in the Air and in the Sea. 33 



arising from difference of temperature, which, as above remarked, 

 must flow at the surface from warmer into colder zones. 



Evaporation, then, also cannot occasion any perceptible cur- 

 rent in the open sea; but in channels connecting an inland sea 

 with the ocean a difference of level between the two seas, arising 

 from greater evaporation of the inland sea, may occasion a strong 

 current. We have instances of this in the Straits of Gibraltar 

 and Babelmandeb. 



Let us now turn to the consideration of the influence which 

 heat may have indirectly on the origination of sea-currents 

 when, through its action upon the aqueous vapour in the atmo- 

 sphere, it generates wind. The action of wind upon the surface 

 of water is familiar not only to the inhabitants of coasts, but to 

 almost every one. Indeed we see in every pond how the water 

 is driven by a strong wind; and if the basin is flat and not very 

 deep, not seldom does the water recede from the windward 

 side, and accumulate on the lee side. Such heapings-up of the 

 water in shallow bays, and at the mouths of great rivers, by 

 strong winds occasion inundations. At a straight coast-line, 

 too, the water may rise considerably by the force of the wind, 

 if the depth increases very gradually and thereby the outflow 

 beneath is checked. 



Indeed, on the open sea the wind often drives the water 

 before it, and thereby forms what are called drift or superficial 

 currents ; but these, as irregular phenomena, do not here come 

 into consideration ; we can only occupy ourselves with those 

 currents which are called forth by constant winds, i. e. the 

 trade-winds. Even the constant action of the trade-winds, 

 however, is hardly able to occasion any very deep-going current, 

 as has already been sufficiently shown by Maury and Muhry. 



According to FitzPtoy's data, the highest waves rise to the 

 height of 60 feet*, measured from the trough to the crest of the 

 wave, therefore 30 feet above the smooth surface of the sea. If 

 we might assume that the entire wave could be driven forward 

 by the wind (which is decidedly assuming too much), thus 

 would be produced a current of 30 feet depth. Through the 

 friction of the water-particles, the effect of the wind upon the 

 current may become sensible somewhat deeper still ; but the 



* This number appears to us very high ; for we have often, in a severe 

 storm, ascending the shrouds, tried to bring our eye into such a position 

 that, at the moment when the ship was exactly in the trough, we could 

 see several wave-crests in a horizontal line. The height of the eye above 

 the ship's water-line then determines the greatest height of the wave. In 

 this way I have only once at Cape Horn (where the waves rise uncommonly 

 high) measured a height of 46 feet, and at the coast of Japan, in a typhoon, 

 one of 38-40 feet; at other times the height was mostlv less than this. 



Phil. Mag. S. 4. Vol. 48. No. 315. July 1874. I) 



