Dec. 12, 1878] 



NATURE 



127 



mou3 Arctic currents which chill it, -we should produce at 

 once a greater increase of heat than is required by the 

 floras. If, further, we impinged the Gulf Stream upon 

 its shores, without cooling it down by floating icebergs 

 upon its back, we might be able to induce at least an even 

 more temperate vegetation to grow there. 



Water is thus seen to be the great factor in distributing 

 heat and cold in northern regions, and not land, as has 

 been generally taught. Humboldt believed the rigorous 

 climate of America to be due to high land stretching to 

 the Pole ; Lyell taught that with great polar seas and an 

 excess of land at the equator, the hottest conditions 

 possible on the globe would be produced, and that with 

 land at the Poles and a great equatorial sea, the coldest 

 conditions would ensue. A study of the isothermal 

 lines leads to the contrarj' belief that the presence of 

 land at the Pole, even if ice capped as Greenland is, 

 would be less productive of cold than a polar ocean with 

 free exits, for air has not the distributing power pos- 

 sessed by ocean streams, and when these are ice-laden 

 the effect is still greater. 



It only remains to call attention to such proof as we 

 have, that these conditions really did exist in eocene 

 time, and that the Arctic currents were actually shut off 

 from the Atlantic in those days by continuous land which 

 connected the two continents of Europe and North 

 America. In the eocenes of Europe and North America 

 we have evidence of a great, and, comparatively speak- 

 ing, sudden rise of temperature, and this was followed in 

 due course by a mingling for the first time of the floras 

 and faunas of the two continents. That there was land 

 communication to the north is further evidenced by the 

 occurrence of types of both kinds in the floras now 

 found upon the spots on which they grew. In further 

 support of this theory we have the fact that no trace of 

 sea-deposit of eocene age has ever been found in the 

 polar area, all the vestiges of strata remaining showing 

 that these latitudes were then occupied by dry land. 



If we may assume that these conditions really did 

 prevail, and that all the outlets into the Atlantic were 

 closed by the elevation of the present sea-bed between 60^ 

 and 70^ (where, I believe, the sea is even now shallower) 

 into land of moderate elevation ; with or without pro- 

 longations south to the 50th latitude ; and the north 

 of Greenland submerged, a temperature would ensue 

 more than adequate to support aU the plants yet found 

 fossil in eocene or miocene Arctic beds. The result 

 would be that the zone of greatest heat would be far 

 north of the equator ; for while the southern hemisphere 

 was still cooled by the Antarctic currents rising to the 

 surface, the North Atlantic would be practically a land- 

 locked sea, cut off" from southern cold by the tropics, 

 from northern cold by land, and heated by the sun like 

 the Gulf Stream or Red Sea. There is no need to suppose 

 that the Gulf Stream washed its northern shores, for the 

 temperature would then be raised in excess of what is 

 required, but its aid may be called in to account for the 

 even warmer previous periods evidenced by the older 

 growths of Gleichenia and cycads. 



It does not necessarily follow that cold did not then 

 exist towards the Pole. Disko is 20° distant from it, and 

 with an inclosed polar sea we should have a rapid lower- 

 ing of temperature on the northern shores of the wide 

 belt of land, and might have even a frozen ocean, perhaps 

 as at present, with outlets on the side of Behring's Straits. 

 The assumption that forests stretched to the Poles is 

 not supported by the evidence. 



The high temperature in these latitudes would be con- 

 fined to the Atlantic ; and that it was under the same laws 

 ^ at pre-ent seems a reasonable supposition, since the 

 American area even then maintained a relative coolness 

 on account probably of the return and cooler currents 

 bemg sheared to the west by the rotation of the earth. 



To recapitulate. I believe the evidence to be in favour 



of the eocene age' of the Arctic floras in question, and 

 not miocene. I think that the temperature acquired by 

 the plants — especially taking into consideration that their 

 affinities with genera belonging to temperate regions is 

 only inferred upon, in many cases, indistinct fragments — 

 may have been over-estimated. There is no inherent 

 impossibility indeed, that these extinct forms may not be 

 the relics of a flora, like our present Arctic flora, specially 

 adapted to bear a rigorous clime, and colour is lent to 

 this by the abundance of the extinct McClintockia, about 

 whose affinities we know nothing — a flora, perhaps, merely 

 requiring the protecting cover of snow and sea-fog during 

 winter. Finally, I believe that a comparatively slight 

 change in the relative distribution of land and water, such 

 as I have described, would alone account by itself for 

 any fluctuations of temperature, of which we have any 

 record preserved, in, at least, the tertiary rocks. 



It does not come within the scope of the present subject, 

 but it is worth consideration, whether wider channels 

 still than those we now possess — some flowing from a 

 more easterly point, so that our land might form the 

 western coast of such a current — would not produce a 

 glacial epoch, intensified by the absence of the Gulf 

 Stream when there was no connecting isthmus (of 

 which there is evidence in recent days) between the two 

 Americas. The present distribution seems, at all events, 

 one productive of more than average cold, as we become 

 aware through the geological record, for the many and 

 wide-existing channels conduct the Arctic waters south, 

 and lower the general temperature of the ocean even to 

 the Tropics. J. Starkie Gardner 



ON GA CrSSIA'S WARNING REGARDING THE 

 SLUGGISHNESS OF SHIP'S MAGNETISM^ 



Practical Rule and Caution 



I. A FTER steering for some time on westerly courses 

 •^*- expect — 

 I. {a) Westerly error if you turn to the north ; 



1. \b) Or easterly error if you turn to the south. 



2. After steering for some time on easterly courses 

 expect — 



2. (a) Easterly error if you turn to north ; 

 2. (^) Or westerly error if you turn to the south. 

 The diagram representing case i {a) illustrates the 

 physical explanation, N and S representing the north and 



south points of the compass card (or true south and true 

 north poles of its needles), and the small letters, s, s, s, 

 true southern polarity, and «, «, «, true northern polarity, 

 induced in the port and starboard ends of deck beams and 

 port and starboard sides of ship while steering east, and 

 remaining for some time after she has been turned to 

 north. 



In the "Admiralty Compass Manual" Gaussin's warn- 

 ing is given with reference to the direction of swinging, 

 in correcting the compass by magnets according to Airy's 



I Being an abstract of- a Communication by Sir Wm. Thomson, F.R.S., to 

 Section A of the British Assjciatian-at iu last meeting (Dublin). 



