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 ITTS, 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- 
