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, the soft parts are external, attached to the bony framework comprising the skeleton, the wing bones being directly connected with the central back bone; so that while these two sorts of animated flying machines are so different in structure, they yet act in much the same manner when on the wing. The difference between them is clearly stated by Marey, some of whose conclusions we now give almost word for word. The flight of butterflies and moths differs from that of birds in the almost vertical direction of the stroke of their wings, and in their faculty of sailing in the air without making any movements; though sometimes in the course they pursue they seem to resemble birds in their flight. The flight of insects and birds moreover differs in the form of the trajectory in space; in the inclination of the plane in which the wings beat; in the role of each of the two alternating (and in an inverse sense) movements that the wings execute; as also in the facility with which the air is decomposed during these different movements. As the wings of a fly are adorned with a brilliant array of colors, we can follow the trajectory or figure that each wing writes in the air. It is of the form of a figure of eight (Fig. 9), first discovered by Professor J. Bell Pettigrew of Edinburgh. [Illustration: 9. Figure cut by an insect's wing.]