316 SCIENTIFIC BALLOOOING. 



naut than as to the mode and measure in which his own proper element 

 conveys the sounds we hear, and mayhap quenches those sounds we 

 don't hear. Some avenue of sound through the void below has some- 

 times admitted, for a moment only, the strains of a band or clang of a 

 bell, which the next moment has been lost utterly. Sometimes, far 

 beyond its proper range, some noise from earth has been caught in the 

 hollow of a cloud, as by a sounding-board, and concentrated loudly 

 upon the ear of the balloonist. Sometimes an intervening cloud far 

 down has damped the roar of a train more effectually than even the 

 mass of a hill has done, when the train had been burrowing through a 

 tunnel. 



In all cases, with one possible exception, sounds heard aloft lose 

 reverberation. Mr. Whymper describes thunder on the mountain side 

 as uttering a " single bang," so to a voyager in the sky will a gun on 

 Plumstead Marsh speak with a single yelp. But Professor Tyndall, On 

 one occasion, convinced himself and those wlio stood around him that 

 reverberation could be found in empty air, and that echo can be 

 returned from an acoustic cloud invisible to the eye. This interesting 

 point is being investigated by the aeronaut, with results that will 

 shortly be more complete. Unquestionably the entire physics of the 

 firmament will shortly have undergone the closest scrutiny, its compo- 

 sition, the proportion of newly found constituents at highest accessible 

 elevations, the amount of carbonic acid it holds, the measurements of 

 its electricity, the condition and character of matter in suspension, the 

 presence or absence of germs. These are questions all important, and 

 on which many facts have been amassed, but on which it may be pre- 

 mature to generalize. 



The balloon has now to be recognized as an indispensable observa- 

 tory. In some ways it affords the student of astronomy and optics 

 opportunities which can not be gained in any station on earth. The 

 extraordinary brilliance and steadiness of celestial objects viewed by 

 optical aid from a balloon 10,000 or 12,000 feet above sea level must 

 be seen to be realized. Indeed, from half that height the full moon, 

 regarded through an ordinary field glass, becomes an object intolerable 

 to gaze upon. But the case is far otherwise on mountain observatories, 

 which can not be wholly free from disturbing currents or from that 

 peculiar stratum of air always and everywhere clinging to earth. 



It is obvious, then, how many questions can be dealt with to great 

 advantage from higher and purer regions. Most important data are 

 being gathered bearing on refraction as influenced by altitude, by 

 temperature and humidity. Spectroscopic observations taken from the 

 earth, and again a few minutes later from some thousands of feet above, 

 are destined to throw very valuable light on those lines which have 

 simply a telluric origin. Again, many doubtful observations needing 

 low powers, and hitherto made from earth, will receive a crucial test 

 when repeated from above; and rare phenomena, such as a total solar 



