July 26, 1895.] 



SCIENCE. 



91 



on the eontrai'}^, the noi-thern limit of the 

 southeast trade winds sometimes passes, 

 with the northern declination of the sun, 

 as much as five degrees above the geograph- 

 ical equator. Between these two currents, 

 whose north and south limits are respect- 

 ively above the line of the Tropic of Cancer 

 and below that of the Tropic of Capricorn, 

 depending upon the annual declination of the 

 sun, and always ranging higher above the 

 Tropic of Cancer than below the Tropic of 

 Capricorn, lies the region of equatorial calms, 

 squalls and variable winds, called by sailors 

 the doldrums, within whose eastern area a 

 counter current, of length varying according 

 to the season, contributes to the Guinea 

 Current on the coast of Africa, also contrib- 

 uted to by the cool southerly current along 

 the northwest coast of Africa, called the 

 North African Current. 



It will readily be seen from the above state- 

 ment and from examination of the map of 

 the Atlantic that, during the summer sea- 

 son of the northern hemisphere, the largest 

 contribution by the Equatorial Currents 

 must be made to the Gulf Stream, for not 

 only is the whole of the North Equatorial 

 Current then, as always, north of the equa- 

 tor, but the South Equatorial Current not 

 only overlaps the equator, but extends then 

 for some distance to the southward in a 

 favorable position for the entrance of much 

 of its water, deflected to the northwest by 

 Cape Eoque, and then known as the Guiana 

 Current, to enter the Carribbean Sea. In 

 both seasons the Atlantic northern hemi- 

 sphere must therefore receive and deliver 

 from near the equator a larger supply of 

 surface-moving water than the southern 

 Atlantic hemisphere ever thence receives 

 and delivers. The slower current in the 

 same direction, on the eastern edge of the 

 Gulf Stream mentioned by Lieutenant Pills- 

 bury, is cleai'ly shown by the observations 

 of the Challenger, in ascending and de- 

 scending the coast of the United States, to 



be the edge of a broad band of water en- 

 veloping the Bermudas. It is therefore 

 part of the great general easterly and north- 

 easterly movement of the Atlantic waters 

 of the northern hemisphere, a portion of 

 which movement eventually impelled by 

 the anti-trade winds gradually goes to 

 constitute the reflux flow towards Africa, 

 called the Northern Connecting Current. 



It has been here said that the body of 

 water which enters the Caribbean Sea as 

 drift currents, produced by the northeast and 

 the southeast trade winds, leaves the resid- 

 uum to be chiefly supplied by the flow of 

 the polar current under and beside the Gulf 

 Stream issuing from the Straits of Florida. 

 To this is to be added the volume discharged 

 into the Gulf of Mexico and the Caribbean 

 Sea bj' the various rivers, and we have 

 what makes, allowing for increase and 

 diminution of volume from temperature and 

 evaj^oration, the sum-total volume to be 

 reckoned for as creating the head of water 

 in the Gulf of Mexico. Taking the temper- 

 ature of the general sea surface on a line 

 just outside of the Greater and Lesser An- 

 tilles to be about 74 F., it is well below that 

 recorded in the Straits of Florida by Lieu- 

 tenant Pillsbury and other authorities, as 

 often over 80 F., one of them giving a record 

 as high as 86 F. That there should be a 

 considerable rise of temperature between 

 the equatorial waters and those of the Gulf 

 of Mexico, in the Straits of Florida, should 

 not surprise anyone who knows that, ac- 

 cording to the investigations of the Chal- 

 lenger, cool water has been found to rise 

 more closely to the surface nearly under the 

 equator than elsewhere in the Atlantic and 

 the Pacific, the temperature of 40° F. reach- 

 ing to within three hundred fathoms of a 

 surface at nearly 80 F. The equatorial sur- 

 face generally is su.rprisingly cool, consider- 

 ing its geographical position, and the equa- 

 torial waters are of a lower salinity than are 

 the tropical waters on either side. We 



