CURRENTS OF THE SEA. 
125 
of human ken ; and being a law of Nature, we know who gave it, 
and that neither chance nor accident had any thing to do with its 
enactment. 
Nature grants us all that this postulate demands, repeating it 
to us in many forms of expression ; she utters it in the blade of 
green grass which she causes to grow in climates and soils made 
kind and genial by warmth and moisture, that some current of the 
sea or air has conveyed far away from under a tropical sun. She 
murmurs it out in the cooling current of the north ; the whales of 
the sea tell of it (§ 65), and all its inhabitants proclaim it. 
233. The fauna and the flora of the sea are as much the crea- 
tures of climate (§ 66), and are as dependent for their well-being 
upon temperature as are the fauna and the flora of the dry land. 
Were it not so, we should find the fish and the algae, the marine 
insect and the coral, distributed equally and alike in all parts of 
the ocean. The polar whale would delight in the torrid zone, and 
the habitat of the pearl oyster would be also under the iceberg, or 
in frigid waters colder than the melting ice. 
234. Now water, while its capacities for heat are scarcely ex- 
ceeded by those of any other substance, is one of the most com- 
plete of non-conductors. Heat does not permeate water as it does 
iron, for instance, or other good conductors. Heat the top of an 
iron plate, and the bottom becomes^warm ; but heat the top of a 
sheet of water, as in a pool or basin, and that at the bottom re- 
mains cool. The heat passes through iron by conduction, but to 
get through water it requires to be conveyed by a motion, which 
in fluids we call currents. 
235. Therefore the study of the climates of the sea involves a 
knowledge of its currents, both cold and warm. They are the 
channels through which the waters circulate, and by means of 
which the harmonies of old ocean are preserved. 
236. Hence, in studying the system of oceanic circulation, we 
set out with the very simple assumption, viz., that from whatever 
part of the ocean a current is found to run, to the same part a cur- 
rent of equal volume is obliged to return ; for upon this principle 
is based the whole system of currents and counter-currents of the 
air as well as of the water. 
237. It is not necessary to associate with oceanic currents the 
idea that they must of necessity, as on land, run from a higher to 
