78 Dr. Fothergill’s account of we islands. 
gent use of the thermometer, by which they may be timely forewarn- 
ed of the approaching danger. It must be acknowledged however, 
that there are two or three circumstances, which form an exception to 
the general rule respecting the thermometer, which, when previously 
understood, can scarcely occasion any embarrassment. 1. It fails in 
rivers and capes. 2, Near sand banks, which rise considerably above 
the surface of the water, and which, on being heated by the sunbeams, 
impart an increased degree of warmth to the surrounding shoal wa- 
ter, and consequently cause the mercury to rise contrary to its wont- 
ed motion in soundings and shallow waters. 3. In marine currents, 
which,. according to their course from warmer or colder climates, 
may produce a sudden-and unexpected variation in the thermometer, 
though perfectly just to its principle of action. In other cases it 
marks the changes with the greatest accuracy. Thus the water out of 
soundings is always 8° or 10° warmer than within soundings. ‘Thus 
also, the current of the gulph stream, from its superior warmth, rais- 
es the mercury from 16° to 24° higher than the adjacent water of the 
coast.* Accordingly the water over the grand bank of Newfoundland, 
from 3 its sepetor coldness, ——— the mereury from 12° to 15° low- 
* This teaaabie siesta is Maetpulshable not city by its warmth, which it 
retains in its course through the ocean for more than 330 leagues, almost to the 
banks of Newfoundland, where its condensed vapour adds to the fogs of that 
gloomy region. The weeds, brought down with it from its source in about 30 
days, mark its course, and its water is never luminous i inthe night. The passage 
from Europe to America is expedited by avoiding to stem its stream, but from 
America to Europe by keeping in it. On this subject see Capt. Williams’ ex- 
periments in three voyages across the Atlantic, who does not indeed appear to 
have met with ice islands, as he only just slightly mentions them. But his exper 
iments, conducted with such uncommon attention, seem worthy the imitation of all 
navigators, and accordingly, in future, 
‘sult. 
I mean to avail myself of their general re- 
‘ 
