WITHOUT THE USE OE THE SOUNDING-LINE. 
675 
effect , but the two not equal. — The rock composing the crust of the earth will be under 
compression, and therefore denser at the depth corresponding to the depth of sea ; but 
sea-water itself will increase in density with depth in a somewhat similar ratio, so that 
the comparison between sea-water and solid rock remains virtually the same for all 
depths. The greater density of the earth towards its centre will, however, greatly 
influence the measure of this dependence as established by the foregoing calculation ; 
but in constructing a measuring instrument it will be safer to rely upon the result of 
actual measurement, in the absence of reliable information regarding the increase of 
density towards the centre, by comparing its indications with those obtained by means 
of the sounding-line. It may here be remarked, however, that the indications of varia- 
tion of gravitation with variation in the depth of water, which have been obtained by 
the use of the instrument, show in excess of what the above calculation gives with the 
mean density of the rock composing the crust of the earth as a factor, and agree more 
nearly with what would result if the upper strata of the earth were of a density equal 
to the mean density of the whole earth. Actual observations, as given in the Table 
further on, confirm, in a remarkable degree, the arithmetical ratio of decrease of gravi- 
tation by depth which results from the foregoing calculation. 
First attempt to construct a Bathometer. — Several years ago I constructed an instru- 
ment in which the gravitation of the earth was represented by a column of mercury in a 
glass tube closed at its upper end, and resting upon a cushion of air enclosed in a large 
bulb, which air, when kept at a perfectly uniform temperature, represented uniform elastic 
force unaffected by gravity or atmospheric density. The principal difficulty that pre- 
sented itself in designing a workable instrument on this principle, consisted in obtaining 
a scale sufficiently large to show such extremely slight variations in the total gravi- 
tation of the earth as would result from ordinary variation in the depth of water. From 
the calculation given under the previous head, assuming the mercury column to have 
a height of 760 millims., each fathom of depth of water would represent a variation of 
potential force in that column equal to a height of *0002059 millim., a quantity which 
it would be impossible to show on any scale. A scale would in reality not even realize 
this quantity of decrease in the upper surface of the column, because a portion of the 
adjustment of height would take place in the air-bulb below, partly from the rise of 
mercury into the bulb, and partly through increase of pressure of the imprisoned air 
due to its compression. I succeeded, however, by means of an arrangement of the 
instrument with three liquids of different densities, in increasing the effect of a change 
of gravitation upon the mercury column three hundredfold, whereby a change of 
10 fathoms depth would be represented by a movement of *6177 millim. of the boun- 
dary between the two liquids in the vertical tube, a quantity sufficiently large to be 
appreciated in the divisions of a scale. This instrument is shown in Plate 63. fig. 2. 
Tests of Instrument. — This instrument was tested by me in 1859 on board H.M.S. 
‘ Firebrand,’ commanded by Capt. Dayman, during a trip undertaken for the Admiralty 
for the purpose of determining a line of soundings across the Bay of Biscay, with a 
5 A 2 
