WITHOUT THE USE OF THE SOUNDING-LINE. 
681 
influence upon the instrument ; but inasmuch as the mercury column exercises a 
preponderating gravitating influence only in the measure of its superior density to the 
atmosphere which the mercury replaces in the tube, it follows that changes in atmo- 
spheric density must exercise an influence upon the readings of the instrument. The 
atmospheric density depends upon barometric pressure, temperature, and admixture of 
aqueous vapour, the amount of which can be easily ascertained by readings of the dry- 
and wet-bulb thermometers and the barometer at the time of taking the bathometrical 
observations. These corrections have been made and applied to the observations taken 
on board the steam-ship ‘Faraday;’ the readings, however, having been taken at sea, 
the air was regarded as saturated with vapour, and the tension of the vapour at the 
temperatures has been employed. In ordinary usage of the instrument these corrections 
might be neglected without serious error, or a table might be constructed giving the 
amount of these corrections for observed changes of the barometer and thermometer. 
Geological Influences . — The readings of the bathometer depend upon the inferior 
density of sea-water as compared with the solid constituents composing the earth’s 
crust, which have been taken, in the calculation at page 673, as 2-763. No account was 
taken, in assuming the above average density of the earth’s crust, of the presence of 
denser materials, such as metallic ores, heavy spar, &c., on the one hand, or of subter- 
ranean cavities on the other. But these abnormal occurrences are not frequent on dry 
land, being chiefly confined to mountainous districts, and may be assumed to be of less 
frequent occurrence in the great depressions constituting the sea-basins. Their relative 
effect upon total gravitation, as measured upon the surface of the water, is less, more- 
over, than it would be if measured upon the solid surface, on account of their greater 
distance from the instrument. The uniform density of the sea is an element eminently 
favourable to the attainment of uniform indications on its surface. 
Geographical Influences . — The configuration of the bottom of the sea below the 
instrument must also exercise a sensible influence upon its readings. The instrument 
would not indicate, for instance, the existence of a local depression surrounded by 
elevated ridges or plateaux, nor would it indicate the existence of a peak. Considerable 
variations must therefore be occasionally expected between the readings of this instru- 
ment, however correctly adjusted, and the results of actual soundings ; but it may be 
observed that broken ground, such as would cause these differences, is comparatively 
rare below the sea, which deepens gradually from the land in such a way that the 
contour lines of uniform depth can generally be distinctly traced ; and the principal 
value of the instrument would consist in its indicating its passage above varying depths. 
The indications of the instrument must coincide very nearly with those of a sounding- 
line upon an even slope, because the comparative proximity of the ground towards the 
rise of the slope will be balanced by the absence of solid matter towards its descent, as 
will be apparent from the accompanying sketch (p. 682). 
Attention has already been called to Sir George Airy’s observation of the greater 
apparent gravitation on islands than on the sea-shore, and there than inland, and also 
MDCCCLXXVI. 5 B 
