INTRODUCTION. 7 
water becomes gradually heavier, and descends into the depths 
until it reaches 49:25 Cent., the point of its maximum density, 
which temperature is found in every clime at the depth of 550 
fathoms. If the water continue to cool until it arrives at zero, 
it is lighter than it was at 4°25, and therefore begins to rise; so 
that freezing, by a wonderful provision, can only take place at 
the surface. When the temperature is above 4°'25, the light and 
warm water rises to the surface and the cold water descends to 
the bottom. If it be under 4°:25, the opposite process takes 
place: the cold layers of water rise, and the warm in their turn 
sink. The former phenomenon takes place in the temperate and 
tropic zones, the latter in the polar regions; and so it happens, 
whether in the warmest or coldest seas, the temperature at ‘great 
depths is the same. The continual ascent of these warm layers 
furthers evaporation, by which clouds are formed. The loss 
the sea thus sustains in the warm latitudes is compensated by 
currents of cold water, which flow in from the poles. 
Again, the rain formed by the condensation of the clouds is 
either warmer or colder than the surface of the sea on which it 
falls: in the first case it will remain on the top, in the second case 
it will sink beneath. 
The waters of the rivers have an effect upon the sea by their 
temperature, their specific gravity, and their impetus, while the 
movements of the air, the winds and storms, have a still more 
manifest influence upon it. 
And lastly, the combined attraction of the sun and moon 
draw daily around the globe two immense waves, which, at the 
periods of the full and new moon, rise to their greatest height, and 
sweep over parts of the shores which are usually dry. These 
great sea movements are known by the name of the tides. 
During one half of the year the highest tides occur during the 
day, and in the other half during the night. The tides in the mid- 
ocean do not rise three feet; but when the tidal wave encounters 
a continent which opposes its onward roll, it rushes up its shores 
with the velocity of a torrent, rising to heights which vary from 
ten to sixty feet. These currents sweep and purify our shores, 
our roadsteads, our ports, and the mouths of our rivers, carrying 
everywhere a vivifying and salutary freshness. Under the influence 
