The ingenuity of Mr. Green has been exerted with the view of discovering other improvements in the art of aerial navigation. One great obstacle to the successful practice in the art is, the difficulty of maintaining the power of the balloon for any length of time undiminished in its progress through the air. It is ascertained by the uniform experience of aeronauts, that, between the earth and two miles above the level of the sea, a variety of currents exist, some blowing in one direction and some in another; and when the aeronaut has risen to the elevation where he meets with a current that will waft him in the desired direction, it is of importance for him to be able to preserve that elevation. But the balloon, in consequence of the increase or diminution of weight to which it is liable from a variety of causes, will not keep at that altitude. The great changes which are constantly taking place in the weight of the atmosphere, the deposition of humidity on the surface of the balloon, and its subsequent evaporation by the rise of temperature, the alternate heating and cooling of the gaseous contents of the balloon, according as it may be exposed to the action of the solar rays or screened from them by the interposition of clouds, not to advert to other agencies, less known though not less powerful, all combine in making the machine at one time to ascend and at another to descend. Thus it may be removed out of a favorable into an adverse current. To overcome this difficulty, and enable the aeronaut to keep the balloon at the same level without expending its power, by discharging gas from the valve to lower it, or by casting out a portion of the ballast to raise it--processes which must in time waste the whole power of the largest balloon, and bring it to the earth--Mr. Green suggested the contrivance of a rope of sufficient length and material trailing on the ground beneath, and if over the sea, the rope is to be tied to a vessel filled with liquid ballast, which floats on the surface. This rope will act as a drag on the balloon, when, from any of the causes we have referred to, it tends to rise, for, in that case, it will draw up a portion of the rope, and, by thus adding to its weight, will be impeded in its upward course; and, on the other hand, when, from opposite causes, it tends to descend, it will, during every foot of its descent, have its weight, and consequently its descending tendency, diminished, by throwing on the earth the labor of supporting an additional portion of the rope. This, however, at best, is a clumsy contrivance, and there are various objections to its practical utility. It could hardly be practicable on land, on account of the damage and danger that would be occasioned by the entanglement of the rope in trees and buildings; and at great elevations above the earth, the weight of the rope would become so considerable as to require for its support a large portion of the ascending power of any balloon.