we 
- 
On North American Spiders. — 113 
air, and never descend except driven by a wind wherefore ’tis cer- 
tain. And ’tis as certain, that what swims and ascends in the air is 
lighter than the air, as that what ascends and swims in water is light- 
er than water. So that if we should suppose any such time, where- 
in the air is perfectly calm, this web is so easily drawn out of the 
spider’s tail, that if the end of it be once out, barely the levity of it 
is sufficient to draw it out to any length; wherefore if it don’t hap- 
pen that the end of this web, bc, catches by a tree or some other 
body, till there is so long a web drawn out, that its levity shall be so 
eat as more than to counterbalance the gravity of the spider, or so 
that the web and the spider, taken together, shall be lighter than such 
a quantity of air as takes up equal space, then according to the uni- 
versally acknowled ged laws of nature, the web and the spider togeth- 
er will ascend, and not descend, in the air: as when a manis at the 
bottom of the water, if he has hold of a piece of timber so great, 
that the wood’s tendency upwards is greater than the man’s tendency 
downwards, he together with the wood will ascend to the surface of 
the water. And therefore, when the spider perceives that the web 
be is long enough to bear him up by its ascending force, he lets go 
his hold of the web a b, Fig. 3, and ascends in the air with the web 
be. If there be not web more than enough, just to counterbalance 
the gravity of the spider, the spider together with the web will hang 
0 equilibrio, neither ascending nor descending, otherwise than as the 
armoves. But if there is so much web, that its greater levity shall 
more than equal the greater density of the spider, they will ascend 
ll the air is gq thin, that the spider and web together are just of an 
*qval weight with so much air. And in this way, Sir, I have multi- 
tudes of times seen spiders mount away into the air, from a stick in 
my hands, with a vast train of this silver web before them; for, if 
® spider be disturbed upon the stick by shaking of it, he will pres- 
‘ bo Y ia this manner leave it. And their way of working may very 
‘Stinetly be seen, if they are held up in the sun, or against a dark 
Be or any thing that is black. 
dis Pik Sir, the only remaining difficulty is, how they first put out 
i a¢of the web 8 ¢, Fig. 3, out of their tails. If once the web 
Out; it is easy to conceive how the levity of it, together with the 
ie of the air may draw it out toa great length. But how should 
x first let out of their tails, the end of so fine and even a string : 
i 5 that the web, while it is in the spider, is a certain cloudy li- 
Wor, With which that great bottle tail of theirs is filled; which im- 
Von. 2K No 1. 15 
