June 27, 1872] 



NATURE 



167 



contrasted with the products of heat in the nether re- 

 gions," Sir Charles says (p. 211) : — "The continual trans- 

 fer of the points of chief development of the earthquake 

 and volcano from one part of the earth's crust to another 

 is established as a general law by the clearest geological 

 evidence. We have also seen that volcanic operations 

 are now in progress on the grandest scale, and also that 

 single currents of lava of modern date are as voluminous 

 as any which can be shown to have ever poured out in 



the earliest eras to which oar geological retrospect can be 

 carried." 



The doctrine of the former greater intensity of the ig- 

 neous forces, connected as it generally was with the 

 hypothesis of the primxval igneous fusion and gradual 

 cooling down of the planet, of course involved the theory 

 of the former higlier temperature of the surface of our 

 earth ; and therefore all indications of a warmer climate 

 over any area in the ancient seas were supposed to point 



Fig. 3.— Bird's- Evi 

 rherasia ; /' the northern entrance, 

 clay-slate, the only non-volcanic r 



VIEW OF THE Gl LF OF SaNTOKIN DURI.NG THE VuLCAKlC EliUl'TION OF FeBRUARV i8£6 



,o6S feet deep ; c Thera ; d Mount St. Ellas, rising i,857 feet above the sea. composed of granular limestone i 

 ick in Santorin ; e Aspronisi ; / Little Kaimeni ; g New Kaimeni ; /; Old Kaimeni ; i Aphroessa ; k George. 



to a universal higher temperature over the globe. But 

 Sir Charles Lyell points out that " the climate of the extra- 

 tropical regions has been by no means always hotter than 

 now ; but on the contrary, there has been at least one 

 period when the temperature of those regions was much 

 lower than at present" (p. 173). 



Space will not allow us tj follow our author while he 

 proves, from an examination of the circumstances under 

 which we find similar and dissimilar climates at the 

 present day, that geographical conditions produce far 

 greater effect upon climate than we have reason to Ijclieve 

 would result from any astronomicil combinations. De- 

 pendent to a great extent upon geographical conditions 

 we have prevalent winds, which materially influence 

 climate, and moreover give rise to most of the great ocean 

 currents. 



About the origin of these, however, there has been soma 

 controversy. Sir Charles considers the various theories 

 very fully, and shows that the great currents are due ta 

 prevalent winds. "That movements," he writes, "of no 

 inconsiderable magnitude should be impressed on a wide 

 expanse of ocean by winds blowing for many months in 

 one direction may easily be conceived, when we observe 

 the effects produced in our own seas by the temporary 

 action of the same cause. It is well known that a strong 

 south-west or north-west wind invariably raises the tide 

 to an unusual height along the west coast of England and 

 in the Channel ; and that a north-west wind of any con- 

 tinuance causes the Baltic to rise 2 ft. and upwards above 

 its ordinary level " (vol. i. p. 492). 



It is clear that when the surface water is being thus 

 driven continuously for a long time in one direction 



■Supposed Section of Barren Island, in the Bay of Beng.al 



against a shore or into a cul dc sac, there must be an 

 undercurrent formed by the head of water thus produced. 

 But in deep basins there is no reason why the water 

 should not remain embayed for ages, and, having been at 

 any time cold, should never receive sufficient from above 

 or below to raise its temperature. 



In the Mediterranean and the smaller seas connected 

 with it there seems to be a great complication of current- 

 producing causes, which have proved a fertile source of 

 speculation and controversy ever since Aristotle puzzled 

 over the currents of the Euripus. That land-locked sea 

 is too small to have any considerable tide generated within 

 itself, but the Atlantic tide rushes in and out with great 

 force. The vast surface current of the Atlantic, and the 



prevailing westerly winds, increase the in-going tide, and 

 check, and generally altogether overpower, the surface 

 part of the out-going tide, so as to give rise to an upper 

 and a lower current in opposite directions through the 

 Straits of Gibraltar. 



In addition to these causes, there is the enormous 

 evaporation during the hot season and the excess of fresh 

 water poured in during the rainy season, which must pro- 

 duce a great effect. But as each tide would, when there was 

 a deficiency of water, bring a little more in, and when an 

 excess of water, take a little moreout, this adjustment being 

 made twice everyday it does not seem likely that either evapo- 

 ration or rain would make any appreciable difference in 

 the currents at the Straits. Some authors have referred 



