264 ON AERONAUTIC SPIDERS. 
tricity. But though each of the alleged causes 
just adverted to appears to be incompetent to pro- 
duce the required effect, yet one abundantly adequate 
may, perhaps, be thought to exist in the physical en- 
dowments of the animals themselves, or in the extreme 
lightness of their filaments; these two last-named 
suppositions, therefore, merit a careful examination. 
If spiders do impel their lines upwards by the 
voluntary exercise of some animal function which has 
hitherto eluded the researches of physiologists, it 
follows that when the communication is interrupted, 
the lines, unless influenced by some other force, must 
necessarily fall. Now the reverse of this uniformly 
ensues ; for if the animals, after having commenced 
their ascent, are suddenly separated from the lines to 
which they are attached, the latter still continue to 
ascend, their motion being accelerated by the dimi- 
nished action of gravity upon them, but the former 
are rapidly precipitated to the ground. The conclu- 
sion is obvious. The buoyancy of the lines cannot 
be occasioned by the beings which produce them ; 
and the ascent of large flakes of web unoccupied by 
spiders, before alluded to, confirms this opinion. 
Perhaps the buoyancy of lines from which spiders 
have been detached, and of webs altogether destitute 
of those animals, may be regarded as facts power- 
fully contributing to establish the accuracy of the 
idea that this secretion is specifically lighter than the 
mixed gases which compose the atmosphere. The 
