TRANSACTIONS OF SECTION E. 705 



near all sea-sbores off which a steady wind hlows the water is cooler than further 

 to seaward. 



This has an important bearing on coral growth, and explains why on all 

 western coasts of the great continents off which the trade winds blow we find an 

 almost absolute dearth of coral, while on the eastern 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. Compared with the area 

 of the oceans they are but few, but our knowledge steadily increases every year. 



The subject of the vertical distribution of temperature has not yet been 

 thoroughly investigated in the light of the whole of the information which we now 

 possess, but Dr. Alex. Buchan has been for some time devoting his spare time to 

 the task, and it is a heavy labour, 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 wiU 

 prove very interesting, in the last volume of the ' Challenger ' series. 



It will readily be understood that observations on temperatures at great depths 

 require great care. In the first place the thermometers must be most carefully 

 manufactured. They must be subjected to rigorous tests, and they must be care- 

 fully handled during the operation. AU 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 facts. 



We have learnt 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 generally spsakin"- 

 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 600 

 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 diflerent parts of the ocean. 



One of the most interesting facts that has been recognised Is that in enclosed 

 hollows of the ocean the bottom temperature is apparently much less than that 

 of the stratum of water at a corresponding depth in the waters outside the sub- 

 marine ridge that forms the enclosing walls, separating them from deeper areas 

 beyond, and is, in all cases that have been observed, equal to that on the ridge. 

 From this fact we are enabled to supplement our imperfect knowledge of depths, 

 because if in a certain part of an ocean we find that the temperature at great 

 depths is higher than we know exists at similar depths in waters apparently con- 

 nected, we can feel certain that there is a submarine ridge which cuts off the 

 bottom waters from moving along, and that the depth on this ridge is that at 

 which ia found the corresponding temperature in the outer waters. As a corollary 

 we also assume that the movement of water at great depths is confined to an 

 almost imperceptible movement, for if there was a motion that we could term, in 

 the ordinary acceptation of the word, a current, it would infallibly surmount a 

 ridge and pour over the other side, carrying its lower temperature with it. 



A notable instance is the bottom temperature of the North Atlantic. This is 

 nowhere below 35° F., although the depths are very great. But in the South 

 Atlantic at a depth of only 2,800 fathoms the bottom temperature is but a little 

 above 32° F., and we are therefore convinced that somewhere between Africa 

 and South America, though soundings do not yet show it, there must be a ridge 

 &t a depth of about 2,000 fathoms. 



"We also come to the same conclusion with regard to the eastern and western 

 portions of the South Atlantic, where similar differences prevail. 



1894. z z 



