by the tenent hairs be added, an increase of pressure is gained,
equal to about one-fourth the weight of a fly. This leaves one-fourth to
be accounted for by slight viscidity of the fluid, by the action I have
so often alluded to, which may be called 'grasping,' by molecular
attraction, and, doubtless, by other agents still more subtle, with
which we have at present scarcely any acquaintance."

_How Insects Fly._ Who of us, as remarked by an eminent ornithologist,
can even now explain the long sustained, peculiar flight of the hawk, or
turkey buzzard, as it sails in the air without changing the position of
its wings? and, we would add, the somewhat similar flight of a
butterfly? It is the poetry of motion, and a marvellous exhibition of
grace and ease, combined with a wonderful underlying strength and
lightness of the parts concerned in flight.

Before we give a partial account of the results obtained by the delicate
experiments of Professor Marey on the flight of birds and insects, our
readers should be reminded of the great differences between an insect
and a bird, remembering that the former, is, in brief, a chitinous sac,
so to speak, or rather a series of three such spherical or elliptical
sacs (the head, thorax and abdomen); the outer walls of the body forming
a solid but light crust, to which are attached broad, membranous wings,
the wing being a sort of membranous bag stretched over a framework of
hollow tubes (the tracheae), so disposed as to give the greatest
lightness and strength to the wing. The wings are moved by powerful
muscles of flight, filling up the cavity of the thorax, just as the
muscles are the largest about the thorax of a bird. Moreover in the
bodies of insects that fly (such as the bee, cockchafer, and dragon
fly), as distinguished from those that creep exclusively, the air tubes
(tracheae) which ramify into every part of the body, are dilated here and
there, especially in the base of the abdomen, into large sacs, which are
filled with air when the insect is about to take flight, so that the
specific gravity of the body is greatly diminished. Indeed, these air
sacs, dilatable at will by the insect, may be compared to the swimming
bladder of fishes, which enables them to rise and fall at will to
different levels in the sea, thus effecting an immense saving of the
labor of swimming. In the birds, as every body knows who has eaten a
chicken, or attended the dissection of a Thanksgiving turkey, t