TEMPEEATUEE OF THE SEA. 85 



uniform temperature to the soutli of the equator describes a long 

 curve, touching the surface of the water at one point only. Ad- 

 mitting with the naturalists of his time, that the sea-water has its 

 greatest density, and in consequence its greatest relative weight, at 

 seven degrees above freezing-point, Sir James Eoss concluded from 

 this that all the deep waters below this line of 39 '2 degrees have 

 ih.e same temperature, and are collected by reason of their con- 

 densation at the bottom of the oceanic basins. 



Nevertheless it has since been proved, by the observations of 

 Neumann * and other scientific men, that if the greatest density of 

 fresh water corresponds in reality to 39 '2 degrees Fahr., the water 

 of the sea only attains this maximum at nearly four degrees below 

 freezing-point (28'4 Fahr.), or even at still lower temperatures, 

 and, in consequence, the conclusions at which Sir James Eoss arrived 

 are negatived. Experiments made in chemical laboratories, however, 

 where substances are treated in small quantities, cannot give a 

 perfectly exact notion of the phenomena which have nature itself for 

 their theatre, and which take place either in the aerial spaces or in 

 the vast oceanic ba^ins. Thus, as the celebrated meteorologist 

 Miihry says, the immense sea, and a bucket-full of salt water, do 

 not obey absolutely the same laws of temperature and density. But 

 before the difference is established, nothing can authorize us in 

 maintaining a superannuated theory against all the experiments of 

 chemists, according to which the volume of salt water in the sea in 

 cooling presents phenomena identical with those of fresh-water lakes. 

 Moreover, during the past years, numerous observers of polar seas 

 have found at great depths, beds of water, at a temperature lower 

 than 39 "5 degrees Fahr.f 



That which remains of the researches of the eminent navigator. 

 Sir James Eoss, is that in the tropical and temperate seas the heat 

 diminishes gradually and constantly to a considerable depth. This 

 is what has been put beyond all question, by soundings taken by 

 Fitzroy and other marine explorers. To the south of the island of 

 Madagascar, the surface of the water having then a temperature of 

 75 "2 degrees Fahr., Fitzroy ascertained that the thermometer fell 

 in the most regular manner, till at the depth of 420 fathoms, where 

 they ceased sounding, the temperature indicated hardly exceeded 

 51 '8 degrees Fahr.:{: 



* Ueier die JDichtigkeit beim Meerwasser. 



t Fitzroy, Weather- Book, p. 81. 



X Adventure and Beagle, Vol. IT., Appendix, p. 303. 



