THE GULF STREAM. 47 



ity is increased or diminished, will impart to the trough of this 

 stream a vibratory motion, pressing it now to the right, now to the 

 left, according to the seasons and the consequent changes of tem- 

 perature in the sea. 



56. Plate YI. shows the limits of the Gulf Stream for March 

 and September. The reason for this change of position is obvi- 

 ous. The banks of the Gulf Stream (§1) are cold water. In 

 winter, the volume of cold water on the American, or left side of 

 the stream, is greatly increased. It must have room, and gains 

 it by pressing the warmer waters of the stream farther to the south, 

 or right. In September, the temperature of these cold waters is 

 modified ; there is not such an extent of them, and then the warm- 

 er waters, in turn, press them back, and so the pendulum-like mo- 

 tion is preserved. 



57. The observations made by the United States Coast Survey 

 indicate that there are in the Gulf Stream threads of warmer, sep- 

 arated by streaks of cooler water. See Plate VI., in which these 

 are shown ; they are marked x, y, z. Figure A may be taken to 

 represent a thermometrical cross section of the stream opposite the 

 Capes of Virginia, for instance ; the top of the curve representing 

 the thermometer in the threads of the warmer water, and the de- 

 pressions the height of the same instrument in the streaks of cool- 

 er water between, thus exhibiting, as one sails from America across 

 the Gulf Stream, a remarkable series of thermometrical elevations 

 and depressions in the surface temperature of this mighty river in 

 the sea. 



58. These streaks, x^ y, ^, are not found in the Gulf Stream as 

 it issues from its fountain, and I have thought them to be an in- 

 cident of the process by which the waters of the Stream gradually 

 grow cool. Suppose a perfect calm over this stream, and that aU 

 the water on the top of it to the depth of ten feet were suddenly, 

 as it runs along in a winter's day, to be stricken by the wand 

 of some magician, and reduced from the temperature of 75° to 

 that of 32°, the water below the depth of ten feet remaining at 

 75° as before. How would this cold and heavy water sink ? Like 

 a great water-tight floor or field of ice as broad as the Gulf Stream, 

 and loaded to sinking ? And how would the warm water rise to 



