
1865.] The Proposed Pendulum Operations for India. 271 
observe not every coincidence, but the first three consecutive coin- 
cidences, and than the 11th, 12th, 13th, then the 21st, 22nd, 23rd, and 
so on; after observing the first two or three, the times of the after 
coincidences can be easily computed with sufficient accuracy to intimate 
when the observer should be ready to note them. 
It is intended to have observations made generally along the Great 
Are at stations 2’’ apart in latitude, and at other points where it may 
be desirable to obtain data regarding local variations in the intensity 
of gravity. 
The pendulum experiments in this country will afford an indepen- 
dent value of the ellipticity of the Indian are, It is also hoped that 
they will throw some light on the existing discordances between the 
astronomical and geodetic latitudes of the Indian survey. 
The amount of the deflections of the plumb line, due to the Hima- 
layas and elevated table lands to the north of India, have been computed 
by Archdeacon Pratt for the different terminal stations of the Indian 
arcs; but these determinations are so much in excess of the results of 
the survey, that it is evident that the effects of the mountain attraction 
must be in a considerable degree compensated, either by a deficiency 
of density in the strata to the north, or by an excess of density in the 
strata to the south of the survey stations, 
Now, the pendulum can undoubtedly be made the means of showing 
whether the compensation is to be attributed to either of these causes ; 
for, whilst the effect of a distant range of mountains on the vibrations 
would be quite inappreciable, any local variation in the density of the 
underlying strata would show itself most unmistakably ; so that by 
taking observations both at a normal station, and at a few points in 
its vicinity symmetrically situated around it, should there be any con- 
siderable excess or defect in the density of the strata to counteract . 
the disturbance due to the mountain mass, the pendulum observations 
would not fail to point it out.* 
* Professer Stokes remarks in his letter on these operations: “The pendu- 
“Tam no doubt indicates only the vertical component of the disturbing force, 
“whereas it is the horizontal component in the plane of the meridian that affects 
“the measures of arcs ; at any one station, of course, a horizontal disturbance 
“may exist without a vertical disturbance, and vice versi; but in a system of 
“stations disturbances of the one kind must necessarily be accompanied by dis- 
“turbances of the other kind. Indeed, it is theoretically possible, from the ver- 
“tical disturbances, supposed to be known, actually to calculate the horizontal 
* diturbances, and that without assuming anything beyond the law of universal 
