180 Mr. G. H. Darwin on Professor Haughtoris [Mar. 14, 



to diminish the angle of the cone described by the instantaneous axis 

 round the principal axis : in other words, the " wabble " set up by the 

 earthquake will gradually die away. 



Then by means of Adams and Delaunay's estimate of the alteration 

 of the length of day, which is attributed to tidal friction, Professor 

 Haughton obtains a numerical value for the frictional effect of the 

 residual tidal current. He then applies this to the 30 6- day tide, and 

 deduces the time required to reduce a " wabble " of given magnitude 

 to any given extent. 



He is of opinion that if, at the present time, the instantaneous axis 

 of rotation of the earth were describing a circle of more than 10 

 feet in diameter at the earth's surface, then the phenomenon could not 

 escape detection by modern astronomical instruments. From the 

 absence of any such inequality he concludes, after numerical calcula- 

 tion, " if Asia and Europe were manufactured per saltum, causing a 

 sudden displacement of the axis of figure through 69 miles, that this 

 event cannot have happened at an epoch less than 641,000 years before 

 the present time, and that this event may have occurred at an epoch 

 much more remote." 



He then passes on to consider the case where the elevation takes 

 place by a number of smaller impulses instead of by one large one. 

 He treats first the case of " 69 geological convulsions, each of which 

 displaced the axis of figure through one mile," and where " the radius 

 of the wabble" is "reduced from one mile to 5 feet in the interval 

 between each two successive convulsions;" and, secondly, the case 

 where " the increase of this radius is exactly destroyed by friction 

 during each wabble, so that the radius of 5 feet continues constant." 



In the first case he finds that the total time occupied by the manu- 

 facture of Europe and Asia is 27f millions of years, and also that " no 

 geological change, altering the position of the axis of figure through 

 one mile, can have taken place within the past 400,000 years." And 

 in the second case, he finds that the same elevation would occupy 

 4,170 millions of years. A little lower he adds : " It is extremely im- 

 probable that the continent of Asia and Europe was formed per saltum, 

 and therefore our minor limit of time is probably far short of the 

 reality." 



It appears from these passages that Professor Haughton is of opinion 

 that a succession of smaller impulses at short intervals will necessarily 

 increase the radius of the "wabble ; " but it is not very clear to me 

 whether he means that the radius of the " wabble " would be the same 

 by whatever series of impulses the principal axis was moved from one 

 position to another. Now, I conceive that it is by no means necessary 

 that a second impulse succeeding a first should augment the radius of 

 the " wabble;" it might, indeed, annihilate it. I admit that by pro- 

 perly timed impulses the radius of the " wabble " might be made as 



