620 



Dr. W. B. Carpenter on the 



[June 13, 



survey of the Tortugas section, and reported very carefully on the pheno- 

 mena it presents. He regards the Gulf-stream current, like that of the 

 Baltic or the Dardanelles, as tending to the restoration of the surface-level 

 of the basin from which it flows, which the inset of the Equatorial current 

 will as constantly tend to raise ; and he gives the following example of 

 " compensating currents," which is peculiarly applicable not merely to the 

 doctrine now under consideration, but also to that of the under-currents of 

 the Straits of Gibraltar and the Baltic Sound. — During the dry season 

 (July), the surface-onflow of brackish water through the Narrows of New 

 York harbour occupies nine out of the twelve tidal hours ; this water chiefly 

 consisting of the water brought down by the Hudson River, with some ad- 

 mixture of salt water. This salt water enters as a lower stratum, the inflow 

 of which is nearly uninterrupted, though not constant in velocity ; and the 

 same condition extends for some distance up the river. On running a line 

 of levels from New York City to Albany, it was found that the bed of 

 the Hudson lies below the mean level of the sea for nearly 100 miles from 

 its mouth ; but that the surface of the fresh water, even in the dry season, 

 is above this level, — not so much so, however, as to equalize the difference 

 of weight between the column of River-water within the Narrows and 

 that of the Sea- water outside ; so that the latter during the summer months 

 flows in along the bed of the stream, while the former overflows into the 

 ocean. During other parts of the year, however, the level within the Narrows 

 being higher, the inner column is the heavier, and the inflow of Sea-water 

 does not take place. 



135. We are now prepared to examine the influence of the body of 

 warm water discharged by the Gulf-stream into the Mid- Atlantic, on the 

 Temperature of the Oceanic area through which it makes its way. — It is 

 obvious that so long as it retains its Northern course, and does not diffuse 

 itself to the East — while it is completely shut in on its Western border by 

 the Arctic current — it cannot directly affect the temperature of the general 

 surface of the Ocean west of the Meridian of 70° ; and we find (as already 

 pointed out) that the Winter Isotherms of 70° and 65° are only carried 

 northwards in the immediate course of the Stream, while the great north- 

 ward movement of the Summer Isotherms of 80° and 75° appears to be 

 quite sufficiently accounted for by the general influences already referred 

 to (§§ 117-119) as modifying the climate of the North Atlantic. 



136. The most striking peculiarity in the Thermal condition of the Gulf- 

 stream, as compared with that of the Ocean through which it flows in the 

 first part of its course, seems to consist, not so much in its more elevated 

 surface-temperature, as in the greater thickness of its superheated stratum ; 

 which, as will be presently shown, has a most important influence on the 

 maintenance of its surface-temperature when it is exposed to an atmosphere 

 colder than itself. Recent inquiries have shown that the high surface-tem- 

 perature of the Gulf-stream is not derived (as commonly supposed) from its 

 circulation within the almost land-locked Gulf of Mexico j for the average 



