38& Mr Watts’ Remarlcs on Captain Keeler's 
action of the weights is such, that the pendulum is much soon^ 
er brought to a state of rest, and, consequently, the arcs of vi- 
bration will decrease much more rapidly, than when the pendu- 
lum is suspended in the reverse position, with the great weight 
below. Hence it is evident, that the degree of uncertainty in 
determining the amplitudes of the arcs of vibration, will in- 
crease in the same proportion; so that, however unobjection- 
able the principle of the pendulum may be, generally consider- 
ed, these circumstances of disparity, in the direct and inverse 
positions of the pendulum, should seem to form an objection to 
Captain Kater’s mode of applying it. 
And, in the third and last place, I imagine that a portion of 
these irregularities might have arisen from some change that 
had taken place in the density and moisture of the atmosphere, 
during the experiments, by the introduction of a quantity of 
aqueous vapour into its different strata ; because this vapour is 
known to possess the same elasticity as dry air, while at the 
same time it has less density : for, according to the experiments 
of Messrs Watt and Saussure, the weight of this vapour is to 
that of air as 10 to 14, when their elastic force and tempera- 
ture are the same. And hence it follows, that the introduction 
of this vapour into the atmosphere renders it equally susceptible 
of sustaining^ the same column of quicksilver as before, with a 
less density ; so that the barometer will not always give informal 
tion when the atmosphere has been suddenly charged with this 
vapour. 
Moreover, as the elasticity of air augments by heat, so that 
with less density, it is capable of sustaining an equal column of 
mercury, I should think that the discrepancies observable in the 
law of the decrease of the arcs of vibration, might partly arise 
from either of these causes, or perhaps from bothi Should this 
be the case, we may naturally conclude, that when the arcs of 
vibration do not decrease in geometrical progression, while the 
times increase in arithmetical, the great weight being below, one 
or the other of the causes just now assigned, operates in produ- 
cing the irregularities in question : and that all those experi- 
ments in which the law of decrease of the arcs of vibration does 
not proceed as stated above, ought to be rejected as insufficient 
for the purpose. 
