AEROSTATIC VOYAGE 55' 



Without Teeming to have loft »ny part of its intenfity. No re- No echo but 

 petition was made except when the trumpet was direded to ^^^^^^^'^^ ^jj^^j 

 thfe earth; and the intervals of retlediion were different ac- to the earth, 

 cording to the elevation of the obfervers. The percuffion im- 

 prelTed on the air by the found every time produced a flight 

 undulation in the aeroftat; whence they deduce an inference 

 in favour of the fuppofed efficacy of cannon in partly modify- 

 ing or averting the difcharge of ftormy clouds. In one of their 

 experi-ments, the found employed ten feconds in its return, 

 which would give a diftance of about two miles out and home, 

 if the farne law of the velocity of found were fuppofed to 

 prevail in the perpendicular courfe as along the furface of the 

 earth, which however does not feem likely. The barometer 

 ftood then at 27 inches, and at their outfet it was at 30 inches 

 on the ground. It would be eafy, and it is furely defirable to 

 make experiments with cannon and ftop watches on the velo- 

 city of albending, and if poliible, defcending found. 



This refiedion of found or echo is a fubject of very great The ftrength of 

 curiofity. There is perhaps no other inftance in nature wi>ere f "^""^ ^^^^ 

 fo extended a wall of refiedlioh can be had. I am difpofed to on the ilijlnefs 

 think that the apparent intenfity of the returned found may in °^" tl^e ^^^^ation. 

 fome rrieafure have depended on the perfe6t filence in which 

 tlie fpeakers were placed. In a ftill night the centinels on the 

 ramparts at Portfmouth may be heard at the Ifle of Wight 

 over a diftance of live miles, and there are numerous inftances 

 of low founds, fuch as the beating of a clock or watch, or the 

 founds of footfteps being heard toconfiderable diftances, when 

 other founds do not a6l on the organ of fenfe. * 



In their defcent to the earth they paffed through various Sudden rife of 

 ftrata of vapours, all of different temperatures, and at the in- f^^ thermometer 

 ftant the earth came in tight, the thermometer ftarted up through ^ 

 feveral degrees, probably becaufe they had quiUed a cold mafs 

 of vapour which obfcured their view, or perhaps becaufe the 

 radiant heat of the earth's furface might at that moment have 

 reached them unimpaired. 



* See a curious paper of M. Perrole on found, with the annota- 

 tions thereon, in the firft Vol. of our quarto Journal, page 41 1. — N. 



Letter 



