WITHOUT THE USE OF THE SOUNDING-LINE. 
687 
which for relatively small values of h may be written 
w It + 2 h 
w'~ It 
or 
w—w' h 
w — ^lt’ 
proving that attraction decreases with elevation in the simple ratio of ^R. The decrease 
on account of depth of sea takes place, as shown on page 674, nearly in the ratio of 
R, or the readings of fathoms on the bathometer may be taken for yards in raising the 
instrument above the sea-level. 
The corrections for latitude necessary for reading depth of sea are also applicable 
for height; but in the latter case another correction will have to be made for the 
attractive force exercised by the mass composing the mountain or elevation above the 
sea-level supporting the instrument, and this will vary greatly with the breadth, being 
a maximum in the case of an elevated plateau. The instrument will, in such cases, 
give indications of height considerably below the real elevation, and it is doubtful on 
that account whether it can be made available for such a purpose. 
Test for Elevation . — Being desirous to test the instrument for height, I decided to 
take it up a tower ; and having obtained the permission of the Board of Works, through 
my friend Dr. Percy, to make use of the Clock Tower for the purpose, the instrument 
was tested on the 18th of December, the readings being as below : — 
Thermometer. Barometer. 
45-0 29-64 
45-63 29-88 
being a difference of 45 - 25 divisions, equivalent to a difference of height of 135 feet, 
the aneroid indicating a difference of 208 feet. This difference of readings may appear 
at first sight excessive, but may be accounted for by disturbance of the instrument in 
taking it by hand up the steep steps of the tower, where little time was allowed to 
insure the complete readjustment of the column. In this case also the reading of the 
instrument gives a result inferior to the indications of theory as compared with its 
indications on board ship, which latter indications I consider are the more reliable, 
because the instrument, when once suspended, is not disturbed, and its indications are 
rendered more delicate through the oscillations of the vessel. 
Modifications in the Instrument . — The instrument, as constructed at present, leaves 
room for such improvements as have partly been, and are likely still, to be suggested 
by experience. It would be possible to eliminate entirely the effect of variation of 
temperature by more carefully proportioning the diameter of the mercury column to 
that of the cup. The influence of variation of density of the atmosphere might also be 
* Including correction for variation in atmospheric density. 
Bathometer. 
(Mean*.) 
At top of tower . . . 1067-75 
At foot of tower . . 1022-5 
