Mr. J. Croll on the Physical Cause of Ocean-currents. 249 



Amount of salt. 



Temperature of 

 freezing-point. 



Temperature of 

 Maximum density. 



0-000123 

 0-0246 

 00371 

 0-0741 



-1-21 O. 

 -2-24 



-2-77 

 -5-28 



+ 1*19 0. 



- 1-69 



— 4-75 

 -16-00 



He found the temperature of maximum density of sea-water, 

 whose density at 20°C. was 1-0273, to be -3°'67 C. (25°*4F.), 

 and the temperature of freezing-point — 2°*55 (27 0, 4F.)*. Some- 

 where between 25° and 26° F. may therefore be regarded as 

 the temperature of maximum density of sea-water of average 

 saltness. We have no reason to believe that the ocean, from 

 the surface to the bottom, even at the poles, is at 27 0, 4F., the 

 freezing-point. An error to the extent of a degree or two, 

 however, will not materially affect the conclusion at which we 

 may arrive. Let us therefore assume the temperature of the 

 ocean at the poles to be 32°, and the surface-temperature at the 

 equator to be 80°. Maury states that at the depth of 7200 feet 

 at the equator the temperature is about 36° (§ 440, eleventh edi- 

 tion). Although this agrees pretty nearly with the results 

 arrived at by several observers who have attempted to deter- 

 mine the temperature of the ocean at great depths in equatorial 

 regions, still 36°, the temperature assigned at 7200 feet below 

 the surface, is probably too high ; for these observations were 

 made with thermometers unprotected from the pressure of the 

 water on their bulbs, which at so great a depth would equal 

 more than 200 atmospheres ; 32°, at a depth of 7200 feet, may 

 probably be nearer the truth than 36°. But we shall assume 

 that we must descend to a depth of, say, 10,000 feet, before the 

 temperature of 32°, that of the poles, is reached. Let us also 

 assume that the temperature decreases at a uniform rate from 

 the surface downwards to that depth. Calculating, then, from 

 Muncke's Table of the Expansion of Sea-water, we have about 

 18 feet as the height at which the water at the equator stands 

 above the level of the ocean at the poles. The distance from 

 the equator to the poles is about 6200 miles. The force im- 

 pelling the water down this slope of 18 feet in 6200 miles would 

 therefore be equal to about j^^o.oiro that of gravity. For ex- 

 ample, the force impelling a cubic foot (64 lbs.) of water at the 

 surface of the ocean would scarcely be equal to the weight of one- 

 fourth of a grain. 



But in reality it would not nearly equal this ; for we have 

 been assuming in our calculations that the temperature of the 

 * Philosophical Magazine, vol. xii. p. 1 (1838). 



Phil. Mag. S. 4. Vol. 40. No. 267. Oct. 1870. ' S 



