232 LIGHT SCIENCE FOR LEISURE HOURS. 



meters not protected against pressure, set this uniform 

 temperature too high. In the western basin of the 

 Mediterranean, as shown by the Porcupine observations 

 of 1870, the uniform temperature is 54 or 55 degrees ; 

 being, in fact, the winter temperature of the entire 

 contents of the basin, from the surface downwards ; and 

 being also, it would appear, the mean temperature of 

 the crust of the earth in that region.' We learn, then, 

 two things viz., first, that where extensive submarine 

 motions are impossible, a constant submarine tempera- 

 ture may be expected to prevail in the same latitudes ; 

 and, secondly, that in the latitude of the Mediterranean 

 the submarine temperature is about 54 or 55 degrees 

 Fahr. Thus, it is clear, in the first place, that the 

 varieties of temperature observed in the depths of the 

 Atlantic must be due to the continual arrival of water 

 of the observed temperatures, at a rate great enough to 

 prevent the deep water from acquiring a constant tem- 

 perature ; and in the second place it becomes possible 

 to tell whence the submarine currents are flowing. If 

 they are cooler than they should be supposing latitude 

 alone in question, then they are travelling from arctic 

 towards tropical regions, and vice versa. On this last 

 point no doubt remains. In a latitude corresponding 

 to that of the Mediterranean basin, the depths of the 

 Atlantic are far colder, even in their warmest por- 

 tions, than they would be if latitude alone were in 

 question. ' In regard to surface-temperature,' says 

 Dr. Carpenter, 'there is no indication of any essen- 

 tial difference between the Mediterranean and the 



