Fisher — Cause of Changes of Level in the Earth? s Crust. 219 



The investigations of Dr. Spencer, Prof. Hull, and Dr. 

 Nansen* have introduced to us the knowledge of much more 

 recent great change of level than I think had previously been 

 suspected. This circumstance seems to favor the explanation 

 now suggested, because the cyclical changes in convection cur- 

 rents in the substratum beneath the crust might be expected 

 to recur at frequent intervals. 



Addendum to a Suggested Cause of Changes of Level in the 

 Earth's Crust, Jan. S, 1906. 



Since forwarding the above I have received an important 

 paper by Lieut.-Colonel Burrard, R.E., F.R.S., Superintendent 

 of the Trigonometrical Surveys of India, " on the intensity 

 and direction of the force of gravity in India,"f in which I 

 meet with a statement that appears of much interest in con- 

 nection with my suggestion about convection currents in the 

 substratum of the earth's crust. 



"Between 1865 and 1873 observations were taken at 31 

 stations in India by Captains Basevi and Heaviside, with the 

 Royal Society's seconds pendulum." In 1903-4 renewed obser- 

 vations were taken by Major Lenox Conyngham with a half 

 second pendulum. " His first station was Dehra Dun. His 

 results there were astonishing, for they show that Basevi's 

 value was no less than 0*103 centimeters too small." [Note] 

 " This extraordinary difference could only mean that Basevi's 

 final value of N [the number of vibrations in 24 hours] was 

 too small by four whole seconds of time. Basevi's observa- 

 tions at Dehra Dun lasted five months, and included 234 inde- 

 pendent swings, taken at pressures varying from half an inch 

 to 28 inches, and at temperatures varying from 48° to 102° 

 Fahr. 



" The force of gravity as observed by 



Basevi and Lenox 



Heaviside Conyngham. 



1865-73. 1904. Difference, 



cms. cms. cms. 



Dehra Dun 978-962 979-065 +0-103 



Madras 978-237 978-281 +0-044 



Bombay 978-605 978-632 +0-027 



Mupooner 978*751 978-795 +0-044" 



Colonel Burrard remarks that these differences are not 

 affected by any constant quantity, and that, " Basevi's and 

 Heaviside's observations were taken with a care that it is diffi- 

 cult for us to equal." " The only faults which have been 



* Bathymetric Features of the North Polar Sea, with a Discussion of the 

 Continental Shelves and previous Oscillations of the Shore Line (Chris- 

 tiana 1904). 



f Phil. Trans. Royal Soc, Series A, vol. ccv, 1905. 



