and Mathematics to Seismology. 175 



They might possess high accuracy if the surface material 

 were bare rock in horizontal strata, and the recording appa- 

 ratus were supported directly on the rock ; but uncertainties 

 are introduced when the load is applied at the surface of 

 ordinary soil, and the support of the apparatus is stonework 

 inside a building whose foundations go to an appreciable 

 depth. 



§ 3. The observed facts on which our investigations are 

 most likely to bear are certain slow changes in the indications 

 of spirit-levels or delicately suspended pendulums. Some of 

 these Prof. Milne is disposed to attribute to meteorological 

 agencies such as rainfall or evaporation. 



A relative excess of moisture to the west, say, of an 

 observatory is, he considers, equivalent to a surface load on 

 that side, tending to make the ground on which the building 

 stands slope downwards from east to west. Such want of 

 symmetry may arise from the peculiarities of the soil, or 

 through the ground being sheltered by trees or modified by 

 cultivation. In sunshine the shadow of the building itself, 

 by retarding evaporation, may set up such a difference as 

 Prof. Milne has in view. 



1 am not sure that an excess of evaporation from the east, 

 say, of a building is necessarily equivalent to the withdrawal 

 of a surface load from that side, at least to the exact extent 

 of the surplus evaporation. The withdrawal of moisture from 

 the soil has a decided influence on its conductivity for heat — 

 not to speak of electricity — and so may exert a very appre- 

 ciable influence on the temperature near the surface, the 

 consequences of which it would be difficult to follow. There 

 is also, presumably, underground circulation both of air and 

 moisture, which may not unlikely counteract to some extent 

 differences of surface evaporation. 



Though these and other uncertainties exist, it is certainly 

 worth while considering the numerical magnitude of the 

 results to be expected from the agencies postulated by Prof. 

 Milne. The theoretical results will also, I hope, suggest the 

 way in which the best use may be made of experimental 

 determinations of the effects of surface loading over limited 

 areas of convenient shape. 



Influence of Surface Load on the observed Level. 



§ 4. This influence is not so simple as might appear at 

 first sight. The weight of the loading material is equivalent 

 to a pressure normal to the surface, which we suppose hori- 

 zontal. But, in addition, we must allow for the fact that the 

 gravitational attraction of the loading material slightly alters 



