Mr. J. Croll on the Physical Cause of Ocean-currents. 263 



I. 



II. 



III. 



Temperature of surface . . . 

 At 10 fathoms 



o 



770 

 710 

 61-5 

 600 

 573 

 56-7 

 55-5 



80-0 

 740 

 64-5 

 63-0 

 603 

 59-7 

 58-5 



20 „ 



30 „ 



40 „ 



50 „ 



100 „ 





Column II. shows the temperature as observed by Dr. Car- 

 penter. If a similar rate of decrease takes place at the equator, 

 which is highly probable, then column III. will show the tem- 

 peratures at the equator to the depth of 100 fathoms. Dr. Car- 

 penter says that in the Atlantic he "found that, after pass- 

 ing through the heated surface-layer, there was a slow nearly 

 uniform descent of temperature down to the ( stratum of inter- 

 mixture/ in which there was another sudden drop of 10°." We 

 may therefore in our calculations assume the decrease of tempe- 

 rature to be uniform below 100 fathoms. We have now a means 

 of determining with more accuracy than before the actual height 

 of the surface of the ocean at the equator above that at the poles. 

 Taking, as before, Muncke's Table of the rate of expansion of sea- 

 water, it turns out that the height of the equatorial column above 

 the polar amounts to little more than 8 feet. But to give Dr. 

 Carpenter's theory full justice, I shall assume the temperature of 

 polar water from the surface to the bottom to be not 32°, but 

 three degrees lower, viz. 29°. This will make the slope from the 

 equator to the pole about 9 feet, or one half what I had made it 

 in my former paper. The distance from the equator to the poles 

 is 32,758,000 feet. But to simplify calculations, let us take 

 the distance in round numbers at 31,500,000 feet. This reduc- 

 tion of the distance is, of course, so far in favour of Dr. Car- 

 penter's theory. We have here an inclined plane 31,500,000 

 feet in length and 9 feet in height. The height of the plane to 

 its length is therefore as 1 to 3,500,000. According to the prin- 

 ciple of the inclined plane, the force of gravity tending to move 

 a pound of water down this plane is x of a pound, or 



g^-Q- of a grain. Were water a perfect fluid and could move 

 without any resistance, a pound of water under the pressure of 

 5 00 °f a g ram would flow down from the equator to the pole 

 with an accelerated motion, and on reaching the pole it would 

 have acquired the same velocity as it would have done supposing 

 it had fallen vertically through a distance of 9 feet. It would reach 

 the pole with an amount of energy in the form of motion equal to 

 9 foot-pounds. Water, however, is not a perfect fluid, but offers 



