182 



KNOWLEDGE. 



[August 1, 1896. 



There would surely be a j^reat future for the man who 

 •will discover a liatidy. accurate, and intolligiblo account 

 of the cause of the tides without using mathematical 

 phraseology ! 



(Kev.) C. RoniNsoN, B.A. 

 AVillow Road, Birmingham, 

 July 1st, lH9(i. 



[Pace Mr. Wohlwill and Mr. Robinson, I did not say, 

 either that the earth is pulled away from its crhit or 

 that the raising of the water is due to a direct lifting 

 power. — V.C.] 



To the Editors of Knowledge. 

 Sirs, — The subject which is mentioned in the corre- 

 spondence of the July Number of KNowLEniiis, the formation 

 of the ocean tides, is one on which we of the general 

 public are sadly in need of fresh instruction and guidance. 

 I have endeavoured, by looking up author after author of 

 those within my reach, to obtain an authoritative ex- 

 planation of the manner in which the great tides 

 are produced ; but I find them discordant one with 

 another, and unsatisfactory or insufficient in themselves. 

 Each explanation is based on one or more of the following 

 ideas or theories, of which the most prevalent is the 

 first one ; — 



(1) The suction theory. According to it, the moon 

 (and the sun, but for brevity sake I will omit it), 

 having more power over the waters nearest her than over 

 the earth as a whole, draws them up — sucks them up in 

 fact — into a heap under her ; the action is described as 

 direct, and is referred to as natural and requiring no 

 elucidation. The authors would seem to have gone back 

 to the old times when we all believed that the " sucker " 

 in the pump drew the water up ; and they fail to see that 

 the tidal power of the moon, being almost infinitesimal 

 compared to the earth's gravity force, could raise towards 

 herself in direct opposition to gravity neither a particle 

 of water, nor a grain of sand, nor any portion of matter, 

 small or large. 



(2) Closely connected with the "suction" is the 

 " slip " theory. It is used to account for the secondary 

 tide ; the solid earth is said to be drawn away from the 

 waters on the side farthest from the moon, and in fact to 

 slip through the water moonwards, leaving a bulge of the 

 ocean behind it. Unfortunately for the theory, the solid 

 earth and the water are held together by a bond many 

 million times stronger than the power which the moon 

 can exert to separate them. 



(3) One of my authors rests the explanation solely on 

 the '• weight " theory. The difi'erence of the moon's attrac- 

 tion on the various parts of the water tends to separate 

 them in the larger portions of the sphere, and to compact 

 them at the quadratures ; that tendency asserts itself by 

 change of weight in the water, which results, through 

 gravity, in rising and sinking, i.e., in the tide bulges and 

 depressions. 



Two other writers eke out the " suction " by the 

 " weight" theory, and one uses the latter idea as if he 

 believed that light bodies have in themselves a power of 

 ascension. None of them descend to the application of 

 the idea ; if they did they would readily perceive its 

 inadequacy to do more than to help slightly the formation 

 of the tide bulges. Our oceans are but a few miles in depth, 

 but, in order to account for a rise of only three feet in 

 their height, a depth of water of between three thousand 

 and four thousand miles would be required. 



(■ll One— perhaps the greatest of my " authorities " — 

 treats the tides as a case of " perturbation," similar to that 



to which heavenly bodies, such as the moon and the earth, 

 are subject in their revolutions. Hut, however true that 

 idea may be in tlio result, it is impossible to identity the 

 course of action of the ocean, tied down as it is by gravity 

 to the earth, with that of bodies moving freely in space. 



(5) A great encyclopaedia describes the force originating 

 the phenomenon, but stops short cautiously at its tewleneirs, 

 and omits to trace for us the course of its action, saying, 

 " Thus wo see that the tidal forces tend to pull the water 

 towards and away from the moon, and to depress the water 

 at riglit angles to that direction." 



I note that most of the authorities recognize the existence 

 of nothing but "force" in the case; that is to say, an 

 agency like gravity, acting on each molecule of matter. 

 The molar result of the action of " force " on matter, 

 namely, " pressure," is considered only in one of its 

 manifestations, "weight." The idea that the " tidal forces " 

 of the moon acting on the water might produce " pressure " 

 in a direction other than the vertical, does not seem to 

 have been taken up by them. They have set themselves 

 the difficult task of explaining to us the action of " force" 

 on water without the aid of hydrostatics. J. Creagh. 



BILLOWS. 



To the Editors of Knowledge, 



Sirs, — From my earliest childhood I have always 

 observed the rhythmic succession of maximum developed 

 waves, with intervals of quiescent water, which I always 

 attributed to the rhythmic gusts in light blows, and to the 

 rhythmic paroxysms of violence always observable in heavy 

 gales. 



I have ever been impressed, daring storms, with the 

 regularity of this motion in heavy rolling, and have often 

 taken its time, which I always find to be every twenty 

 minutes — that is, three times in every hour ; this rhythmic 

 oscillation, with some five, six, or seven seas, then easy 

 steaming. Some call these largest waves " tenth waves," 

 but they are nearer " fiftieth" in such gales. 



When the sea rises in tempests, and the exits leading 

 out on deck have to be battened down, the passenger 

 vouchsafed on deck by the captain imagines that the 

 waves are the highest possible ; but they are only the 

 highest at that particular time and phase in that par- 

 ticidar storm. So the gallant Scoresby thought when he 

 measured what he believed to be the highest billows. 



My good friend Captain Atkin, of the Cunard Steamship 

 Company, related his experiences in the heaviest hurricane 

 which he ever met on the North Atlantic some five winters 

 ago, the like of which he does not desire ever again to run 

 into. From his bridge — a coign of vantage not possessed 

 in Scoresby's time — with the steamer in the trough of the 

 sea, he observed the crests of the tremendous surges con- 

 siderably higher than the top of the funnel, and but little 

 below the masthead. Again, a well-posted American sea 

 captain, lately returned from a passage around the Cape 

 of Good Hope, using Dr. Scoresby's method, measured 

 mountainous billows of the type of Mr. Daniell's realistic 

 and ideal picturing on page 1G4 of your last issue, on the 

 South Atlantic, from fifty-eight feet up to seventy-two 

 from trough to crest; but he did not get the distance 

 from crest to crest, or their velocity. H. P. Curtis. 



SEA .sicivNKSS. 

 To the Editors of Knowledge. 



Sirs, — I followed Mr. Moy's advice recently, and secured 

 a berth lin^itliwise of the sttauier, whereon I lay carefully 

 on my left side, with my head towards the engine room. 

 The cabin was amidship, and the engine room aft, so my 



