[119] The artificial currents produced by the wing during its descent may be readily seen by partially filling a chamber with steam, smoke, or some impalpable white powder, and causing the wing to descend in its midst. By a little practice, the eye will not fail to detect the currents represented at _d_, _e_, _f_, _g_, _h_, _i_, _l_, _m_, _o_, _p_, _q_, _r_ of fig. 129, p. 253.
If fig. 129 be made to assume a horizontal position, instead of the oblique position which it at present occupies, the manner in which _an artificial current_ is produced by one sweep of the wing from right to left, and utilized by it in a subsequent sweep from left to right, will be readily understood. The artificial wave wing makes a horizontal sweep from right to left, _i.e._ it passes from the point _a_ to the point _c_ of fig. 129. During its passage it has displaced a column of air. To fill the void so created, the air rushes in from all sides, viz. from _d_, _e_, _f_, _g_, _h_, _i_; _l_, _m_, _o_, _p_, _q_, _r_. The currents marked _g_, _h_, _i_; _p_, _q_, _r_, represent the reflex or _artificial currents_. These are the currents which, after a brief interval, force the flame of the candle from right to left. It is those same currents which the wing encounters, and which contribute so powerfully to its elevation, when it sweeps from left to right. The wing, when it rushes from left to right, produces a new series of artificial currents, which are equally powerful in elevating the wing when it passes a second time from right to left, and thus the process of making and utilizing currents goes on so long as the wing is made to oscillate. In waving the artificial wing to and fro, I found the best results were obtained when the range of the wing and the speed with which it was urged were so regulated as to produce a perfect reciprocation. Thus, if the range of the wing be great, the speed should also be high, otherwise the air set in motion by the right stroke will not be utilized by the left stroke, and _vice versâ_. If, on the other hand, the range of the wing be small, the speed should also be low, as the short stroke will enable the wing to reciprocate as perfectly as when the stroke is longer and the speed quicker. When the speed attained is high, the angles made by the under surface of the wing with the horizon are diminished; when it is low, the angles are increased. From these remarks it will be evident that the artificial wave wing reciprocates in the same way that the natural wing reciprocates; the reciprocation being most perfect when the wing is vibrating in a given spot, and least perfect when it is travelling at a high horizontal speed.