e72 Dr. G. Johnstone Stoney on 



orating used to print the impressions on the three positives. 

 So well did they perform, that it seemed as if it might be 

 possible in this way to build up satisfactory gratings of large 

 size for spectroscopic work. Starting with a 1-inch grating 

 of 2000 lines, I have bailt up a grating 8 inches square, 

 which, when placed over the object-glass of a telescope, 

 showed the dark bands in the spectrum of Sirius with great 

 distinctness. No especial precautions, other than the use of 

 the flat glass plate, were taken to insure absolute parallelism 

 of the lines, and I have not Lid time to thoroughly test the 

 grating. The spectra, however, are of extraordinary brilliancy; 

 and on the whole the field seems promising. This matter will, 

 however, be deferred to a subsequent paper. 



Physical Laboratory of the 

 University of Wisconsin, Madison. 



XXXIII. Denudation and Deposition. 

 By G. Johnstone Stoney, M.A., D.Sc, F.R.S* 



IN a lecture to the Royal Geographical Society, of which a 

 copious extract is given in ' Nature ' of the 2nd of 

 February, 1899, Dr. J. W. Gregory discusses many of the 

 causes which may have led to the existing form of the earth. 

 But there is one important factor in the problem left unnoticed, 

 namely, the conspicuous alterations of level which may be 

 attributed to the earth's compressibility, and which seem to 

 have been brought about wherever either denudation or 

 deposition have continued over wide areas and for a long time. 



Dr. Gregory makes a convenient division of the earth into 

 three parts : — (1) the unknown internal centrosphere ; (2) the 

 rocky crust or litho sphere ; (3) the oceanic layer, or hydro- 

 sphere. These, with the atmosphere, which may be added as 

 a fourth part, make up the whole earth. 



If now we imagine a pyramid whose base is a square centi- 

 metre of the surface of the solid part of the earth and whose 

 vertex is the earth's centre, it has a volume of about 212 

 cubic metres, which is the same as 212 millions of cubic centi- 

 metres. This pyramid passes first through the lithospheric 

 yhell, or outer crust, and then halfway across the centrosphere 

 to the centre of the earth. All the materials of which it 

 consists are compressible. Those which lie within the outer 

 shell consist mainly of carbonates, silicates, and aluminates, 

 a id have probably a coefficient of compressibility about equal 

 to that of glass ; while the compressibility of the centrosphere 

 is unknown, and may be either more or less. The observed 

 form of the earth's surface seems to suggest that the average 

 * Communicated by the Author. 



