TEMPEEATURES. 



219 



The soundings made by Commander Bartlett after I left the 

 " Blake," and by the " Albatross," to determine the ridges 

 which unite the various islands between Sombrero and Trinidad, 

 show plainly that the cold water of the bottom of the Caribbean 

 can only come in through the passage between Sombrero and 

 the Virgin Islands. This passage has a depth of about 1,100 

 fathoms, with a bottom temperature of 38° ; the water which 

 passes over the ridge connecting Santa Cruz with Porto Rico 

 has a depth of about 900 fathoms and a temperature of 39^°. 

 The five-hundred-fathom line, as I have stated, marks a large 

 bank formed of all the islands to the south of Sombrero, in- 

 cluding Dominica, with a narrow oceanic bay of 575 fathoms 

 between it and Martinique ; the five-hundred-fathom line again 



CasxpeclLe'BaiLk ^ ^ 

 Tu.ca.tan. ' gj\._g 





Fig. 143. 



uniting all the islands to the south of it into one large spit, as a 

 part of South America. Thus the bulk of the water forced into 

 the Caribbean Sea has a comparatively high temperature, — an 

 average, probably, of the temperature of the three-hundred- 

 fathom fine. The cold water of the Atlantic is, however, also 

 forced into the western basin of the Caribbean through the 

 Windward Passage, and this passes through the Yucatan Chan- 

 nel, between Cape San Antonio and the Yucatan Bank. 



The greatest depth of the Yucatan Channel (Fig. 143) is 

 somewhat more than 1,100 fathoms, so that the temperature of 



