454 Mr. Blackwall's Observations io 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 upvvard 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 thej;' 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 Hakes of web unoccupied by 

 spiders, before alluded to, confirms this opinion. 



Perhaps the buoyancy of lines from Avhich 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 wliich the aqueous vapour in the atmosphere begins to be condensed into 

 water, and tlie temperature of the air. See the first series of the Memoirs of' the Lite- 

 rary and P/iilosop/iicdl Socicti/ of Manchester, vol. v. part ii. p. .588. The electrical 

 state of the atmosphere is shown by Bennet's gold-leaf electrometer. 



com pa- 



