247 
1879.1 J. T. Walker —On Indian Pendulum Observations. 
terior of the earth’s crust, immediately below the mountain masses. In 
this case pendulum observations, taken at stations on the Himalayas, and 
probably also on the table-lands of much lower elevation which are situated 
between those mountains and the sea, would show more or less diminution 
in the vertical force of gravity at each station (reduced to the sea-level) as 
compared with what would be found at stations actually situated on the 
sea-level. It was in order to throw light on this subject that the pen¬ 
dulums were employed in India, at a series of consecutive stations along 
the axis of the Peninsula, from Cape Comorin up to and then on the 
Himalayan masses, as well as at points on the coast and on islands contigu¬ 
ous thereto. 
On making a comparison of the observed with the calculated results, we 
find a considerable diminution in the vibration-numbers of the pendulums— 
that is to say, in the force of gravity—at the Himalayan and the higher 
Continental stations, relatively to what is met with at the Coast and the 
Island stations. We may not, however, attribute this deficiency of gravity 
wholly to local causes, because Sir George Airy has already pointed out 
many years ago, in his discussion of pendulum observations, see the Ency¬ 
clopaedia, Metropolitans Art. Figttbe oe the Eaeth— that gravity appears 
to be greater at oceanic stations than at continental stations, on the evidence 
of the vibration-numbers of pendulums which had been swung at several 
stations in various parts of the world, on the coasts and islands of the 
Atlantic and Pacific, as well as on the continents of Europe and America. 
Subsequently, in 1849, Professor Stokes showed* that these differences 
between observation and theory might be due to a general raising of the 
level of the sea in the vicinity of continents, over the level at oceanic 
islands, because of the greater density of the continent than of the ocean. 
He proved that “ if we set a circle of land -J-th of a mile high, of 1000 
“ miles radius, surrounding one station, against a circle of sea 35 miles 
“ deep, surrounding another station, we get a difference of about 3'5, near- 
“ ly, in the number of vibrations performed in one day by a seconds pendu- 
“ lum.” The principal part of this correction is, however, due to the depth 
of sea. “ Thus it would require a uniform elevation of about 2T miles, in 
“ order that the land elevated above the level of the sea should produce as 
“ much effect as is produced by the difference between a stratum of land 
“ 3-5 miles thick and an equal stratum of water.” 
It is clear from Professor Stokes’s investigations that whenever the 
results of the pendulum'observations in India came to be compared with 
those of pendulum observations at distant oceanic stations, it was to be 
expected that the observed vibration-numbers might be found to be gener- 
* See his paper on the Variation of Gravity at the Surface of the Earth, in the 
Transitions of the Cambridge Philosophical Society. Volume VIII, Part V. 
