1894.] of the interior of the earth. 137 



that the exact form of the tidal spheroid at the mean under- 

 surface of the floating crust would not generally be maintained, 

 just as it is not maintained at the surface of the ocean owing to 

 the deflection caused by coast lines; whence arise the irregularities 

 known as the establishment of ports. The result would be, that 

 the upper surface of the crust would not conform to the rule of 

 the equipotential surfaces, and ocean tides would be possible, for 

 it is highly improbable that the establishments of the tide in the 

 substratum should exactly agree with those of the ocean. 



If there is truth in this suggestion, it would seem likely that, 

 where there is an exceptionally large unbroken area of ocean, 

 there would also be a comparatively smooth extent of under- 

 surface to the crust, and the form of the tidal equipotential 

 surfaces might be more nearly preserved both for the subjacent 

 liquid and for the water. In such an area the ocean tides would 

 consequently be small. Now this is the case in the central parts 

 of the Pacific, scarcely any tide being observable at the Sandwich 

 Islands \ 



If however tidal waves derived from the main equatorial tide 

 in the substratum by reflection at the mountain roots were to 

 be propagated into higher latitudes, they would cause undulations 

 every six lunar hours in the crust ; and it is a question whether 

 these would not be noticed at observatories. 



And first of the effect upon the clock. Suppose the point of 

 suspension of a pendulum to be carried downwards, or upwards, 

 with a mean velocity of h feet in six hours. The average accele- 

 ration of the bob with reference to the point of suspension would 

 \)Q g + 6/6 X 60^ feet per second. In 12 hours the gain would 

 compensate the loss ; each of which would be 6/64 seconds. For 

 example, if the crust tide from highest to lowest was six feet, the 

 gain or loss in six hours would be about Yo^h of a second. This 

 would not be noticed. 



As regards the levels, the passing wave would scarcely affect 

 the direction of gravity at all, because the direction of gravity 

 would still be towards the centre of the earth. But the supports 

 of instruments would be slightly tilted. To estimate roughly how 

 much, suppose a tidal wave, which when n days old arrives in 

 latitude \, to be represented by the formula 



6 2?i7r 

 V = H cos — — - X ; 



that is, suppose it to be modified in its passage as if it had a 

 height 6/2 at the equator, and ran up as a simple harmonic wave 

 of n loops to the place in question. 



1 Herschel's Physical Geography, § 78. 



