414 Mr. (jr. H. Darwin on Variations in the Vertical 



per square centimetre, that of copper 540, and of steel 843 

 millions. 



I will take vl g—?>x 10 8 ; so that the superficial layers of 

 the earth are assumed to he more rigid than the most rigid 

 glass. It will he easy to adjust the results afterwards to any 

 other assumed rigidity. 



With these data we have 



gwh _ 5 - 67 



also 848,000 5-67 „ 



7T X 10 8_U U 



It seems not unreasonable to suppose that 1500 miles 

 (2*4 x 10 8 centim.) is the distance from the place where the 

 barometer is high (the centre of the anti-cyclone) to that 

 where it is low (the centre of the cyclone). Accordingly the 

 wave-length of the barometric undulation is 4*8 x 10 8 centim., 

 and 6 = 4-8 x 10 8 -h6-28 centim., or, say, b = Sx 10 8 centim. 



Thus, with these data, 



•4—6 = 4*5 centim. 

 2v 



We thus see that the ground is 9 centim. higher under the 

 barometric depression than under the elevation. 



If the sea had time to attain its equilibrium slope, it would 

 stand 5 X 13*6, or 68 centim. lower under the high pressure 

 than under the low. But as the land is itself depressed 9 cen- 

 tim., the sea would apparently only be depressed 59 centim. 

 under the high barometer. 



It is probable that, in reality, the larger barometric inequa- 

 lities do not linger quite long enough over particular areas to 

 permit the sea to attain everywhere its due slope, and therefore 

 the full difference of water-level can only be attained occa- 

 sionally. 



On the other hand, the elastic compression of the ground 

 must take place without any sensible delay. Thus it seems 

 probable that the elastic compression of the ground must exer- 

 cise a very sensible effect in modifying the apparent depres- 

 sion or elevation of the sea under high and low barometer. 



It does not appear absolutely chimerical that at some future 

 time, when both tidal and barometric observations have attained 

 to great accuracy, an estimate might thus be made of the 

 average modulus of rigidity of the upper 500 miles of the 

 earth's mass. 



Even in the present condition of barometric and tidal infor- 



