CAUSES or CtJEKENTS. 
23 
times they produce several contrary effects. Winds that 
are light, hut which, like the trade-winds, arc continually 
acting on the whole of a zone, cause a real movement of 
transition, which we do not observe in tho heaviest tempests, 
because these last arc circumscribed within a small space. 
" hen, in a. great mass of water, the particles at the surface 
acquire a different specific gravity, a superficial current is 
formed, which takes its direction towards tho point where 
tne water is coldest, or whei’e it is most saturated with 
muriate of soda, sulphate of lime, and muriate or sulphate 
ot magnesia. In the seas of the tropics we find, that at 
great depths the thermometer marks 1 or 8 centesimal 
degrees. Such is the result of the numerous experiments 
of commodore Ellis and of M. Peron. The temperature of 
the air in those latitudes being never below 19 or 20 degrees, 
it is not at tho surface that the waters can have acquired a 
degree of cold so near the point of congelation, and of the 
maximum ol tho density of water. The existence of this 
cold stratum in the low latitudes is an evident proof of the 
existence of an under-current, which runs from the poles 
towards the equator: it also proves that the saline sub- 
stances which alter the specific gravity of the water, are 
distributed in the ocean, so as not to annihilate the effect 
produced by tlie differences of temperature. 
Considering the velocity of the molecules, which, on 
account of the rotatory motion of the globe, vary with the 
parallels, we may be tempted to admit that every current, in 
the direction from south to north, tends at the same time 
eastward, while the waters which run from the pole towards 
the equator, have a tendency to deviate westward. "W e may 
also be led to think that these tendencies diminish to a 
certain point the speed of the tropical current, in the same 
manner as they change the direction of the polar current, 
which in July and August, is regularly perceived during the 
melting of the ice, on the parallel of the bank of New- 
foundland, and farther north. Very old nautical observa 
tions, which I have had occasion to confirm by comparing 
the longitude given by the chronometer with that which the 
pilots obtained by their reckoning, are, however, contrary 
to these theoretical ideas. In both hemispheres, the polar 
currents, when they are perceived, decline a little to the 
