220 Fisher — Cause of Changes of Level in the Earth' 's Crust. 



found with their work are such as would tend to produce con- 

 stant error." Is it not possible that the large difference in the 

 force of gravity at Delira Dun may be to some extent due to a 

 real change of density beneath that station in the interval of 

 between thirty and forty years ? We know that during 1905 a 

 severe earthquake was experienced at Dehra Dun ; and the 

 stress which caused it must have been accumulating since 

 Basevi's time. May not that process have been accompanied 

 by a gradual change of density which Conynghanrs observa- 

 tions revealed in progress ? 



Taking the mean force of gravity to be 978 centimeters, a 

 change of density amounting to 1/J per cent in a depth of 50 

 miles in the underlying magma at Dehra Dun would have 

 been quite su 4V icient to cause the whole of the observed dif- 

 ference in gravity, supposing the higher density of the two to 

 be 3. 



It seems, therefore, that if this change of gravity at Dehra 

 Dun cannot be accounted for by errors of observation, it 

 amounts to a proof that changes of density, and therefore 

 movements, have taken place in the substratum of the earth's 

 crust in sub-Himalayan regions within the last thirty years. 



