_3d_, The great rapidity with which wings, especially insect wings, are made to vibrate, and the difficulty experienced in analysing their movements. _4th_, The great weight of all flying things when compared with a corresponding volume of air. _5th_, The discovery of the balloon, which has retarded the science of aërostation, by misleading men’s minds and causing them to look for a solution of the problem by the aid of a machine lighter than the air, and which has no analogue in nature. Flight has been unusually unfortunate in its votaries. It has been cultivated, on the one hand, by profound thinkers, especially mathematicians, who have worked out innumerable theorems, but have never submitted them to the test of experiment; and on the other, by uneducated charlatans who, despising the abstractions of science, have made the most ridiculous attempts at a practical solution of the problem. Flight, as the matter stands at present, may be divided into two principal varieties which represent two great sects or schools-- _1st_, The Balloonists, or those who advocate the employment of a machine specifically lighter than the air. _2d_, Those who believe that weight is necessary to flight. The second school may be subdivided into (_a_) Those who advocate the employment of rigid inclined planes driven forward in a straight line, or revolving planes (aërial screws); and (_b_) Such as trust for elevation and propulsion to the vertical flapping of wings. _Balloon._--The balloon, as my readers are aware, is constructed on the obvious principle that a machine lighter than the air must necessarily rise through it. The Montgolfier brothers invented such a machine in 1782. Their balloon consisted of a paper globe or cylinder, the motor power being super-heated air supplied by the burning of vine twigs under it. The Montgolfier or fire balloon, as it was called, was superseded by the hydrogen gas balloon of MM. Charles and Robert, this being in turn supplanted by the ordinary gas balloon of Mr. Green. Since the introduction of coal gas in the place of hydrogen gas, no radical improvement has been effected, all attempts at guiding the balloon having signally failed. This arises from the vast extent of surface which it necessarily presents, rendering it a fair conquest to every breeze that blows; and because the power which animates it is a mere lifting power which, in the absence of wind, must act in a vertical line. The balloon consequently rises through the air in opposition to the law of gravity, very much as a dead bird falls in a downward direction in accordance with it. Having no hold upon the air, this cannot be employed as a fulcrum for regulating its movements, and hence the cardinal difficulty of ballooning as an art.