INTENSITY AND DIRECTION OF THE FORCE OF GRAVITY IN INDIA. 297 



" Attendu que c'est seulement par cette etude qu'on pourra obtenir une repre"- 

 sentation exacte de la distribution des masses dans 1'ecorce terrestre et de la 

 forme du ge"oide dans ces contre"es." 



In India itself our view of the subject has been modified by our recent discoveries 

 that the direction of gravity is liable to a constant deflection throughout large 

 regions, and that the density of the earth's crust may differ constantly from the 

 mean surface value throughout great areas. In 1895, when General WALKER'S 

 paper was written, it was believed that deflections of the plumb-line were accidental 

 and due to small local pockets of exceptional density studding every part of the 

 country. It was considered proper to treat deflections by minimum squares,* and it 

 was held that the true direction of the normal to the mean figure could be discovered 

 by grouping stations round a centre, and by assuming that in the mean of the group 

 the effects of local attraction are cancelled. 



There are now grounds for believing that the direction of gravity may be deflected 

 through 8 seconds of arc or move over an area of thousands of square miles. To 

 assume, therefore, that its mean direction as deduced from a group of contiguous 

 stations coincides with the normal, is seen to be hardly more justifiable than to 

 assume that the mean direction of the magnetic needle, as observed at several 

 stations in Surrey, gives the true direction of north. 



The investigation of the laws governing the deflection of gravity in India has been 

 impeded by many difficulties. Political considerations have erected a barrier round 

 Nepal and Bhutan, which geodetic operations have been unable to pass. Nepal and 

 Bhutan include almost the whole of the central and southern Himalayas. Geodesists 

 wish to approach the Himalayas from the south, and, by working gradually towards 

 their centre of mass, to discover their influence on the plumb-line. Being excluded 

 from Nepal and Bhutan, they have had to attack the mountainous area at its south- 

 west salient at Dehra Dun (see Plate 16). 



They have, moreover, been generally confined to deducing the direction of gravity 

 from latitude observations, which give only the meridional component. It is true 

 that our longitude observations show the direction of gravity in the prime vertical, 

 and if we could observe both the latitude and longitude of points on the Himalayan 

 snows, it would be possible to calculate the actual direction of gravity from its two 

 measured components. But until wireless telegraphy can be utilised for longitude 

 determinations, our longitude stations will have to be located near telegraph offices 

 instead of on mountain tops. We have observed astronomical azimuths at numerous 

 stations and their results will in the future be available for plumb-line discussions, but 



* When arcs of meridian are employed to determine the figure of the mean spheroid they are not 

 regarded as fixed in latitude. Their most probable positions in latitude are found by the method of 

 minimum squares. Each arc is moved up or down its meridional ellipse until a position is found for it in 

 which the squares of the deflections of the plumb-line are a minimum ; by this method large deflections 

 may be eliminated that exist in nature. 



VOL, CCV. A. 2 Q 



