J. T. Walker —On Indian Pendulum Observations. 
249 
1879.] 
These figures show a comparative excess of gravity at the Island sta¬ 
tions which is equivalent to 3-25 vibrations of a seconds pendulum, and 
which diminishes to an excess of 024 at the Coast stations, and becomes a 
defect of 2’27 at Inland stations under 2000 feet in height, of 509 at 
Inland stations between 2000 and 7000 feet, and of 2F44 at More, where 
the height is 15,400 feet. It will be found that the ratios of the mean 
defects of the vibration-numbers to the average heights of the elevated 
masses, as exhibited at the two groups of Inland stations and at More, are 
very fairly accordant, a change of one vibration accompanying changes of 
height of 533, 751 and 718 feet respectively. And if we multiply the 
mean of these values by 3’25—the mean excess at the three Island stations 
—we get 2,1G8 feet, which probably does not differ very materially from 
the mean depression of the circumjacent ocean-bed below the level of the 
islands. Thus the amount of elevation above or depression below the actual 
sea-level has obviously a very important bearing on the discrepancies be¬ 
tween theory and observation. 
Now it is to be remembered that the vibration-numbers at the Island 
and Coast stations have not, in a single instance, been increased for the 
deficiency of density of the sea as compared with the land; because satis¬ 
factory data of the surrounding sea-depths were not available. It will, 
however, be obvious that in order to institute a fair comparison between the 
force of gravity at these and at the inland stations, the vibration-numbers 
at the coast stations should be somewhat augmented, and those at the 
island stations should be augmented in a still greater degree, on this account. 
Moreover the vibration-numbers at the higher inland stations should also 
be increased, not on account of the density of the sea, but because the 
observations have been reduced to the apparent sea-level, which is presum¬ 
ably raised above the normal sea-level by the attraction of the continental 
masses. The extreme increase would be at Minicoy Island, for deficient 
density, on the one hand, and at More, for elevation, on the other. Tor 
Minicoy we may take the increase as from 3 to 4 vibrations. For More, if 
we assume the apparent sea-level to have been raised 1000 feet by Hima¬ 
layan attraction,* we get an increase for height and mass of 2 to 3 vibra- 
* In the 4th edition of his Figure of the Earth (1871), Archdeacon Pratt shows 
(Arts. 200 and 201) that if z is the height through which the sea-level immediately 
below a station on a table-land is raised by attraction, h the height of the table-land, 
d the length of the mean horizontal diameter of the table-land through the station, and 
c the earth’s radius, then 
* 
z •= 5-M approximately, 
8c 
and taking h = 15,500 feet, d = 670 miles and e = 3,956 miles 
he gets z = 985 feet. 
