670 DYNAMICAL GEOLOGY. 



passed freely into the Pacific (the West India islands being also in the 

 depths of the ocean, as would be necessary for the result), a great 

 change would thereby be produced in the temperature of both the 

 Atlantic and the Pacific, — a loss of heat to the former and a gain to 

 the latter. (See Physiographic Chart ) But no facts yet observed 

 prove this supposition to have been a realized fact since the opening 

 of the Silurian age. A shallow-water connection across the isthmus 

 between the two oceans probably existed as late as the Cretaceous, as 

 has been inferred from the parallel series of representative species now 

 existing on the two sides. 



■ Besides the general system of currents, which has been considered, there are currents 

 between the ocean and some confined seas opening into it, which are due to the evap- 

 oration going on over the surface of those seas. The consequent diminution of water 

 causes a flow at surface from the ocean, to supply the loss. This happens at the Straits 

 of Gibraltar, opening into the Mediterranean. At bottom, there is a flow outward, of the 

 denser water. In many seas of this kind, the accessions from rivers more than supply 

 the amount removed by evaporation; and these produce an out-current at the entrance. 

 The Black Sea, by losing much of its salt, is rendered less dense than Sie ^Egean, to 

 the south, and hence there is an under-current into it at the Dardanelles. 



2. Ordinary Wind-Waves and Currents. 



1. Waves. — The winds are an almost incessant wave-making power. 

 Even in the calmest weather, there is some breaking of wavelets 

 against the rocky headlands or exposed beaches ; and, with ordinary 

 breezes, the beaches and rocks are ever under the beating waves, night 

 and day, from year to year. Most seas, moreover, have their storms ; 

 and in some, as those about Cape Horn, gales prevail at all seasons. 

 The breakers on the shores of the Pacific are especially heavy, on 

 account of its extent and depth. Through a large part of the ocean, 

 the winds are nearly constant in direction either for the year or half- 

 year. 



The highest waves measured by Scoresby rose 43 feet above the 

 trough between them, or 21^ feet above the main water plane or plane 

 of rest. In a wave each particle of water moves in a circle about 

 its centre of rest, — a circle of 21^ feet radius in a wave of 43 feet. 

 But these circles at a depth of only one wave-length have a radius 

 1 -535th of that at the surface, and at a depth of two wave-lengths, 

 l-300,000th; so that if, for the 43-foot waves, the wave-length — or 

 the distance between the crest of two consecutive waves — is 300 feet, 

 the circle at a depth of one wave-length will have a diameter of 4-10ths 

 of an inch, and at two wave-lengths, 1-1 200th of an inch. Conse- 

 quently the movement of the heaviest waves is exceedingly slight at 

 a depth of 100 fathoms. 



Waves made by winds as they advance toward a shore over a shelv- 



