and Mathematics to Seismology, 185 



Suppose the load to arise from a sheet of water 1 cm. deep, 

 or that 



p = l gramme wt. 



As in case (iii.) of § 7, let us put 



77 = - 5, n = llx 10 7 grammes wt. per sq. cm. 



Then we have approximately, in absolute measure, 



^ =(2ttx 11 X 10 7 )" 1 10^(10100/163) ; 



or, as unit angle = 206 x 10 3 seconds of arc approximately, 

 slope at O=0 // '0012 approximately. . . (25) 



The result would be the same if the side and the least 

 distance of the loaded square were altered in the same pro- 

 portion, e. g. if the side were altered to 1000 and the shortest 

 distance to 10 metres. 



The slope increases directly as the load. It would, however, 

 require an abnormally large differential rainfall or evaporation 

 to appreciably influence by direct pressure a level inside a 

 building situated on strata similar to the material of our 

 calculation. 



§ 15. The differential effect of barometric pressure during 

 the approach or retirement of a deep cyclonic depression 

 would appear a more probable disturbing cause. We might 

 very easily have a mean differential excess or diminution of 

 pressure of 1 or 2 cm. of mercury over an area whose greatest 

 dimension was very large compared to the shortest distance 

 from the observing station, and consequently effects 10 or 20 

 times that appearing in (25) might not unreasonably be 

 expected in disturbed weather. 



In the case of a large cyclonic area it would be desirable 

 to apply a formula applicable to a loaded spherical surface, 

 but (17) would probably give a very fair idea of the order of 

 magnitude of the result. 



§ 16. As an illustration of a different kind, suppose in fig. 5 

 that O is a station near a long straight portion AB of a tidal 

 river, and that we desire the difference of slope at O at high 

 and low tide. It will suffice to take the difference of level 

 at high and low water as the same all along AB. Suppose 

 this difference to be 5 metres, and assume r) and n to be the 

 same as in the last example. 



Taking first c = 4 x 26, we get approximately from (21) 



d w (ir\\ 5xl0 2 x7 , 1A , ,., ftE . x 



Ty < at °) = 2x22xllx -10>g* 10 X W 1 *)' 

 Phil. Mag. 8. 5. Vol. 43. No. 262. March 1897. Q 



