THE PHYSICAL CONDITION OF THE OCEAN. 351 



A remarkable point recently bronglit to light by the researches of 

 Mr. John Murray in Scotch lochs is the eifect of wind on the surface 

 temperature. It has been observed that wind driving off a shore drifts 

 the surface water before it. This water is replaced by the readiest 

 means, that is to say, by water from l)elow the surface rising to take 

 its place. As the lower strata are in all cases cooler than the surface a 

 lowering of the temperature results, and we find in fact that near all sea- 

 shores off which a steady wind blows the water is cooler than farther to 

 seaward. This has an important bearing on coral growth, and explains 

 why on all western coasts of the great continents off" which tlie trade 

 winds blow we find an almost absolute dearth of coral, while on the east- 

 ern coasts, on which warm currents impinge, reefs abound, the coral 

 animal flourishing only in water above a certain temperature. 



Observations of the temperature of the strata of water between the 

 surface and bottom have been of late years obtained in many parts. 

 Comiiared with the area of the oceans they are but few, but our knowl- 

 edge steadily increases every year. The subject of the vertical distri- 

 bution of temperature has not yet been thoroughly investigated in the 

 light of the whole of the information which we now possess, but Dr. 

 Alexander Buchau has been for some time devoting his spare time to the 

 task, and it is a heavy labor, for the data obtained here and there over 

 the world by different ships of all maritime nations are very difficult to 

 collect and to appraise; but I understand that before long we shall have 

 the result, which will prove very interesting, in the last volume of the 

 Challenger series. It will readily be uiulerstood that observations on tem- 

 peratures at great depths require great care. In the first place, the ther- 

 mometers must be most carefully manufactured. They must be subjected 

 to rigorous tests, and they must be carefully handled during the oper- 

 ation. All observations are not of the same value, and the discussion 

 therefore presents considerable difficulty and demands much discretion. 

 In the meantime we can state certain known tacts. We have learned 

 that the depth of the warm surface water is small. In the equatorial 

 current between Africa and South America, where the surface is of a 

 temperature of 78°, at 100 fathoms it is only 55°, a difference of 23°, and a 

 temperature of 40° is reached at 400 fathoms. In this region, so far as 

 knowledge goes, the fall in temperature as we descend is most rapid; 

 but, generallj^ speaking, the same variations prevail everywhere. In 

 the tropical Pacific the temperature falls 32° from the surface, where it 

 stands at 82°, to a depth of 200 fathoms, 40° being reached at from 

 500 to ()00 fathoms below the surface. Below the general depth of from 

 400 to 600 fathoms the temperature decreases very slowly, but there 

 is considerable variation in the absolute amount of it when we get to 

 great depths in different parts of the ocean. 



One of the most interesting facts that has been recognized is that in 

 inclosed hollows of the ocean the bottom temperature is apparently 

 much less than that of the stratum of water at a corresponding depth 



