1872.] 



' Shearwater 3 Scientific Researches, 



553 



since u a pressure of one fourth of a grain on the cubic foot of water would 

 " be totally inadequate to overcome the mere molecular resistance of the 

 " water to go into motion. " Now not only Mr. Hawksley, but every high 

 Mathematical authority whom I have consulted, has assured me that the 

 " viscosity'' of water is so far from having been satisfactorily determined, 

 that the assertion that it would be adequate to prevent water whose level 

 has been disturbed to the extent just mentioned, from ever finding it again, 

 is totally inadmissible. The very small degree in which this "viscosity" 

 operates to bring to a stand the motion of water upon itself, is shown (as 

 Mr. Hawksley has suggested to me) by the long persistence in the open sea 

 of the "swell" produced by a gale of wind (provided that after the subsi- 

 dence of the gale there be no counteracting wind), and by the propagation 

 of this "swell" to a great distance*. 



23. Now, so far from asserting (as Captain Maury has done) that the 

 trifling difference of level arising from inequality of Temperature is adequate 

 to the production of "ocean-currents," I simply affirm that as fast (or as 

 slowly) as the level is disturbed by change of Temperature, it will be re- 

 stored by Gravity ; and I venture to think that as this is a direct corol- 

 lary from the fundamental conception of a fluid, the onus probandi of the 

 contrary rests on Mr. Croll. "When he shall have shown experimentally 

 that water is so viscid a fluid that it does not find its own level when time 

 is given to it to do so, he will have obtained a definite foundation for rea- 

 soning which is at present simply baseless. 



24. But, further, there is an experimentum crucis in constant progress, 

 which appears to me completely to disprove Mr. Croll's position. It will 

 not, I presume, be denied by him that the semi-diurnal Tide is produced by 

 the attraction of the Moon, augmented or partly neutralized (as the case 

 may be) by that of the Sun. Now, according to Sir John Herschelf, the 

 Moon's maximum of power to disturb the Earth's waters is only about 

 1-1 1,400,000th part of gravity; while that of the Sun does not exceed 

 1 -25,736,400th part of gravity. Yet the Moon's attractive force, even 

 when partly counteracted by that of the Sun (as at neap-tides), suffices to 

 raise and to propagate a vast wave, which, though neither high nor strong 

 in the open Ocean, rises and becomes more rapid where its onward move- 

 ment is checked, and especially where a large body of water is forced to 

 flow in a narrow channel. Now since the disturbance of Gravity produced 

 by difference of Temperature is many times greater than that produced by 

 the combined action of the Sun and Moon, it is for Mr. Croll to show win- 

 such a vast force is without effect. 



25. Mr. Croll proceeds to say (loc. cit. p. 251) : — " Suppose that at the 

 "Equator we have to descend 10,000 feet before water equal in density to 



* I happen to have myself been subjected for three days continuously, in the middle 

 of the Atlantic, to a heavy swell with a dead calm, — one of the most disagreeable of all 

 nautical experiences. 



t ' Outlines of Astronomy,' 3rd Ed., p. 496, note. 



2s2 



