ANNIVERSARY ADDRESS OF THE PRESIDENT. U 



certain how much of that retardation is due to tidal friction,— how 

 much to meteors, — how much to possible excess of melting over ac- 

 cumulation of polar ice during the period covered by observation, 

 which amounts, at the outside, to not more than 2600 years. 



3. The effect of a different distribution of land and water in mo- 

 difying the retardation caused by tidal friction, and of reducing it, 

 under some circumstances, to a minimum, does not appear to be 

 taken into account. 



4. During the Miocene epoch the polar ice was certainly many feet 

 thinner than it has been during or since the Glacial epoch. Sir W. 

 Thomson tells us that the accumulation of something more than a foot 

 of ice around the poles (which impKes the withdrawal of, say, an inch 

 of water from the general surface of the sea) will cause the earth to 

 rotate quicker by one-tenth of a second per annum. It would ap- 

 pear, therefore, that the earth may have been rotating, throughout 

 the whole period which has elapsed from the commencement of the 

 Glacial epoch down to the present time, one, or more, seconds per an- 

 num quicker than it rotated during the Miocene epoch. 



But, according to Sir W. Thomson's calculation, tidal retardation 

 win only account for a retardation of 22" in a century, or ^-^ (say -J-) 

 of a second per annum. 



Thus, assuming that the accumulation of polar ice since the Mio- 

 cene epoch has only been sufScient to produce ten times the effect of 

 a coat of ice one foot thick, we shall have an accelerating cause which 

 covers all the loss from tidal action, and leaves a balance of |- of a 

 second per annum in the way of acceleration. 



If tidal retardation can be thus checked and overthrown by other 

 temporary conditions, what becomes of the confident assertion, based 

 upon the assumed uniformity of tidal retardation, that ten thousand 

 million years ago the earth must have been rotating more than twice 

 as fast as at present, and, therefore, that we geologists are " in di- 

 rect opposition to the principles of Natural Philosophy" if we spread 

 geological history over that time ? 



II. The second argument is thus stated by Sir W. Thomson : — " An 

 article, by myself, published in ' MacmiUan's Magazine ' for March 

 1862, on the age of the sun's heat, explains results of investigation 

 into various questions as to possibilities regarding the amount of heat 

 that the sun could have, dealing with it as you would with a stone, 

 or a piece of matter, only taking into account the sun's dimensions, 

 which showed it to be possible that the sun may have already illu- 

 minated the earth for as many as one hundred million years, but at 

 the same time rendered it almost certain that he had not illuminated 

 the earth for five hundred millions of years. The estimates here are 

 necessarily very vague ; but yet, vague as they are, I do not know 

 that it is possible, upon any reasonable estimate founded on known 

 properties of matter, to say that we can believe the sun has really 

 illuminated the earth for five hundred million years." (Z. c. p. 20.) 



I do not wish to "Hansardize" Sir William Thomson by laying much 

 stress on the fact that, only fifteen years ago, he entertained a totally 

 different view of the origin of the sun's heat, and believed that the 



