INTENSITY AND DIRECTION OF THE FORCE OF GRAVITY IN INDIA. 291 



The differences between the observed and computed values in Table I. correspond 

 very nearly to the differences between the observed and computed values of N, as 

 formerly given by General WALKEK. That the correspondence is not exact is due to 

 the adoption by HELMERT and WALKEK of different constants in CLAIRAUT'S law. 



The physical meaning of BASEVI'S pendulum results was for many years the subject 

 of controversy.* The deficiency of gravity which he had found to exist in Himalayan 

 regions was attributed by some authorities to the elevation of the level surface above 

 the surface of the mean spheroid, and by others to the defective density of the under- 

 lying crust; by the former the surface of the geoid was held to depart largely in 

 certain places from that of the spheroid, and by the latter the two surfaces were 

 assumed to be almost identical. In his ' Schwerkraft im Hochgebirge,' published in 

 1890, Professor HELMERT gave a mathematical solution of the problem, and his 

 writings have closed the controversy. 



A graphical interpretation of the results of Table I. is given in Plate 14, the method 

 by which the several ordinates are computed being explained in Table II. below. The 

 first figure of the Plate shows the height above sea-level, as determined by spirit 

 levelling, of the surface of India along its central meridian. The second figure shows 

 the deficiency of matter in the underlying crust, as deduced from BASEVI'S pendulum 

 results. The third figure gives the differences between the ordinates in the two 

 upper figures, and shows the surface of India as it would be if the crust were 

 everywhere of equal density. An examination of the figures of this Plate brings to 

 light four significant facts : 



(1) That there exists in the earth's crust throughout India a general deficiency 



of matter as compared to Europe ; t 



(2) That the apparent excess of matter above sea-level, which the eye observes 



at More (Station 43) under the form of mountains, is largely compensated 

 by subjacent deficiencies ; 



(3) That an extraordinary deficiency of matter underlies the stations of Dehra 



Dun, Kaliana and Nojli (Nos. 38, 37, 36), stations situated not in the 

 Himalayas, like Mussooree (No. 41), but in the plains at the foot of 

 the Himalayas ; this deficiency leads one to beliere that the pressure 

 of the Himalaya Mountains upon the crust is diminishing the density of 

 the latter under the surrounding plains ; 



(4) If we disregard the evidence of fig. 1, and if we consider only the distribution 



of mass in the surrounding crust, we see that stations in the plains of 



* See preface to Vol. V. of ' Account of Operations of the Great Trigonometrical Survey of India.' 



t The peninsula of India is composed of crystalline and volcanic rocks ; the great age of the former and 



the great weight of the latter would lead us to expect a high value for g ; that g should be abnormally 



small is, from a geological point of view, surprising. 



2 P 2 



