect the leading features of the physical geography of the globe. Notwithstanding the apparent uncertainty of the seasons, it is found that the mean temperature of particular localities is very constant, provided we compare observations made at different periods fot· a series of years. Yet, there must be exceptions to this rule, and even the labours of man have, by tl1e drainage of lakes and marshes, a. nd ht he felling of extensive forests, caused such chan<Tcs 0 In t e atmosphere as raise our conception of the important • Ad. Brongniart, Cousid. Generales sur la Nat. de Ia Veget., &c. Ann. des Sciences Nat., Nov. 1828. . |