44 THE PHYSICAL GEOGRAPHY OF THE SEA. 
level, than Nature is sure with her efforts to restore equilibrium in 
both sea and air whenever, wherever, and by whatever it be dis- 
turbed. Therefore, though the waters of the Gulf Stream do not 
extend to the bottom, and though they be not impenetrable to the 
waters on either hand, yet, seeing that they have a waste of waters 
on the right and a waste of waters on the left, to which 2) they 
offer a sort of resisting permeability, we are enabled to compre- 
hend how the waters on either hand, as their specific gravity is in- 
creased or diminished, will impart to the trough of this stream a 
vibratory motion, pressing it now to the right, now to the left, ac- 
cording to the seasons and the consequent changes of temperature 
in the sea. > 
54. Plate VI. 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 warmer 
waters, in turn, press them back, and so the pendulum-like motion 
is preserved. 
55. 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 VL, in which these 
are shown. Figure A may be taken to represent a thermomet- 
rical 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 depressions the height 
of the same instrument in the streaks of cooler 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. 
56. As a rule, the hottest water of the Gulf Stream is at or near 
the surface ; and as the deep sea thermometer is sent down, it 
shows that these waters, though still far warmer than the water on 
either side at corresponding depths, gradually become less and 
less warm until the bottom of the current is reached. There is 
reason to beheve that the warm waters of the Gulf Stream are no- 
