748 



THE POPULAR SCIENCE MONTHLY. 



COHRESPONDEISrCE. 



"THE TIDES." 

 To the Editor of the Popular Science Monthly. 



I HAVE read with a good deal of inter- 

 est Prof. Schneider's article on " The 

 Tides," in the July number of the Month- 

 ly. I was pleased with his method of ap- 

 proaching the problem, because it deals 

 with the planetary bodies as we actually see 

 them in motion, not demanding that effort 

 of the imagination required in studying the 

 problem simply as one of static equilibrium. 

 He has succeeded in rendering tolerably in- 

 telligible from a new standpoint a subject 

 which is perhaps left for the average reader 

 in a somewhat unsatisfactory state in our 

 popular works on astronomy and physical 

 geography. 



In proportion as he has done this por- 

 tion of his work well, is any error of state- 

 ment into which he may have been led lia- 

 ble to prove mischievous. This is my only 

 excuse for venturing to offer any criticism 

 on the work of one who has really done 

 valuable service in presenting familiar 

 truths in new aspects. 



Nowhere is our author more clearly 

 wrong than in his own criticism of the com- 

 monly-accepted theory of the causation of 

 the tides. He admits, apparently, that the 

 attraction of the moon, or of the sun, is 

 capable of lifting into a tidal protuberance 

 the waters that lie, in popular parlance, 

 directly beneath them ; but that the earth 

 itself should be drawn away from the 

 waters upon its opposite surface, he pro- 

 nounces preposterously absurd. " It has 

 been proved experimentally," he says, 

 "that all bodies on the surface of the earth 

 are heavier at midnight than at any other 

 hour of the twenty-four." He cites no au- 

 thority for this statement, which is simply 

 inconsistent with the observed fact that at 

 midnight, leaving out of account the influ- 

 ence of the moon, the tide is rising instead 

 of falling. The state of the tide, however, 

 as we shall perhaps have occasion to indi- 

 cate hereafter, is not a trustworthy meas- 

 ure of the local variations in that gravita- 

 tive force which manifests itself as weight. 

 Unless, therefore, delicate experiments with 

 the pendulum have actually demonstrated 

 the existence and amount of such diurnal 

 variations, we can only infer them from 

 our knowledge of the forces which may 

 produce them. 



It is in his attempt to do this that our 

 author falls into the fatal confusion of 

 thought Avhich leads him to pronounce ab- 



surd a theory which to the clear-sighted 

 Newton was simply the truth. This confu- 

 sion seems to arise wholly from a careless 

 use of the term weight or gravity. On the 

 side of the earth facing the sun, all particles 

 of matter feel the attraction of the earth 

 and that of the sun as forces acting in op- 

 posite directions. " The weight of a body 

 situated at this point then will be diminished 

 by precisely the amount of the sun's attrac- 

 tive force." Yes, if meanwhile the earth's 

 centre remain stationary. But this is not 

 the fact ; the whole mass of the earth has 

 simultaneously yielded to the solicitation 

 of the same attraction. If these motions 

 are equal, they can produce no change in 

 weight, for weight is simply the force with 

 which a body tends to approach the earth's 

 centi'e, not simply the force with which it 

 advances through space in the direction of 

 that centre. Prof. Schneider himself points 

 out the distinction, but proceeds immedj- 

 ately to ignore it in his reasoning. He 

 says: "As the particles of the eartti most 

 remote from the sun feel its attraction plus 

 that of the earth herself, they are drawn 

 with greater force toward the centre of the 

 earth than any other particles. Hence," 

 he triumphantly asserts, " it cannot be true 

 that the whole earth is drawn away from 

 the waters, and that any tide is produced 

 by the waters being left behind." 



Having thus convincingly (?) shown the 

 necessity for a more satisfactory hypothesis 

 regarding the causation of the tides, he pro- 

 ceeds to offer one of his own. The first 

 thing we remark, however, in examining 

 this is that it embodies all that was con- 

 tained in the old " absurd " hypothesis, 

 while it compHcates the problem by com- 

 pelling us to consider not only the attrac- 

 tion of the sun or moon, but also the antag- 

 onizing force which prevents the earth from 

 moving in the direction of the attracting 

 body. It is true that by an ingenious mis- 

 statement of his own theory the writer avoids 

 what to him seems paradoxical in that which 

 he rejects. One of the tides that on the 

 side of the earth facing the disturbing ))ody 

 he tells us, is produced by centripetal, 

 the other by centrifugal force. In an ex- 

 planatory paragraph he admits, although 

 he does not distinctly state, what is the fact, 

 viz., that in each case coinciding effects are 

 produced by simultaneous variations in op- 

 posite directions of both these forces. Rec- 

 ognizing, however, the supreme value of 

 directness and simplicity of statement in 

 all popular expositions of scientific truth, 



