254 THE PHYSICAL GEOGRAPHY OF THE SEA. 
shine, to day and night 507). Few persons have ever taken the 
trouble to compute how much the fall of a single inch of rain over 
an extensive region in the sea, or how much the change even of two 
or three degrees of temperature over a few thousand square miles 
of its surface, tends to disturb its equilibrium, and consequently to 
cause an aqueous palpitation that is felt from the equator to the 
poles. Let us illustrate by an example : The surface of the At- 
lantic Ocean covers an area of about twenty-five millions of 
square miles. Now, let us take one fifth of this area, and sup- 
pose 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 vapor, was left behind to 
disturb equilibrium, weighed sixteen millions more of tons, or near- 
ly twice as much as all the ships in the world could carry at a car- 
go each. It 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 Mis- 
sissippi River during the year were taken up in one mighty meas- 
ure, and cast into the ocean at one effort, it would not make a 
greater disturbance in the equilibrium of the sea than would the 
fall of rain supposed. Now this is for but one fifth of the At- 
lantic, 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 144) 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 w^hole 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 vapor of which they 
were formed was taken up at still other places, we shall be en- 
abled to appreciate the better the force and the effect of these 
pulsations in the sea. 
651. Between the hottest hour of the day and the coldest hour 
of the night there is frequently a change of four degrees in the tem- 
perature of the sea.* Let us, therefore, to appreciate the throb- 
* Vide Admiral Smyth's Memoir of the Mediterranean, p. 125* 
