294 Oti the PractlcaliUtij 



marche do not accord with anv general rule. The density of 

 the water at 32 and 35 degrees N. is as strong as that of the 

 water of the tropics. M. Ilunaboldt, whose attention was di- 

 rected in his celebrated voyage to all the great phenomena of 

 nature, has observed, after comparing the results of his experi- 

 ments with those obtained by other voyagers, " that the experi- 

 ments hitherto published do not justify any opinion being en- 

 tertained that the sea is Salter under the equator than under 

 30 and 44 degrees of latitude." — {Relat. Hist. i. 74.) — Ac- 

 cording to Pages, the saltness of the sea is constant and equal 

 at four centimes from 45° N. to 50° S. However, according 

 to the experiments of Bladh on the specific weight of sea water, 

 it would appear that the w-atcr is sailer towards the tropics than 

 even under the equator. Some similar experiments of Mr. John 

 Davy, which have been published in the Philosophical Trans- 

 actions for 1817, prove that from 30° to 35^ on one side as 

 well as the other of the equator, the specific weight and conse- 

 quently the saltness is exactly the same, but that it is a little 

 stronger under the northern than under the southern tropic. 

 And lastly, M. Humboldt, by tlie use of an excellent aerometer 

 constructed by Dollond, thought he could discern that the water 

 is less salt between the tropics than from the coasts of Spain to 

 Teneriffe. 



From this discordance between the results, which may depend 

 on the process of analysis, or on local causes ; from the circum- 

 stance especially that the results of different voyagers range of- 

 ten in contrary directions, we may conclude that the saltness of 

 the great ocean, if it is not the same every where, presents at least 

 very slight variations. But every incertitude which remains on 

 this subject, will doubtless be removed on the return of M. Frey- 

 cinet, who is to bring home, in vessels closed with emery, water 

 taken in a great number of places in the two hemispheres. 



Looking in the mean time to the subject in a theoretic point 

 of view, it is easy to be convinced, that in general tiiere can be 

 but little difference between the saltness of the sea at one place 

 and at another, if we except some local causes, and particularly 

 the river waters, which mav occasion a good deal of vaiiation. 



LI. Oti the Practicahility of a direct Passage over the North 

 Pole-. 

 ^^j Hull, Feb. 27, 1818. 



Mr. Hydrographer, — When the public attention is so ge- 

 nerally directed to the expedition now fitting out to explore the 

 Arctic regions, it is to be supposed that individuals who take a 



* From the Iifaval Chronicle for March 1818. 



lively 



