1889-90.] Professor Crum Brown on Optical Activity. 
185 
the replacement of H 2 by 0 diminishes k although it increases the 
mass of the radical. 
On the Mean Level of the Surface of the Solid Earth. 
By Hugh Robert Mill, D.Sc. 
(Read June 2, 1890.) 
In a paper “ On the Height of the Land and the Depth of the 
Ocean,” read to the Society on 19th December 1887, and published 
in the Scottish Geographical Magazine , vol. iv. pp. 1-41, Dr John 
Murray gives a very detailed estimate of the volumes of oceanic 
hollows and continental protuberances. I have recently had recourse 
to Dr Murray’s calculations in order to determine the position of 
the contour-line which should separate the portions of the solid 
earth above the general level of the surface from those below that 
level. The results of this determination were published in the 
Scottish Geographical Magazine , vol. vi. pp. 182-187 ; but since then 
the theoretical importance of the particular contour-line in question 
has induced me to give further consideration to the subject. 
The term “ lithosphere ” has been applied to the solid part of the 
earth, and “ hydrosphere ” to the oceans collectively, corresponding 
etymologically with “ atmosphere ” by which the gaseous envelope is 
familiarly known. If the lithosphere were smooth and the earth at 
rest, it would be entirely and uniformly covered by the hydrosphere. 
The amount of dry land emergent above the surface of the hydro- 
sphere depends in the first instance on the configuration of the 
lithosphere, in the second on the volume of the hydrosphere. 
The surface of the hydrosphere is usually adopted as the standard 
level from which heights and depths are estimated; theoretically 
this is unsatisfactory, on account of the distortion of surface caused 
by the gavitational attraction of the ridges on the lithosphere. The 
only perfect reference surface is that of the geoid, or smooth figure 
which would result if the ridges of the lithosphere were laid to rest 
in the hollows. In order to arrive at this mean level of the litho- 
sphere (shortly mean-sphere level), the requisite data are the 
volumes of the hydrosphere and of the emergent land, together with 
the areas occupied by each. From calculations given at length in 
