There are birds of many kinds, and therefore a great variety of wings and modes of flight. Birds with short, broad, rounded wings, with the under surface slightly concave and the upper surface correspondingly convex, usually have comparatively heavy bodies, and race through the air with rapid wing-beats and rather labored flight, and compass only short distances. Among the birds of this kind of aerial movement may be mentioned the American meadowlark, the bob-white, and the pheasant. Other species propel themselves in rapid, gliding, and continued flight by means of long, narrow, and pointed wings, like the swifts, swallows, and goatsuckers, while many others, notably herons, hawks, vultures, and eagles, are distinguished by a vast alar expansion in proportion to their weight, and hence are able to sustain themselves in the air by sailing, with only a slight stroke at rare intervals. Such birds as the stormy petrel and the frigate-bird have wings that are broad, convex, and of great length in contrast with the lightness and small bulk of their bodies, for which reason they are able to sustain themselves in the air for days without rest. It is even thought that some of these wonderful birds of the limitless ocean sleep on the wing, though how such an hypothesis could be proved it would be difficult to say. Even in this day of scientific research and astuteness, it must not be supposed that everything about the mechanics of avicular flight is understood. We may readily comprehend how a bird, without fluttering its wings, can poise in the air; but how can it move forward or in a circle, and even mount upward, without a visible movement of a pinion? And this some birds are able to do without reference to the direction of the ethereal currents. That, I venture to say, is still a mystery. It almost seems as if some of the masters of aerial navigation in the bird world were gifted with the ability to propel themselves forward by a mere act of volition. An interesting article on the subject of bird flight appeared not long ago in one of the foremost periodicals of the country, a part of which is here quoted to show what a puzzling problem we have before us: Recent developments in aerial navigation have renewed interest in the comparative study of the mechanical principles involved in the flying of birds. There is one exceedingly puzzling law in regard to birds and all flying creatures, the solution of which may work far-reaching influences in the construction of flying craft.