336 
ARCHDEACON PRATT ON THE CONSTITUTION 
below the sea-level under mountains and plains there is a deficiency of matter, approxi- 
mately equal in amount to the mass above the sea-level ; and that below ocean-beds there 
is an excess of matter, approximately equal to the deficiency in the ocean when compared 
with rock ; so that the amount of matter in any vertical column drawn from the surface 
to a level surface below the crust is now, and ever has been, approximately the same in 
every part of the earth. 
2. The process by which I arrived at this hypothesis I will explain. In the Philoso- 
phical Transactions for 1855 and 1858 I showed that the Himalayas and the Ocean 
must have a considerable influence in producing deflection of the plumb-line in India. 
But by a calculation of the mean figure of the earth, taking into account the effect of 
local attraction, it appeared that nowhere on the Indian Arc of meridian through Cape 
Comorin is the resultant local attraction, arising from all causes, of great importance*. 
This result at once indicated that in the crust below there must be such variations of 
density as nearly to compensate for the large effects which would have resulted from 
the attraction of the mountains on the north of India and the vast ocean on the south, 
if they were the sole causes of disturbance, — and that, as this near compensation takes 
place all down the arc, nearly 1500 miles in length, the simplest hypothesis is, that 
beneath the mountains and plains there is a deficiency of matter nearly equal to the 
mass above the sea-level, and beneath ocean-beds an excess of matter nearly equal to the 
deficiency in the ocean itself. 
3. The compensation, should the hypothesis be true, is not complete, but approximate ; 
of matter in tile ocean ; and in a paper in the same volume of the Phil. Trans, (p. 779) I showed by calculation 
that the vast ocean stretching down to the south pole would produce considerable effects in the southern parts 
of India, such as the Survey altogether failed to detect. This seemed to imply that, as beneath mountain-regions 
there is a deficiency of matter, so beneath ocean-beds there must be an excess, in order to account for the defi- 
ciency in the effect of the ocean (which of itself would be large) not being discernible. The thought of an excess 
of matter below the ocean-bed accords with the remark which Sir John Herscidrl once made, that the ocean- 
bed of the Pacific must be more dense than the average surface of the solid crust, otherwise the protuberant 
ocean would be drawn away and would flow to other parts of the surface. 
My calculations had shown, then, that a considerable effect on the plumb-line must result from each of the 
following causes taken separately: — (1) the mountain-region, (2) the ocean, (3) any widespread, though slight, 
deficiency or excess of matter in the solid crust. As there is reason to believe that resultant local attraction is 
nowhere in India very great, and is generally small, it must follow that, generally speaking, below high ground 
there is a deficiency of matter, and below ocean-beds an excess. 
But there had not been at that time any physical hypothesis proposed to account for these two conditions of 
the crust, and to connect them together as results of one and the same cause. An hypothesis was, however, 
suggested in 1864, in my paper in the 4 Proceedings of the Royal Society ’ alluded to at the beginning of this note, 
and referred to in the text, viz. that all the varieties we see in the earth’s surface (in mountains, plains, and 
ocean-beds) have arisen from the earth’s mass having contracted unequally in a vertical direction, in passing 
from a fluid to a solid state— a necessary result of its fluid origin being that the amount of matter in any ver- 
tical column down to a level surface is the same and always has been the same, whatever the changes in length 
it may have undergone. — Calcutta, April 6, 1871. 
* See Proc. Royal Soc. No. 64, 1864; but especially Phil. Mag. January and February, 1867. 
