SCIENTIFIC BALLOONING. 315 



cations of disturbance overnight, and at Earl's Court, for two hours 

 before closing - time, the captive balloon had had to be hauled down. 

 Still on the day in question there was nothing - of a nature unusual 

 save a fitful, gusty wind, and, perhaps, a feeling of languor in the air. 



The ascent was made at -1 p. m. from the Crystal Palace, and the 

 balloon's course lay directly over London at a mean height of 4,000 feet. 

 The thermometer indicated nothing abnormal, there being a fall of 

 half a dozen degrees in the dry bulb, and an amount of general 

 moisture, shown by the wet, comparable with that recorded below 

 previously to starting, only fluctuating constantly within small limits. 

 One matter of consideration only was remarkable, namely, sound, and 

 this was noteworthy by its absence. The cheer of the crowd lost its 

 wonted heartiness, lesser sounds were mute, even whistles forgot their 

 shrillness, and the raucous rattle of the giant city was reduced to a 

 mere dull hum. But not till the quiet country to the north of London 

 was reached did the full measure of acoustical opacity in the atmos- 

 phere betray itself. This is well tested by means of echo. A 

 hunting-horn forms a convenient instrument for evoking echoes, and 

 very frequently it is easy to hear the sound of a blast returned to a 

 balloon across an iuterval of upward of _},o00 feet. Of course the 

 nature of the country over which the aerouaut is traveling at the 

 moment greatly influences the result. The waves of sound recoil from 

 trees more readily than from fields, most readily of all from the surface 

 of still water; but days have been found when fields in open country, 

 irrespective of their character, have clearly responded to the horn, 

 though lying a full half mile below. There was, then, much significance 

 in the fact that on the day now being described echoes refused to be 

 aroused even at the range of a few hundred feet. The explanation 

 apparently lay in the unequal nature of the medium through which 

 the sound had to travel. The air was presumably, as it were, broken 

 up in patches, and barred the passage of sound much as glass when 

 broken up will impede the passage of light. We may, indeed, conceive 

 the air to have been invisibly mottled, after the manner, say, of a finely 

 divided mackerel sky; a transient condition of things, no doubt, and 

 we apparently see how three days later the moister masses had settled 

 in a general low-lying layer. 



We may here note that Mr. Glaisher's published statements with 

 regard to sounds heard from a balloon are interesting and valuable. 

 He reports that the whistle of a train is audible at 10,000 feet, the train 

 itself at 8,200; the bark of a dog at 5,900; shouting of men and women 

 at 5,000, and so on; but since the writing of that report, atmospheric 

 refraction and reflection of sound have become established facts. So 

 also the strengthening of sound by resonance; its extinction by inter- 

 ference, and that curious modification it occasionally undergoes whereby 

 the same sounds may vary in relative intensity on different days. 



No more important inquiry can come within the province of the aero- 



