324 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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 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 pulsations in the sea. 



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

 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 the throbbings of the sea-heart, 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 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 intensi- 

 ty, and raises the temperature two degrees. At night the clouds 

 interpose, and prevent radiation from this fifth, whereas the re- 

 maining four 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 up to the stars in a cloudless sky, and serve to lower 

 the temperature of the surface-waters, by radiation, two degrees. 

 Here, then, is a difference of four degrees, which we will suppose 

 extends only ten feet below the surface. The total and absolute 

 change made in such a mass of sea water by altering its temper- 

 ature two degrees, is equivalent to a change in its volume of three 

 hundred and ninety thousand millions of cubic feet. 



923. Do not the clouds, night and day, now present themselves 

 to us in a new light ? They are cogs, and rachets, and wheels in 

 that grand and exquisite machinery which governs the sea, and 

 which, amid all the jarring of the elements, preserves in harmony 

 the exquisite adaptations of the ocean. 



924. It seems to be a physical law, that cold-water fish are more 

 edible than those of warm water. Bearing this fact in mind as 

 we study Plate IX., we see at a glance the places which are most 



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



