ON AERONAUTIC. SPIDERS. 267 
adjusting the suspensory filaments, and not in pro- 
pelling the adventurous aéronauts through the atmo- 
sphere. It is manifest, therefore, that, in the strict 
sense of the word, spiders do not fly. 
The various directions in which spiders sail through 
the atmosphere admit of an easy explanation. A 
direction parallel to the horizon will be given by a 
current of air moving in that plane; a vertical one 
by the ascent of air highly rarefied ; and directions 
intermediate between these two will, in general, 
depend upon the composition of forces. When the 
horizontal and vertical currents are equal in force, the 
line of direction will describe an angle of 45° nearly 
with the plane of the horizon; but when their forces 
are unequal, the angle formed with that plane will be 
greater or less accordingly as one current or the other 
predominates. 
Founded on results obtained from an experiment 
which has been frequently made, but never conducted 
with sufficient care, is the belief, entertained by many 
eminent naturalists, that spiders can forcibly propel 
or dart out lines from their papillae. Now as this 
process would, contrary to my own experience, imply 
the exercise of a physical power peculiar to these 
creatures, and as attempts have been made to ex- 
plain on this principle the fabrication of the nets of 
Geometric Spiders in situations where their ordinary 
mode of proceeding could not be employed, I deter- 
mined to repeat the experiment from which so strange 
