Royal Society. 537 



10 s on the time shown by the earth's rotation. But this would also 

 accelerate the moon's mean motion by half the same proportional 

 amount ; and therefore a layer of meteor-dust accumulating at the 

 rate of -J^ of a foot per century, or 1 foot in 3000 years, would suf- 

 fice to explain Messrs. Adams and Delaunay's result. I see no 

 other way of directly testing the probable truth of M. Dufour's 

 very interesting hypothesis than to chemically analyze quanti- 

 ties of natural dust taken from any suitable localities (such dust, 

 for instance, as has accumulated in two or three thousand years 

 to depths of many feet over Egyptian, Greek, and Roman monu- 

 ments). Should a considerable amount of iron with a large 

 proportion of nickel be found or not found, strong evidence for 

 or against the meteoric origin of a sensible part of the dust 

 would be afforded. 



Another source of error in the earth as a time-keeper, which 

 has often been discussed, is its shrinking by cooling. But I find 

 by the estimates I have given elsewhere* of the present state of 

 deep underground temperatures, and by taking ioqVoo as tne 

 vertical contraction per degree Centigrade of cooling in the earth's 

 crust, that the gain of time by the earth, regarded as a clock, 

 would not in a century amount to more than o 1 ^ of a second, 

 or ^o'oo of the amount estimated above as conceivably due to 

 tidal friction. 



LXX VI 1 1 . Vrocecdings of Learned Societies, 



ROYAL SOCIETY. 



[Continued from p. 4/7-] 

 March 15, 1866. — Lieut. -General Sabine, President, in the Chair. 

 ^HE following communication was read 



1 



' On a possible Geological Cause of Changes in the Position of 

 the Axis of the Earth's Crust." By John Evans, F.R.S., Sec. G.S. 



At a time when the causes which have led to climatal changes in 

 various parts of the globe are the subject of so much discussion, but 

 little apology is needed for calling the attention of this Society to 

 what possibly may have been one of these causes, though it has 

 apparently hitherto escaped observation. 



That great changes of climate have taken place, at all events in the 

 northern hemisphere of the globe, is one of the best-established facts 

 of geology, and that corresponding changes have not been noticed to 

 the same extent in the southern hemisphere may possibly be con- 

 sidered due rather to a more limited amount of geological observa- 

 tion than to an absence of the phenomena indicative of such alte- 

 rations in climatal conditions having occurred. 



The evidence of the extreme refrigeration of this portion of the 



* " Secular Cooling of the Earth." Transactions of the Royal Society of 

 Edinburgh, 1862 ; and Philosophical Magazine, January 1863. 



