454 Mr. BrackwaArLL's Observations to ascertain 
when the evaporating force is unusually great, or the electricity 
of the atmosphere is remarkable for its intensity *. | 
But though each of the alleged causes just adverted to appears 
incompetent to produce the required effect, yet one abundantly 
adequate may perhaps be found in the physical endowments 
of the animals themselves, or in the extreme lightness of their 
webs: these two last-named suppositions therefore merit a care- 
ful examination. 
If the spiders do impel their lines upward by the voluntary 
exercise of some animal function which has hitherto eluded the 
researches of physiologists, it follows, that when the communi- 
cation is interrupted, the lines, unless influenced by some other 
force, must necessarily fall. Now the reverse of this uniformly 
ensues : for if the insects, 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 their diminished gravity, but the former are rapidly pre- 
cipitated to the ground. The conclusion is obvious. The buoy- 
ancy of the lines cannot be occasioned by the beings which pro- 
duce 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 these insects, may 
be regarded as facts powerfully contributing to establish the 
idea that this animal secretion is specifically lighter than the 
mixed gases which compose the atmosphere. The fallacy of 
this notion, however, is easily proved by experiment. In the 
* The evaporating force may be determined by the atmometer, or from the tempe- 
rature at which the aqueous vapour in the atmosphere begins to be condensed into 
water, and the temperature of the air. See the first series of the Memoirs of the Lite- 
rary and Philosophical Society of Manchester, vol. v. partii. p. 588. The electrical 
state of the atmosphere is shown by Bennet's gold-leaf electrometer. 
compa- 
