Page:Popular Science Monthly Volume 3.djvu/477

Rh before the Fauna, as a whole, could, with any propriety, be termed Cretaceous.

I have now indicated some of the chief lines of Biological inquiry, in which the Challenger has special opportunities for doing good service, and in following which she will be carrying out the work already commenced by the Lightning and Porcupine in their cruises of 1868 and subsequent years.

But biology, in the long-run, rests upon physics, and the first condition for arriving at a sound theory of distribution in the deep sea is, the precise ascertainment of the conditions of life; or, in other words, a full knowledge of all those phenomena which are embraced under the head of the "Physical Geography of the Ocean."

Excellent work has already been done in this direction, chiefly under the superintendence of Dr. Carpenter, by the Lightning and the Porcupine, and some data of fundamental importance to the physical geography of the sea have been fixed beyond a doubt.

Thus, though it is true that sea-water steadily contracts as it cools down to its freezing-point, instead of expanding before it reaches its freezing-point as fresh water does, the truth has been steadily ignored by even the highest authorities in physical geography, and the erroneous conclusions deduced from their erroneous premises have been widely accepted as if they were ascertained facts. Of course, if sea-water, like fresh water, were heaviest at a temperature of 39° Fahr., and got lighter as it approached 32°, the water of the bottom of the deep sea could not be colder than 39°. But one of the first results of the careful ascertainment of the temperature at different depths, by means of thermometers specially contrived for the avoidance of the errors produced by pressure, was the proof that, below 1,000 fathoms in the Atlantic, down to the greatest depths yet sounded, the water has a temperature yet lower than 38° Fahr., whatever be the temperature of the water at the surface. And that this low temperature of the deepest water is probably the universal rule for the depths of the open ocean is shown, among others, by Captain Chimmo's recent observations in the Indian Ocean, between Ceylon and Sumatra, where, the surface-water ranging from 85° to 81° Fahr., the temperature at the bottom, at a depth of 2,270 to 2,656 fathoms, was only from 34° to 32° Fahr.

As the mean temperature of the superficial layer of the crust of the earth may be taken at about 50° Fahr., it follows that the bottom layer of the deep sea in temperate and hot latitudes is, on the average, much colder than either of the bodies with which it is in contact; for the temperature of the earth is constant, while that of the air rarely falls so low as that of the bottom water in the latitudes in question; and, even when it does, has time to affect only a