THE PROBLEMS OF THE DEEP SEA. 463 



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 con- 

 dition 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, 1 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 care- 

 ful ascertainment of the temperature at different depths, by means of 

 thermometers specially contrived for the avoidance of the errors pro- 

 duced by pressure, was the proof that, below 1,000 fathoms in the 

 Atlantic, down to the gi-eatest 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 observa- 

 tions 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 aver- 

 age, much colder than either of the bodies with which it is in con- 

 tact ; 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 compara- 



1 "Proceedings of the Royal Society," 1870 and 1872. 



