INTENSITY AND DIRECTION OF THE FORCE OF GRAVITY IN INDIA. 313 



(6.) TJie Zero of Vertically. 



Tables IVA., IVs., IVc., IVo., and IVE. show in terms of the old datum, namely, 

 with Kalianpur as zero, the angles of inclination in the meridian that have been 

 determined in different parts of India between the level surface and the surface of the 

 Clarke- Bessel spheroid. The difference between any two of these angles of inclination 

 is affected only by changes of spheroid, but the absolute value of every angle is based 

 on the assumption that the level and spheroidal surfaces are parallel at Kalianpur. 

 Any alteration in the assumed inclination of the two surfaces at this our initial station 

 will affect the inclinations as deduced at other stations by a constant quantity. The 

 direction of gravity at Kalianpur has been adopted by the Survey of India as the 

 datum, from which deflections of the plumb-line at all stations are measured ; the 

 direction of gravity is, we know, always perpendicular to the level surface ; at 

 Kalianpur it has been assumed to be perpendicular to the spheroidal surface also. I 

 propose now to deduce a new value for the deflection of the plumb-line at Kalianpur, 

 and to exhibit the values of the deflections of the plumb-line in India that will be 

 obtained, if the deduced direction of gravity at Kalianpur be substituted for the 

 original one in other words if our zero or datum be corrected. 



The direction of gravity throughout the first, second, and fourth regions appears to 

 be under the influence of abnormal attractions ; there is, I believe, no other area in 

 the world in which the deflection of the plumb-line undergoes at once such large and 

 such systematic variations as it does in the two first regions of Plate 15 ; these 

 observed peculiarities, too, have been discovered to exist in the neighbourhood of 

 extraordinary mountain masses, and though the connection between the observed 

 phenomena and the visible protuberances is obscure, there can be little doubt that the 

 latter are in some indirect way the cause of the former. 



The direction of gravity in the fourth region also is probably influenced by the high 

 mountains of Central Asia, though their effects are not directly perceptible. We 

 will, therefore, omit from present consideration the results obtained in the first, 

 second, and fourth regions, and we will confine our attention to those of the third 

 region only. 



The third region is in the form of a trigon with its apex at Cape Comorin ; its 

 length from north to south is 1100 miles, and its greatest breadth 1300 miles; its 

 area is 750,000 sq. miles. This trigon is one of the oldest portions of land surface 

 now existing on the earth ; it is mostly composed of ancient gneiss, and though a 

 large part was covered in the cretaceous period by volcanic overflows, it suffers now 

 but slightly from earthquakes and is exceptionally stable. This trigon appears to be 

 as free from abnormal sources of disturbance and to be as suitable for the determination 

 of the absolute direction of gravity as any area of land can be. If we examine the 

 results "Eeferred to Kalianpur as zero" in Table IVc., we find that out of 

 85 determinations of the direction of gravity made within the third region, 80 show a 



VOL. CCY. A. 2 S 



