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MR WATT ON THE 
quite across. Now when the first part of the hne has ac- 
tually ceased to move, it could not again set it in motion 
by ejecting a continuation of the thread behind it, without 
the assistance of some gas or fluid. Indeed, as the thread 
of the spider seems, when first emitted, rather lighter than 
the air, it is impossible that any impulse, without the assist- 
ance of some such cause as I have alluded to, could give it 
such a rapid motion forward *. 
All authors, therefore, who treat of this subject, as far as 
I have noticed, ascribe the extension of the thread to the 
action of the wind, and the hind legs of the spider, and 
they do not suppose that the insect has the power of eject- 
ing its thread when the air is perfectly tranquil. However, 
it is evident from what has been remarked, and also from 
a glance at the webs of the largest geometric spiders, that this 
supposition is incorrect, as the long lines of these webs radi- 
ate regularly from the centre in all directions; and I never 
could observe that they used their hind legs in drawing out 
their threads ; they only use one of their hind legs in guid- 
ing and measuring the distances of the threads when form- 
ing the circular lines or warp of the web. The surprising 
rapidity, indeed, with which they shoot out their threads 
precludes this idea. One clear evening a friend of mine 
and I both observed a large geometric spider on the post 
that supported the gate of a park : it turned its back to the 
opposite post, and in a few seconds shot out a line that 
struck against it. The instant it struck the opposite post, 
it turned round, pressed its spinners against the post it 
stood upon, and thus fastened the thread ; and then ran 
across it. Many species of spiders that draw out their 
threads, seem to have no power to shoot out their lines in 
this manner. 
• The powers of the Bombardier, the Ink fish, and the Torpedo, 
are analogous to this property of the spider. 
