298 , Description of the 



time to avoid the danger {i.e. if his vessel is not so much crippled as 

 to render it impossible for him to use any means to get off shore) ; 

 and various circumstances induce me to hope that the thermo- 

 meter will ultimately be found not only an indicator of an ap- 

 proach to the coasts of the United States, but also, that it will 

 point out the proximity of land or soundings in all places to the 

 northward of the Tropics, and probably also to the southward of 

 them, though my own experience does not warrant me in the 

 hope that it will within them. 



Besides these advantages, the use of the thermometer has al- 

 ready been ascertained as illustrative of several of the oceanic 

 currents. The venerable and indefatigable geographer, Major 

 Rennell, in a letter to myself dated iUh of November last, ob- 

 serves, " The current from the Indian OCcan round the Cape (f 

 Good Hope differs llhfrom the Ocean ivater, i. e. is warmer ; 

 the EQUATORIAL cuRRKNT Colder by 5 or 6 deg. than the Gui- 

 nea current, which brushes it in passing, &c." Thus far al- . 

 ready established. We may surely venture to hope that as ther- 

 niometrical observations are multiplied, their utility will become 

 more obvious, and be more generally acknowledged. 



In my own experience I have found a difference of 12 deg. in 

 the temperature of the water in a few hours. Running out of 

 the Delaware, in 9 fathoms water, the mercury stood at 60 deg. 

 (in October 1817); as the water deepened, it rose to G4 deg.; 

 and as we entered into the Gulf stream, it suddenly increased to 

 72 deg. 



In September ISIS, when bound from New Orleans to Gi- 

 braltar, in the ship Asia, of Scarborough, then under my com- 

 mand, a fever broke out on board the ship, and for a considerable 

 number of days after we cleared the Strait of Florida, we had 

 only four men and a boy fit for duty, and three out of that num- 

 ber merely convalescents. I was myself confined to bed, and my 

 mate in the same situation ; we were the only two navigators on 

 board, and both unable to make up any reckoning, and some days 

 unable even to crawl on deck to take an observation at noon. 

 In this dilemma I trusted entirely to n)y thermometers, having 

 given orders that the instant the mercury fell two or three de- 

 grees, the ship's head should be wore round off shore. In this 

 way I kept the ship from danger, and also availed myself of the 

 Gulf stream current to carry us to the northward ; and 1 really 

 believe that, under Divine Providence, the safety of the ship, car- 

 go, and crew was attributable to my thermometers. 



On the same voyage, on approaching very near the Azore* 

 Islands, the mercury sank 2 deg., and when we made the coast 

 of Portugal, it rapidly fell from. 69 deg. to 6l deg., at which it 

 stood -when we rounded Cape St. Vincent, at the distance of 



about 



