42 



HYDROGRAPHY. 



tion is required in keeping the ship on her course, to secure all the 

 progress possible, as a mile, or even a smaller distance, in position, 

 may make a great difference in time when a favorable breeze is 

 taken, and the voyage is accelerated or retarded accordingly. 



The winds on this route, from latitude 29° to 27°, are no doubt some- 

 what influenced by the proximity of the Canary Islands, and the 

 approach to the trade winds. I am satisfied that attention to the 

 trimming of the sails and sailing of the vessel, whilst passing through 

 these latitudes, will greatly expedite a voyage. 



We encountered on this passage, between latitude 24° 30' and 

 18° 25' north, on the meridian of 20° 40' west, an extent of 360 miles 

 of water as green as that of but 50 fathoms in depth. In passing 

 through it, we repeatedly sounded in from 150 to 300 fathoms, with- 

 out obtaining bottom. 



The width of this discolored water I have ascertained to be about 

 150 miles, on the parallel of 21° north latitude. In passing into it the 

 temperature fell 2° of Fahrenheit, and rose the same amount on passing 

 out. The line of demarcation was very well defined, the sea on either 

 side being of a deep blue. It was noticed in the voyage of Frazer, in 

 1712, but, considering the great number of navigators who have since 

 traversed that part of the ocean, it does not seem to have been often 

 observed. There seems to be an impression by those who have 

 noticed it that soundings could be had, but the lead has been often 

 tried, as was the case with us, to various depths, without success. 

 Many have imputed the discoloration to animalcula, but I am con- 

 fident that this was not the case at the time we passed through it, 

 for careful examination failed to discover any in water taken from the 

 surface as well as that from a depth of fifty fathoms ; and I would also 

 urge against its being the effect of animalcula, the long continuance of 

 it in the same place. Whether it is the reflection of the bottom, as 

 some suppose, or a peculiarity of the surface waters, emanating from 

 the rivers of Africa, I leave for others to decide ; it is a phenomenon 

 well worthy of investigation. While in it we saw no phosphorescence 

 of the water, but on passing out, it was very evident. To determine 

 its actual limits, both in breadth and width, is a work of no great diffi- 

 culty, and if observations were repeated, in a few years the question 

 would be settled as to its being permanent. 



In the Pacific as well as Atlantic Ocean there are many places of 

 discolored waters, betokening banks of large extent, which have served 



