It is difficult for an inexperienced person to conceive of the effect of even a light wind upon so small an object as a rifle-ball, when shot from the gun. The difficulty arises from the impossibility of taking in the idea of such rapid flight, or of the resistance produced by it, by comparison with anything within the limits of our experience. We may attain a conception of it, however, by trying to move a stick through the water. Moving it slowly, the resistance is imperceptible; but as we increase the velocity, we find the difficulty to increase very rapidly, and if we try to strike a quick blow through the water, we find the resistance so enormous that the effort is almost paralyzed. Mathematically, the resistance increases in the ratio of the square of the velocity; and although the air is of course more easily displaced than water, the same rule applies to it, and the flight of a ball is so inconceivably rapid that the resistance becomes enormous. The average initial velocity of a cannon- or rifle-ball is sixteen hundred feet in a second, and a twelve-pound round shot, moving at this rate, encounters an atmospheric resistance of nearly two hundred pounds, or more than sixteen times its own weight. Perhaps a clearer idea may be attained by the statement of the fact, that, were it possible to remove this resistance, or, in other words, to fire a ball in a vacuum, it would fly ten miles in a second,--the same time it now requires to move sixteen hundred feet. Bearing in mind this enormous resistance, it will be more readily apparent that even a slight motion of the element through which the ball is struggling must influence its course. For this reason it is that the best time to shoot, as a general rule, is in the morning or evening, when the air is most apt to be perfectly calm. It will often be found, after making very satisfactory shots at sunrise, that by ten o'clock, even on what would be called a calm day, it is impossible to attain to anything like the accuracy with which the day's work was begun; and, owing to the irregular motion of the air, the difficulty cannot be overcome, except to a limited degree, by making allowance for it.