OF INSECTS. 
189 
plan was suggested, of producing propulsion by reaction in 
this way: By the force of a steam-engine, a stream of 
water was to be shot out of the stern of a boat; the im¬ 
pulse of which stream upon the water in the river, was to 
push the boat itself forward; it is, in truth, the principle 
by which sky-rockets ascend in the air. Of the use or 
practicability of the plan, I am not speaking; nor is it my 
concern to praise its ingenuity: but it is certainly a con¬ 
trivance. Now, if naturalists are to be believed, it is 
exactly the device which nature has made use of, for the 
motion of some species of aquatic insects. The larva of 
the dragon-fly, according to Adams, swims by ejecting 
water from its tail; is driven forward by the reaction of 
water in the pool upon the current issuing in a direction 
backward from its body. [PI. XXXIII. fig. 6.] 
VII. Again: Europe has lately been surprised by the 
elevation of bodies in the air by means of a balloon. The 
discovery consisted in finding out a manageable substance, 
which was, bulk for bulk, lighter than air; and the appli¬ 
cation of the discovery was, to make a body composed of 
this substance bear up, along with its own weight, some 
heavier body which was attached to it. This expedient, 
so new to us, proves to be no other than what the author 
of nature has employed in the gossamer spider. We fre¬ 
quently see this spider's thread floating in the air, and 
extended from hedge to hedge, across a road or brook of 
four or five yards width. The animal which forms the 
thread has no wings wherewith to fly from one extremity 
to the other of this line; nor muscles to enable it to spring 
or dart to so great a distance: yet its Creator hath laid 
for it a path in the atmosphere; and after this manner. 
Though the animal itself be heavier than air, the thread 
which it spins from its bowels is specifically lighter. This 
is its balloon. The spider, left to itself, would drop to the 
ground; but being tied to its thread, both are supported. 
We have here a very peculiar provision: and to a contem¬ 
plative eye it is a gratifying spectacle, to see this insect waft¬ 
ed on her thread, sustained by a levity not her own, and 
traversing regions, which, if we examined only the body of 
the animal, might seem to have been forbidden to its nature. 
I must now crave the reader’s permission to introduce 
into this place, for want of a better, an observation or two 
upon the tribe of animals, whether belonging to land or 
water, which are covered by shells. 
I. The shells of snails are a wonderful, a mechanical, 
