GEOLOGY. 267 



" The aneroid barometer was set with the mercurial on leaving New 

 York in October. It commenced at once to differ, indicating higher ; 

 and, though its daily fluctuations agreed well, this difference steadily 

 increased until, by the time we got back to New York, seven months 

 after, it had reached as high as six tenths of an inch above it ; thus 

 acquiring an error of very nearly one tenth of an inch a month. This 

 leads me to doubt whether this ingenious instrument can ever be suf- 

 ficiently trusted to take the place of the mercurial, though it is so 

 much to be desired." 



ON THE CURRENTS OF THE ATLANTIC, AND THE EXISTENCE OF 



THE NORTHWEST PASSAGE. 



LIEUT. MAURY said, that in studying the system of oceanic circula- 

 tion he had found it necessary to set out with a very obvious and 

 simple principle, viz. that from whatever part of the ocean a current 

 was found to run, to the same part a current of equal volume \vas 

 obliged to return. Upon this principle was established the whole 

 system of currents and counter-currents. It is not necessary to asso- 

 ciate with oceanic currents the idea that they must of necessity, as on 

 land, run from a higher to a lower level. So far from this being the 

 case, some currents of the sea actually run up hill, while others run 

 on a level. The Gulf Stream was of the first class. The bottom of 

 this stream he had shown some years since to be an inclined plane, 

 running upwards. If the Gulf Stream was 200 fathoms deep in the 

 Florida Pass, and but 100 fathoms off Hatteras, it is evident that the 

 bottom would be uplifted 100 fathoms within that distance, and that, 

 while the bottom of the Gulf Stream was up hill, the top preserved 

 the water level, or near]) 1 - so. The currents which run from the At- 

 lantic into the Mediterranean, and from the Indian Ocean into the Red 

 Sea, were the reverse of this. Here the bottom of the current was a 

 water-level, and the top an inclined plane running down hill. The 

 Red Sea, for example, lies for the most part in a rainless and riverlesr, 

 district. It may be compared to a long, narrow trough. It is about 

 1,000 miles long, extending nearly north and south, from lat. 12 or 13 

 to the parallel of 30 N. The evaporation from its surface is immense, 

 and may be safely assumed to equal a rate of two tenths of an inch 

 per day. Now, if we suppose the current w 7 hich runs into this sea to 

 average from mouth to head 20 miles a day, it would take the water 50 

 days to reach the head of it. If it lose two tenths of an inch from its 

 surface daily by evaporation, by the time it reached the Isthmus of 

 Suez, it would have lost 10 inches from its surface. Thus the waters 

 of the Red Sea ought to be lower at the Isthmus of Suez than at the 

 Straits of Babelmandel. They ought to be lower from two causes, 

 viz. evaporation and temperature ; for the temperature of that sea is 

 necessarily lower at Suez, in latitude 30 3 , than at Babelmandel, in 

 latitude 13. To make this quite clear, suppose the channel of the 

 Red Sea to have no water in it, and a wave ten feet high to enter the 

 straits, and flow up this channel at the rate of 20 miles a day for 50 

 days, losing daily by evaporation two tenths of an inch, it is easy tc 



