252 Mr. W. Crouch on Physical Cunditions of the Sea. 



islands. As regards density, it had always been assumed that 

 such would largely increase at extreme depths, but it now 

 appears that it scarcely increases at all beyond 2000 fathoms. 



Referring to tides, and consequently including "bores," he 

 said that in the Amazon, at the equinoxes, for three successive 

 days they have five waves from twelve to fifteen feet high, which 

 follow one another regularly up the river ; and as to the general 

 circulation of the ocean. Sir W. Thompson had said, " We have 

 now good reason to believe that the indraught of water at a low 

 temperature into the Atlantic and Pacific from the southern 

 seas is to a great extent due to an excess of precipitation over 

 evaporation in the water hemisphere, and a corresponding excess 



of evaporation over precipitation in the land hemisphere 



That, in fact, a part of the circuit of general ocean circulation 

 passes through the atmosphere. 



Mr. Crouch then passed on to ocean temperatures, and their 

 causes. It had been estimated by the ' Challenger ' expedition 

 that the direct rays of the sun penetrated to a considerable depth, 

 probably in clear water to 600 ft. Highest recorded temperature, 

 90° in Red Sea. The temperature of the sea falls very slowly 

 from 39° to 28-4° (its freezing point), because its whole depth 

 cools together. 



Touching upon the composition of the ocean-bed and the 

 OJobigenna ooze of the Pacific, he alluded to the continuity of 

 the deposit of the chalk, as shown by the various forms, tests, 

 and remains from this grey ooze. In the channel between the 

 Caroline and Ladi-one Islands, at a depth of 4575 fathoms, the 

 bottom was of red clay, binding together the siliceous tests of 

 Radiolarians and Diatoms. The lecturer, in describing the 

 colour of the sea, said that the true normal colour, as may be 

 seen in the Tropics and in the Mediterranean, is of a deep rich 

 blue ; but this may change to purple, red, brown, yellow, to even 

 black. Dr. Carpenter was of opinion that the peculiar blue colour 

 of the Gulf Stream was probably due to the very finest particles of 

 river silt from the Mississippi. Colour is often caused, too, by 

 the nature of a shallow bottom. 



The Red Sea owes its tint to a series of minute Algae, which at 

 times give it the appearance of clotted blood. This Alga, 

 Trichodesmium, Prof. Moseley describes as like minute fragments 

 of chopped hay. It has also recently been observed that the 

 density of sea-water has a most important bearing upon the 

 question of colour, and that frequently the brown waters of a 

 river will flow for many miles over the clear blue waters of the 

 ocean without mingling, owing to the great difference in density. 

 In Arctic seas, the masses of pelagic life alter the colour to such 

 an extent that whalers are able to tell the locality of whales by 

 this presence of their food. 



