314 SCIENTIFIC BALLOONING. 



moisture unseen and unsuspected, but which proved the true herald 

 of the short break that immediately after occurred in the hot, dry 

 season. 



A fair indication of the moisture present in a clear heaven may, 

 perhaps, often be found in the tint of the blue sky — toned to gray in 

 dry, east winds, pale during continuance of drought, and deep blue 

 when storms are imminent. However, enough has been said to show 

 that we must, as a general rule, by no means regard even our clearest 

 skies as homogeneous or uniform. Moisture will lie around or above 

 in pools or shallow seas, and close observation in addition to delicate 

 instrumental aid is needed to measure even approximately its varying 

 constitution. But there is a subtler test that now claims our notice, 

 and which is capable of far greater development than has been accorded 

 to it. A few records gathered from a long series of observations will 

 introduce and justify this new division of our subject. 



From a high ridge in Berkshire there is occasionally to be heard 

 the sound of the firing of guns at Aldershot, 30 miles to the eastward. 

 These guns are chiefly noticed in the summer time, when there is very 

 rarely an east wind to help the sound. Occasionally the reports are 

 mistaken for distant thunder, and thus cause alarm at a time when hay 

 harvest is in progress. There is, however, a saying in the district that 

 the " guns are worse than thunder," and this because they forecast 

 not a passing or local storm, but rather the approach of generally 

 unsettled weather. It is easy to prove that it is a continuity of an 

 uniform moisture-laden air stretching across that part of the country 

 that is the cause of the phenomenon. The testimony of seamen and 

 other trained observers goes to show that homogeneous moist air or 

 mist is the readiest vehicle of sound; that dry air seldom or never 

 conveys sound so readily; while an atmosphere of varying density 

 renders all sound capricious. 



From a sheltered quiet lawn the Aldershot guns had not been 

 noticed all through the late summer until far on in one afternoon in 

 the middle of August, when their sound rolled out with great dis- 

 tinctness, the weather to all appearance remaining unchanged and the 

 barometer standing Arm and high. In the night, however, thunder 

 was heard for some two hours, the first time for many weeks, and in 

 the morning the guns were heard again more distinctly than before. 

 In this case sound had been the clearest, and, indeed, the only telltale 

 of a humid layer of the atmosphere brooding over the countryside. 



Arguing, however, by the light of such statistics as have been 

 given above, there was no proof here of the true condition of the air 

 at higher elevations; but, as it happened, only three days previously, 

 the writer, during an aerial trip, had had occasion to note some 

 remarkable acoustic conditions prevailing aloft. Weather conditions, 

 as indicated by hot suns and clear skies, by readings of temperature 

 and pressure, remained, indeed, unchanged, but there had been indi- 



