Another wonder detected, equally striking though less uncommon, was of an acoustical nature, the locality this time being over Paris. The height of the balloon at this moment was not great, and, moreover, was diminishing as it settled down. Suddenly there broke in upon the voyagers a sound as of a confused kind of murmur. It was not unlike the distant breaking of waves against a sandy coast, and scarcely less monotonous. It was the noise of Paris that reached them, as soon as they sank to within 2,600 feet of the ground, but it disappeared at once when they threw out just sufficient ballast to rise above that altitude. It might appear to many that so strange and sudden a shutting out of a vast sound occurring abruptly in the free upper air must have been more imaginary than real, yet the phenomenon is almost precisely similar to one coming within the experience the writer, and vouched for by his son and daughter, as also by Mr. Percival Spencer, all of whom were joint observers at the time, the main point of difference in the two cases being the fact that the "region of silence" was recorded by the French observers as occurring at a somewhat lower level. In both cases there is little doubt that the phenomenon can be referred to a stratum of disturbed or non-homogeneous air, which may have been very far spread, and which is capable of acting as a most opaque sound barrier. Attention has often been called in these pages to the fact that the action of the sun on an inflated balloon, even when the solar rays may be partially obscured and only operative for a few passing moments, is to give sudden and great buoyancy to the balloon. An admirable opportunity for fairly estimating the dynamic effect of the sun's rays on a silk globe, whose fabric was half translucent, was offered to the French aeronauts when their balloon was spread on the grass under repair, and for this purpose inflated with the circumambient air by means of a simple rotatory fan. The sun coming out, the interior of the globe quickly became suffocating, and it was found that, while the external temperature recorded 77 degrees, that of the interior was in excess of 91 degrees. CHAPTER XVII. ADVENTURE AND ENTERPRISE. A balloon which has become famous in history was frequently used in the researches of the French aeronauts mentioned in our last chapter. This was known as "The Giant," the creation of M. Nadar, a progressive and practical aeronaut, who had always entertained ambitious ideas about aerial travel.