466 Mr. J. Prestwich on Tables of [June 18, 



ture of between 39 and 40, a point that had been investigated and 

 disproved by Marcet in 1819, approximately determined by Ermann in 

 1822, and which was finally settled by Despretz, in 1837, at 25-4 F. 



While the law of the decrease of temperature with the depth, in both 

 the great oceans, to a point but little above the zero of Centigrade was 

 being established, experiments had been carried on in polar seas showing, 

 on the contrary, that the temperature at depths was higher than the 

 average surface-temperature. The careful experiments of Scoresby and 

 of Martins fully established this for the Arctic seas, and those of Ross, 

 after correction, establish the same fact for the Antarctic Ocean. In 

 one part, however, of the Arctic seas this rule has not been found to 

 hold good ; for, in Baffin's Bay, the experiments of John Eoss, Sabine, and 

 Parry, at depths of from 600 to 6000 feet, agree in showing a decrease 

 of temperature of from 30 to 32 near the surface, to 29 and 28-5 at 

 the greatest depths attained. There are also two instances given of yet 

 lower temperatures. 



Nor were observations wanting in inland seas. Those of Saussure, 

 D'Urville, and Berard had indicated generally that, in the Mediterranean, 

 the temperature decreased to a depth of about 1200 feet, after which it re- 

 mained uniform at from 54 to 55F. ; and, in 1844, Airne instituted a series 

 of experiments which resulted in showing that the diurnal influence ceased 

 to be sensible at a depth of from 16 to 18 metres, and the annual variation 

 at a depth of from 300 to 400 metres, below which the temperature 

 remained constant at 12- 6 C. (54- 6) ; and this he showed to be the mean 

 winter temperature of the area of the Mediterranean, over which his 

 observations extended. These observations were confirmed, for the 

 ' Eastern Mediterranean, by those of Admiral Spratt. His first experiments 

 in the Grecian archipelago showed, at a depth of 1200 feet, a temperature 

 of 54 0> 5 to 55 F., while the later ones, at greater depths in the open sea, 

 give, after correction, a temperature of about 55. In the Red Sea, 

 Captain Pullen found that while the surface-temperature varied from 77 

 to 86 F., it fell to 70 or 71 F. at 1200 to 1400 feet, below which 

 it remained uniformly the same to the greatest depth he attained of 

 4068 feet. Some curious results were obtained in 1803-6 by Dr. Homer 

 in the Sea of Okhotsh. The surface-temperature was 46-4 F. ; and the 

 author finds (after correcting the original readings) at 360 feet a tempera- 

 ture of 28, and at 690 feet of 28-6, which is almost exactly that determined 

 by Despretz as the temperature of sea-water at the moment of congelation. 

 The cause of the decrease of temperature with the depth in the great 

 oceans was early investigated by physicists. Humboldt concluded that 

 " the existence of these cold layers in low latitudes proves the existence 

 of an undercurrent flowing from the poles to the equator." D'Aubuissou 

 and Pouillet took the same view. D'Urville went further, and remarked 

 that " it is rather a transport nearly in mass, and very slow, of the deep 

 waters of high latitudes towards the equator," and that from his zone of 



