HYDROGRAPHY. 321 



fathoms is covered with the debris of shells of foraminifera, 

 now everywhere known as globigerina ooze. 



Nearer the coast the deposit consists chiefly of mud, 

 washed down by rivers, or of disintegrated rock ; while be- 

 yond 2000 fathoms a chemical change seems to take place in 

 the globigerina by which the carbonate of lime is removed 

 and a sort of red clay remains, frequently mixed with vol- 

 canic remains, such as felspar and pumice. The results from 

 temperature observations would require too much room to 

 be given in detail, but a very important fact was established, 

 viz. : that a great many of the observed anomalies in bottom 

 and serial temperatures may be accounted for by the fact 

 that, as yet, no registering thermometer has been devised 

 whose indications may be strictly relied upon. The Miller- 

 Casella thermometer, where the bulb containing the expand- 

 ing fluid is surrounded by another bulb containing diluted 

 spirit and vapor, is by far the best, but its action leaves a 

 great deal to be desired. 



Avast number of interesting and valuable data were gath- 

 ered regarding the currents of the Atlantic, and it seems as 

 if the vexed question of the cause of the Gulf Stream might 

 now be considered as settled. Nearly all intelligent naviga- 

 tors and physicists will agree with Sir Wyville Thomson in 

 his theory that the permanent winds blowing eternally in one 

 direction send the heated surface-water in a constant stream 

 to the westward, and this stream, split by Cape St. Roque, 

 directs its northern branch round the Gulf of Mexico, and be- 

 coming contracted and condensed by the Straits of Florida, 

 it makes itself manifest as the Gulf Stream. At the same 

 time it is considered probable that the general circulation of 

 the water in the Atlantic is kept up by an excess of evapora- 

 tion in the northern portion, balancing an excess of precipi- 

 tation over evaporation in the southern, or water hemisphere. 



It is too soon to generalize with regard to the chemical 

 composition of the waters collected from every part of the 

 ocean by the Challenger, but they are being carefully ana- 

 lyzed, and will doubtless give most valuable results. 



An important exploring and surveying expedition has 

 been despatched by the English Admiralty, under Sir George 

 Nares, in his old ship the Alert, of Arctic celebrity. The of- 

 ficers of the Alert have been most carefully selected, and 



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