cxviii GENERAL SUMMARY OF SCIENTIFIC AND 



The publication of the report of the operations of the 

 United States steamer Tuscarora has already been referred 

 to ; and Dr. Carpenter, in discussing her observations made 

 in the deep seas between the United States and Japan, infers 

 the general want of that sub-surface stratum of above 40 

 Fahr. which in the North Atlantic, with the same or yet 

 higher parallels, has a thickness of at least 500 fathoms. The 

 true cause of this peculiarity is that the North Pacific de- 

 rives its deep stratum of glacial water, which nearly fills its 

 basin, from the polar area of the opposite hemisphere, the in- 

 let at Behring Strait being too narrow and too shallow to 

 admit a flow of water of any appreciable importance. This 

 northward flow of the water from the equator must have, as 

 its complement, a movement of the superficial stratum from 

 the northernmost limit of this flow toward the equator, and 

 thence toward the southern pole. The glacial current when 

 it reaches the North Pacific comes nearer the surface than it 

 does in the Southern Ocean, even in the higher latitudes ; 

 and this, modifying still further the reflex surface flow to- 

 ward the equator, appears to account for the well-known 

 moderation of the climate of the Sandwich Islands, though 

 they lie within the Tropic of Cancer. 



The /Shearwater, a British vessel, completed in 1875 a four 

 years' course of surveying service, having been put in com- 

 mission on the 20th of July, 1871, and returned to Sheerness 

 on the 23d of July, 1875. She was originally commanded 

 by Captain Nares, who, when transferred to the Challenger, 

 was succeeded by Captain W. J. L. Wharton. While on 

 her original duty in the Mediterranean she was employed 

 for a time in prosecuting certain important physical inves- 

 tigations, under Dr. Carpenter, and after two years' service 

 there she was sent to Zanzibar to survey that island and 

 the opposite coast. In February, 1874, the vessel proceeded 

 to the Cape of Good Hope, and left Cape Town July 14, 

 with the Rodriguez transit party, remaining for some time 

 at Rodriguez on specific service. She then transported the 

 transit party to the Mauritius, and again proceeded to Zan- 

 zibar to continue her work there. 



According to Nature, during the four years she has been 

 in commission the vessel has surveyed in detail 790 miles of 

 coast-line, and made many soundings over an area of 5900 

 square miles. 



