62 THE PHYSICAL GEOGRAPHY OF THE SEA. 



their reckoning. He himself was 5°. Therefore, in approaching 

 the coast, the current of warm water in the Gulf Stream, and of 

 cold water on this side of it, if tried with the thermometer, would 

 enable the mariner to judge with great certainty, and in the worst 

 of weather, as to his position. Jonathan Williams afterward, in 

 speaking of the importance which the discovery of these warm 

 and cold currents would prove to navigation, pertinently asked the 

 question, " If these stripes of water had been distinguished by the 

 colors of red, white, and blue, could they be more distinctly dis- 

 covered than they are by the constant use of the thermometer ?" 

 And he might have added, could they have marked the position 

 of the ship more clearly ? 



When his work on Thermometrical Navigation appeared. Com- 

 modore Truxton wrote to him : " Your publication will be of use 

 to navigation by rendering sea voyages secure far beyond what 

 even you yourself will immediately calculate, for I have proved 

 the utility of the thermometer very often since we sailed together. 



" It will be found a most valuable instrument in the hands of 

 mariners, and particularly as to those who are unacquainted with 



astronomical observations ; these particularly stand in need 



of a simple method of ascertaining their approach to or distance 

 from the coast, especially in the winter season ; for it is then that 

 passages are often prolonged, and ships blown off the coast by 

 hard westerly winds, and vessels get into the Gulf Stream with- 

 out its being known ; on which account they are often hove to by 

 the captains' supposing themselves near the coast when they are 

 very far off (having been drifted by the currents). On the other 

 hand, ships are often cast on the coast by sailing in the eddy of 

 the Stream, which causes them to outrun their common reckoning. 

 Every year produces new proofs of these facts, and of the calam- 

 ities incident thereto." 



82. Though Dr. Franklin's discovery was made in 1775, yet, for 

 political reasons, it was not generally made known till 1790. Its 

 immediate effect in navigation was to make the ports of the North 

 as accessible in winter as in summer. What agency this circum- 

 stance had in the decline of the direct trade of the South, which 

 followed this discovery, would be, at least to the political econo- 

 mist, a subject for much curious and interesting speculation. I 

 have referred to the commercial tables of the time, and have com- 



