| Address to the Royal Geological Society. 15 
because it did not attract my notice until after the above con- 
elusion had been arrived at. Sir William states that though the 
lunar semi-monthly declinational and monthly elliptic tides, as 
discussed by the Tidal Committee of the British Association, of 
which he is a member, indicated either no yielding or more pro- 
bably a very small yielding of the body of the earth, yet that the 
absence from all the results of any indication of a 18°6—year tide, 
connected with the revolution of the moon’s nodes, could not be 
easily explained without assuming or admitting a considerable 
degree of yielding. If there be no perceptible ocean tide, 
answering to the unquestionably existing 18'6-year variation 
of tidal force it shews that the body of the earth and tke 
ocean go together ; or so nearly together that the difference is not 
perceptible. The difference of straining force, connected with the 
last-mentioned tide, is far less than those connected with the two 
former, and yet the body of the earth recognises it far more 
sensibly because of its long period. 
Though this fact is to me more striking as a confirmation, in 
consequence of the order in which it occurred to me, others might 
prefer to make it a substantive argument and say—If our grandly 
deliberate globe, which refuses to be hurried and stands sensibly 
as obstinate as steel to a fortnightly or a monthly change of 
force, will, nevertheless, yield considerably to a much smaller 
18-6-year change of force, when she has reasonable time given her, 
what may we not expect of her when she an indefinite length of 
time in which to accommodate herself to the continued and ever 
similarly directed decrement of the centrifugal force of rotation ? 
But, besides, it our earth be, as a whole, a viscous body capable of 
yielding in this manner there is something to help her to respond, 
through her viscosity, to a suitable straining force. Ifa viscous 
solid is very slowly giving way to a continued gentle pressure 
the movement is promoted if small, reciprocating agitations, or 
vibrations, be set up in the yielding mass. Now the semi-diurnal 
tidal straining of the earth is an action, which, though small in 
amount, is there, quantum valeat, and which is of the kind 
required to help our viscous globe, as we shall take leave to call 
it, to answer to the continual decrease of the centrifugal force. 
Some of the other variations of tidal force which affect our earth 
may be equally important in this way, on account of their much 
longer periods, and notwithstanding their smaller amount. 
