Page:Physical Geography of the Sea and its Meteorology.djvu/72

46 resisting permeability, we are enabled to comprehend how 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 temperature in the sea.

126. Limits of the Gulf Stream in March and September.—Plate VI. shows the limits of the Gulf Stream for March and September. The reason for this change of position is obvious. The banks of the Gulf Stream (§70) 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.

127. Reluctance of layers or patches to mingle.—In the offings of the Balize, sometimes as far out as a hundred miles or more from the land, puddles or patches of Mississippi water may be observed on the surface of the sea with little or none of its brine mixed with it. This anti-mixing property in water has already (§ 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 found in juxtaposition. The patches of white, black, green, yellow, and reddish waters so often met with 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 strikingly developed in the neighbourhood of the Gulf Stream, that persons have been led to suppose that the Gulf Stream has forks in the sea, and that these are they.

128. Streaks of warm and cool.—Now, if any vessel will take up her position a little to the northward of Bermuda, and steering thence for the Capes of Virginia, will try the water-thermometer all the way at short intervals, she will find its readings to be now higher, now lower; and the observer will 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 bifurcations of the Gulf Stream, and view them rather in the light of thermal streaks of water which have, in the