282 THE POPULAR SCIENCE MONTHLY. 



this occasion being the hearing of the rockets at the Mouse Lighthouse, 

 8| miles east by south, and at the Chapman Lighthouse, 8^ miles west 

 by north ; that is to say, at opposite sides of the firing-point. It is 

 worthy of remark that, in the case of the Chapman Lighthouse, land 

 and trees intervened between the firing-point and the place of observa- 

 tion. " This," as General Younghusband justly remarked at the time, 

 " may prove to be a valuable consideration if it should be found neces- 

 sary to place a signal-station in a position whence the sea could not be 

 freely observed." Indeed, the clearing of such obstacles was one of 

 the objects which the inventor of the rocket had in view. 



On the 13th of December, 1876, and again on the 8th of March, 1877, 

 comparative experiments on firing at high and low elevations were exe- 

 cuted. The gun-cotton near the ground consisted of ^-pound disks 

 suspended from an horizontal iron bar about 4|^ feet above the ground. 

 The rockets carried the same quantity of gun-cotton in their heads, and 

 the height to which they attained, as determined by a theodolite, was 

 from 800 to 900 feet. 



The latter day was cold, with occasional squalls of snow and hail, 

 the direction of the sound being at right angles to that of the wind. 

 Five series of observations were made on board the Vestal, at dis- 

 tances varying from 3 to 6 miles. The mean value of the explosions 

 in the air exceeded that of the explosions near the ground by a small 

 but sensible quantity. At Windmill Hill, Gravesend, however, which 

 was nearly to leeward, and h\ miles from the firing-point, in nineteen 

 cases out of twenty-four the disk fired near the ground was loudest, 

 while in the remaining five the rocket had the advantage. Toward the 

 close of the day the atmosphere became very serene. A few distant 

 cumuli sailed near the horizon, but the zenith and a vast angular space 

 all round it were absolutely free from cloud. From the deck of the 

 Galatea a rocket was discharged, which reached a great elevation, and 

 exploded with a loud report. Following this solid nucleus of sound 

 was a continuous train of echoes, w T hich retreated to a continually 

 greater distance, dying gradually off into silence after seven seconds' 

 duration. These echoes were of the same character as those so fre- 

 quently noticed at the South Foreland in 1872-'73, and called by me 

 "aerial echoes." 



On the 23d of March the experiments were resumed, the most note- 

 worthy results of that day's observations being that the sounds were 

 heard at Tillingham, 10 miles to the northeast ; at West Mersea, 15f 

 miles to the northeast by east ; at Brightlingsea, 17^ miles to the north- 

 east ; and at Clacton Wash, 20^ miles to the northeast by \ east. The 

 wind was blowing at the time from the southeast. Some of these 

 sounds were produced by rockets, some by a 24-pound howitzer, and 

 some by an 8-inch maroon. 



In December, 1876, Mr. Gardiner, the managing director of the 

 Cotton-Powder Company, had proposed a trial of this material against 



