TIDE-EIPS AND SEA DRIFT. 393 



the change even of two or three degrees of temperature over a 

 few thousand square miles of surface, tends to disturb its equili- 

 brium, and consequentl}^ to cause an aqueous palpitation that is 

 felt from the equator to the poles. Let us illustrate by an 

 example : The surface of the Atlantic Ocean covers an area of 

 about twenty-five millions of square miles. Now let us take one 

 fifth of this area, and suppose a fall of rain one inch deep to take 

 place over it. This rain would weigh three hundred and sixty 

 thousand millions of tons ; and the salt which, as water, it held 

 in solution in the sea, and which, when that water was taken up 

 as vapour, 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, to occupy what time it 

 might in falling, it is calculated to exert so much force — which is 

 inconceivabl}^ great — in disturbing the equilibrium of the ocean. 

 If all the water discharged by the Mississippi River during the 

 year were taken up in one mighty measure and cast into the 

 ocean at one effort, it would not 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 was but one inch, whereas the average for 

 the year is (§ 757 j sixty 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 equilibrium of the whole 

 ocean as is here supposed 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 vapour of which they were formed was taken up 

 at still other places, we shall be the better enabled to appreciate 

 the force and effect of these irregular movements in the sea. 



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

 Ditto of cloud and hour of the night there is frequentl}^ a change of 

 sunshine. ^^^^ degrees in the temperature of the sea.* Let 



us, therefore, the more thoroughly to appreciate those agitations 

 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 processes. 

 And to make the case as strong as, with truth to nature, we may^ 

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



