‘76 PRESIDENTS ADDRESS—-SECTION C. 
varying pressure caused by tidal action, and the resulting surface 
movements might be small, or even nothing: there would 
be a density tide only, which might be unable to move the 
superjacent crust. 
Sir W. Thomson had assumed in his investigations that the 
interior of the earth is perfectly elastic, which is no doubt incor- 
rect. Professor Darwin undertook the calculations on the sup- 
position that it is viscous or elastico-viscous, and found that in 
these cases the semi-diurnal hodily tides would lag so much 
behind the ocean tides that the two could not be compared, 
especially as the ocean tides are much affected by the distribution 
of the land-masses, which would not be the case with bodily tides. 
If an earth tide followed an ocean tide at a distance of less than 
half an oscillation, the time of apparent high ocean tide would 
be accelerated ; and if an earth tide followed at a greater distance 
than half an oscillation, the time of apparent high water would 
be retarded. But owing to the friction on the sea bottom and 
the varying depth of the sea, neither the height nor the time of 
actual high water at any place can be calculated from theory ; 
and observations give only the time and height of apparent high 
water, consequently, acceleration or retardation cannot be proved. 
Sir W. Thomson always recognised the difficulties connected 
with the semi-diurnal tides, but thought that the longer period 
lunar fortnightly tide might be used for the purposes of obser- 
vation, as with it the irregularities must be much less. He 
calculated that at Iceland and at Teneriffe the fortnightly ocean 
tides ought to be five inches if the earth be perfectly rigid, three 
inches if as rigid as steel, and one inch if the rigidity be that of 
glass. Subsequently, Professor Darwin found that the observed 
fortnightly and monthly tides at various Indian and European 
ports are about two-thirds of the calculated height, and, conse- 
quently, he thought that this proves the effective rigidity of the 
earth to be as great as that of steel. These results were obtained 
by employing the Newtonian or equilibrium or tidal theory, which 
supposes that each particle of the earth takes up the position of 
equilibrium. But owing to their inertia they never have time 
to do this completely, for the directions of the external attractions 
are always changing, and Professor Darwin has lately found 
that even the fortnightly oceanic tide could not be more than 
one half the equilibrium height, so that the observed tides seem 
to be actually greater than theory will account for. But, 
according to Mr. Love, ‘‘the Tidal Committee of the British 
Association appears to be still doubtful whether there really 
is an appreciable fortnightly (oceanic) tide,’* and both Professor 
Darwin and Sir W. Thomson have come to the conclusion 
that the equilibrium theory cannot be used for the purposes of 
calculation. 
* Quoted by the Rey. O. Fisher. 
