Mr. A. Lawrence Rotch, director of the Blue Hill Observatory, in his valuable book, _The Conquest of the Air_, gives this practical summary of a long series of studious observations: “At night, however, because there are no ascending currents, the wind is much steadier than in the daytime, making night the most favorable time for aerial navigation of all kinds.... A suitable height in the daytime, unless a strong westerly wind is sought, lies above the cumulus clouds, at the height of about a mile; but at night it is not necessary to rise so high; and in summer a region of relatively little wind is found at a height of about three-fourths of a mile, where it is also warmer and drier than in the daytime or at the ground.” Notwithstanding all difficulties, the fact remains that, once they are overcome, the air is the ideal highway for travel and transportation. On the sea, a ship may sail to right or left on one plane only. In the air, we may steer not only to right or left, but above and below, and obliquely in innumerable planes. We shall not need to traverse long distances in a wrong direction to find a bridge by which we may cross a river, nor zigzag for toilsome miles up the steep slopes of a mountain-side to the pass where we may cross the divide. The course of the airship is the proverbial bee-line--the most economical in time as well as in distance. Chapter III. LAWS OF FLIGHT. The bird--Nature’s models--Man’s methods--Gravity--The balloon--The airship--Resistance of the air--Winds--The kite--Laws of motion and force--Application to kite-flying--Aeroplanes. If we were asked to explain the word “flying” to some foreigner who did not know what it meant, we should probably give as an illustration the bird. This would be because the bird is so closely associated in our thoughts with flying that we can hardly think of the one without the other. It is natural, therefore, that since men first had the desire to fly they should study the form and motions of the birds in the air, and try to copy them. Our ancestors built immense flopping wings, into the frames of which they fastened themselves, and with great muscular exertion of arms and legs strove to attain the results that the bird gets by apparently similar motions. However, this mental coupling of the bird with the laws of flight has been unfortunate for the achievement of flight by man. And this is true even to the present day, with its hundreds of successful flying machines that are not in the least like a bird. This wrongly coupled idea is so strong that scientific publications print pages of research by eminent contributors into the flight of birds, with the attempt to deduce lessons therefrom for the instruction of the builders and navigators of flying machines.