224 THREE CRUISES OF THE " BLAKE." 



miles in a depth of 56 fathoms. This cold water may come 

 from the colder water of the deeper channel on the east of the 

 island. The water is also colder near San Domingo, where we 

 find at 339 fathoms 49°, while seven miles to the eastward, 

 toward Porto Rico, it is 51° in 337 fathoms. This section is 

 very similar to that of the Anegada Channel. 



The temperature sections of the passages between the Wind- 

 ward Islands on the eastern side of the Caribbean — Dominica, 

 Martinique, St. Lucia, and St. Vincent — are in striking con- 

 trast to those of the northern side, the Anegada, Mona, and 

 Windward Passages. At a depth of 75 fathoms there is a dif- 

 ference of no less than 9° ; at 200 fathoms, one of 10° ; and one 

 of 5° at 300 fathoms. The equilibrium is only restored at a 

 depth of 600 fathoms, where we have again the temperature of 

 40°. We find repeated here on a smaller scale the variations of 

 temperature at given depths near the equator and in the more 

 temjDerate zones. A temperature of 41° has been observed in 

 the Atlantic near the equator at a depth of only about 250 

 fathoms, while in the temperate latitudes both north and south 

 of the equator we must go to a depth of at least 600 fathoms 

 in one case, and of over 400 fathoms in the other, to obtain the 

 same temperature. In other words, there must be an active 

 lateral and upward flow of the cold water, and this establishes 

 one of the primary causes of the equatorial oceanic circulation. 

 It also shows that the temperature of the deepest point of the 

 ridge which separates an inland sea, and thereby forms a med- 

 iterranean, is not necessarily that of the bottom of the enclosed 

 sea. It is very e\ddent that, supposing the ridge to have been 

 only 300 fathoms deep, we could have a bottom temperature of 

 50°, which would indicate an oceanic ridge of 350 fathoms, on 

 the same principles that have led Dr. Carpenter and others to 

 speak of the bottom temperature of enclosed seas as assuming 

 that minimum temperature. 



The section across the Windward Passage (Fig. 149) bears a 

 close resemblance to the oceanic temperature section of that 

 region. The belt of water between 400 and 800 fathoms is, 

 however, considerably warmer. It also resembles the section 

 between Jamaica and San Domingo (Fig. 150), though the belt 



