Movements oj the Ground. 199 



material (to take an extreme case) to be entirely transferred 

 to the north polar zone, and re-formed there as a sheet of ice 

 of uniform thickness. Then, to cause such a displacement of 

 the centre of gravity, the sheet melted from one zone and 

 collected on the other would have to be more than 3800 feet 

 in thickness. By such a transference of the material, the 

 centre of gravity would indeed be still further displaced in 

 consequence of the displacement of the ocean-waters^. But, 

 attributing the greatest possible value to such a cause, it is 

 evidently insufficient to produce a deviation of the vertical of 

 '04 of a second, which is the smallest change of inclination 

 perceptible in M. Plantamour's N.S. level. 



Non-Periodic Movements. — To the geologist, perhaps, the 

 most interesting of the movements observed by M. Plantamour 

 will be those which seem as yet to be non-periodic in their 

 occurrence. It will be seen from fig. 1 that these movements 

 are not confined to the winter of 1879-80. The last two 

 columns of the Table, which give the coordinates of what may 

 be called the centre of the year, show also that this centre 

 undergoes a displacement greater than can be accounted for 

 by mere differences of temperature from one year to another. 

 The extraordinary movement of the second year is continued, 

 though to a less extent, during the third year, and the centre 

 is again shifted from the fifth to the sixth and from the sixth 

 to the seventh year. From the third to the eighth years 

 inclusive the centre has moved constantly towards the north. 

 It is not unlikely, of course, that these movements may be 

 local or accidental in their origin, and independent of any 

 secular or wide-spread action. But it is at least possible that 

 they are due to great earth-movements : that they represent, 

 for our finite time, the infinitesimal changes which culminate 

 in a great mountain-chain. 



To observe these movements with a greater chance of 

 success it would be advisable to separate them from the 

 strictly periodic movements caused by the annual and 

 diurnal changes of external temperature. The latter diminish 

 as the distance from the surface of the ground increases, and 

 must be insensible at a depth of about 60 or 70 feet. At this, 

 or a greater, depth levels might be observed in a deserted or 

 unfrequented mine ; and they need not, for this purpose, be 

 read oftener than once a week. If this were done at two or 

 three places on either side of a recently-formed mountain- 

 chain, like the Apennines or Alps, the results could hardly 

 fail to throw light on some of the great problems in the theory 

 of terrestrial evolution. 



* See Dr. OroU's ' Climate and Time.' 



