Page:Africa by Élisée Reclus, Volume 3.djvu/22

 expedition in a part of the Azorian Atlantic presenting great inequalities of depth, and where four soundings revealed 500, 95, 150, and 830 fathoms in rapid succession. Nevertheless, the same naturalists observed that the pate was of a sea-green colour between the Canaries and Cape Verd group, where, instead of meeting with comparatively shallow water, they determined a mean depth of no

less than 1,500 fathoms. In fact, this phenomenon of shifting colours in seawater has been explained by physicists in the most diverse ways. While Toynbee asserts that a green tint prevails when the surface is cool and the atmosphere moist, the observations of the Gazelle would seem to show that the contrast between blue and greenish hues is due to the different degrees of salt held in solution, blue water being the most dense.