There are many insects which one would little suspect being furnished with apparatus suited to swift and more or less continuous flight. House flies frequent the insides of our windows, buzzing sluggishly in and out of the room. But what different creatures are they when they accompany your horse on a hot summer's day. A swarm of these little pests keep pertinaciously on wing about the horse's ears; quicken the pace up to ten or twelve miles an hour, still they are there; let a gust of wind arise and carry them backwards and behind, the breeze having dropped, their speed is redoubled, and they return to their post of annoyance to the poor horse, even when urged to its fastest pace. But this example gives only a partial proof of the fly's power of flight, as the following will show. The writer was travelling one day in autumn by rail at about twenty-five miles an hour, when a company of flies put in an appearance at the carriage window. They never settled, but easily kept pace with the train; so much so, indeed, that their flight seemed to be almost mechanical, and a thought struck the writer that they had probably been drawn into a kind of vortex, whereby they were carried onward with little exertion on the part of themselves. But this notion was quickly dispelled. They sallied forth at right-angles from the carriage, flew to a distance of thirty or forty feet, still keeping pace, and then returned with increased speed and buoyancy to the window. To account for this look at the wings of a fly. Each is composed of an upper and lower membrane, between which the blood-vessels and respiratory organs ramify so as to form a delicate network for the extended wings. These are used with great quickness, and probably six hundred strokes are made per second. This would carry the fly about twenty-five feet, but a seven-fold velocity can easily be attained, making one hundred and seventy-five feet per second, so that under certain circumstances it can outstrip the fleetest racehorse. If a small insect like a fly can outstrip a racehorse, an insect as large as a horse would travel very much faster than a cannon-ball.