44 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



resisting permeability, we are enabled to comprehend bow the 

 waters on either hand, as their specific gravity 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 temperatm^e in the sea. 



126. Plate VI. shows the limits of the GuK Stream for March 

 stream ?a March"^^ ^^^ September. The reason for this change of posi- 

 and September. tion is obvious. The bauks of the Gulf Stream 

 (§ 70) are cold water. In mnter 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 tempera- 

 ture of these cold waters is modified ; there is not such an extent 

 of them, and then the warmer waters, in tm^n, press them back, 

 and so the pendulum-like motion is preserved. 



127. In the offings of the Balize, sometimes as far out as a hun- 

 Reiuctance of layers drod milcs or more from the land, puddles or patches 

 or patches to mingle, ^f Mississippi watcr maybe observed on the sm'- 

 face of the sea with little or none of its brine mixed with it. This 

 anti-mixing property in water has abeady (§ 98) been remarked 

 upon. It may be observed from the gutters in the street to the 

 rivers in the ocean, and everywhere, wherever two bodies of 

 water that differ in colour are fomid in juxtaposition. The patches 

 of white, black, green, yellow, and reddish waters so often met mth 

 at sea are striking and familiar examples. We have seen, also, that 

 a like proclivity exists (§99) between bodies or streams of water that 

 differ in temperature or velocity. This peculiarity is often so strik- 

 ingly developed in the neighbourhood of the Gulf Stream, that 

 persons have been led to suppose that the GuK Stream has forks in 

 the sea, and that these are they. 



128. Now, if any vessel will take up her position a little to the 

 streaks of warm and uorthward of Bermuda, and steering thence for the 

 ^^°^' Capes of Virginia, will try the water-thermometer 

 all the way at short intervals, she mil find its readings to be now 

 higher, now lower ; and the observer ^ill discover that he has 

 been crossing streak after streak of warm and cool water in regular 

 alternations. He will then cease to regard them as bifm-cations 

 of the Gulf Stream, and view them rather in the light of thermal 

 streaks of water which have, in the plan of oceanic ckculation 

 and in the system of imequal heating and cooling, been brought 

 together. 



