ITS 



PROFESSOR EDWARD HULL, LL.D., F.R.S., OX 



the twenty-four hours, is due to the attraction of the moon, 

 augmented, under certain conditions, hy that of the sun, by 

 which the ocean waters are raised to a small extent above the 

 normal level in the form of a wave which, owing to the rotation 

 of the earth, moves along from east to west till, obstructed by 

 some barrier of land thrown across its path, such as that of 

 Africa or America, its course is deflected or destroyed. This 

 elevatory force is applied within the plane of the Ecliptic, and 

 has its maximum effect along a line drawn from the centre of 

 the moon to that of the globe, but diminishes towards the great 

 circle which has the moon at its pole where the force ultimately 

 becomes tangential. Of the several great oceans on the Earth's 

 surface, only one, the Pacific — as it covers nearly half the 

 globe at the equator — offers sufficient expanse for the formation 

 of a full tidal wave. The other oceans, such as the Atlantic and 

 Indian, present insufficient surfaces towards the moon for the 

 full development of the tidal wave ; and still less does the 

 Mediterranean, though both are influenced to some small 

 extent. 



Existing Theories. — The solution of the problem for the 

 existence of a tidal wave on opposite sides of the globe has been 

 often attempted, but with unsuccessful results, as admitted by 

 writers themselves. The favourite theory, and one generally 

 adopted, may thus be stated : " The attraction of the moon is 

 strongest on the earth's surface next the moon, less at the 

 centre, and less again on the parts beyond ; so that the solid 

 body of the earth, which is attracted as though it was condensed 

 into its own centre, is more powerfully attracted than the ocean 

 water on the off' side from the moon, and is drawn away from 

 the water." 



A recent writer on this subject, Mr. J. A. Hardcastle, rejects 

 the theory that " on the side towards the moon the water is 

 drawn away from the earth, while on the other side the earth 

 is drawn away from the water."* 



The latest writer I have met with is Mr. Arthur R Hinks, 

 recently Chief Astronomical Assistant at Cambridge Observatory ,f 

 but he skips rather lightly over the subject of the double tides, and 

 apparently does not accept any of the accepted theories, or give 

 one by himself. He merely remarks that " the subject is 



* Joum. Brit. Astron. Assoc., Dec, 1912, p. 141. Mr. Hardcastle's letter 

 is misleading ; for, although headed " Tide on the other side," it deals 

 with " tide on this side," as the last paragraph of it shows. 



t "Astronomy," Borne University Lil/rory, p. 58. 



