In Fig. 55 is shown the outlines of the fly with outstretched wings. Fig. 56 represents it with the wing folded, and Fig. 57 is a view of a wing with the relative size of the top of the body shown in dotted lines. _Fig. 56. Common Fly. Folded Wings._ The first thing that must attract attention, after a careful study is the relative size of the body and wing surface. Each wing is slightly smaller than the upper surface of the body, and the thickness of the body is equal to each wing spread. _Fig. 57. Relative size of wing and body._ The weight, compared with sustaining surface, if expressed in understandable terms, would be equal to sixty pounds for every square foot of surface. STREAM LINES.--The next observation is, that what are called stream lines do not exist in the fly. Its head is as large in cross section as its body, with the slightest suggestion only, of a pointed end. Its wings are perfectly flat, forming a true plane, not dished, or provided with a cambre, even, that upward curve, or bulge on the top of the aeroplane surface, which seems to possess such a fascination for many bird flight advocates. It will also be observed that the wing connection with the body is forward of the line A, which represents the point at which the body will balance itself, and this line passes through the wings so that there is an equal amount of supporting surface fore and aft of the line. Again, the wing attachment is at the upper side of the body, and the vertical dimension of the body, or its thickness, is equal to four-fifths of the length of he wing. The wing socket permits a motion similar to a universal joint, Fig. 55 showing how the inner end of the wing has a downward bend where it joins the back, as at B. THE MONOPLANE FORM.--For the purpose of making comparisons the illustrations of the monoplane show a machine of 300 square feet of surface, which necessitates a wing spread of forty feet from tip to tip, so that the general dimensions of each should be 18 1/2 feet by 8 1/2 feet at its widest point. First draw a square forty feet each way, as in Fig. 58, and through this make a horizontal line 1, and four intermediate vertical lines are then drawn, as 2, 3, 4, 5, thus providing five divisions, each eight feet wide. In the first division the planes A, B, are placed, and the tail, or elevator C, is one-half the width of the last division. _Fig. 58. Plan of Monoplane._ The frame is 3 1/2 feet wide at its forward end, and tapers down to a point at its rear end, where the vertical control plane D is hinged, and the cross struts E, E, are placed at the division lines 3, 4, 5. The angles of the planes, with relation to the frame, are usually greater than in the biplane, for the reason that the long tail plane requires a greater angle to be given to the planes when arising; or, instead of this, the planes A, B, are mounted high enough to permit of sufficient angle for initiating flight without injuring the tail D.