§ 758. TIDE-RIPS AND SEA DRIFT. 409 



sea, and which, when that water was taken up as vapor, was left 

 behind to 'disturb equilibrium, weighed sixteen millions more of 

 tons, or nearly twice as much as all the ships in the world could 

 carry at a cargo each. This rain might fall in an hour, or it might 

 fall in a day ; but, occupy what time it might in falling, this rain 

 is calculated to exert so much force — which is inconceivably great 

 — in disturbing the equilibrium of the ocean. If all the water 

 discharged by the Mississippi Eiver during the year w^ere taken 

 up in one mighty measure, and cast into the ocean at one effort, 

 it would n.ot make a greater disturbance in the equilibrium of the 

 sea than would the supposed rain-fall. Now this is for but one 

 fifth of the Atlantic, and the area of the Atlantic is about one 

 fifth of the sea-area of the world ; and the estimated fall of rain 

 w^as but one inch, whereas the average for the year is (§ 757) six- 

 ty inches ; but we will assume it for the sea to be no more than 

 thirty inches. In the aggregate, and on an average, then, such 

 a disturbance in the equilibrum of the whole ocean as is here sup- 

 posed occurs seven hundred and fifty times a year, or at the rate 

 of once in twelve hours. Moreover, when it is recollected that 

 these rains take place now here, now there ; that the vapor of 

 which they were formed was taken up at still other places, we 

 shall be enabled to appreciate the better the force and the effect 

 of these irregular movements in the sea. 



758. Between the hottest hour of the day and the coldest 

 Ditto ofcioud and sun- ^our of the night there is frequently a change 

 ^^^°^ of four degrees in the temperature of the sea."^ 



Let us, therefore, the more thoroughly to appreciate those agita- 

 tions of the sea which take place in consequence of the diurnal 

 changes in its temperature, call in the sunshine, the cloud without 

 rain, with day and night, and their heating and radiating proc- 

 esses. And to make the case as strong as to be true to nature 

 we may, let us again select one fifth of the Atlantic Ocean for 

 the scene of operation. The day over it is clear, and the sun 

 pours down his rays with their greatest intensity, and raises the 

 temperature two degrees. At night the clouds interpose, and 

 prevent radiation from this fifth, whereas the remaining foui- 

 fifths, which are supposed to have been screened by clouds, so as 

 to cut off the heat from the sun during the day, are now looking 



* Mde Admiral Smyth's Memoir of the Mediterranean, p. 125. 



