2G8 FRAGMENTS OF SCIENCE. 



produced near the surface of the earth with others 

 produced at the elevation attainable by the gun-cotton 

 rockets. Wind and weather, however, are not at our 

 command; and hence one of the objects of a series of 

 experiments conducted on December 13, 1876, was not 

 fulfilled. It is worthy, however, of note that on this 

 day, with smooth water and a calm atmosphere, the 

 rockets were distinctly heard at a distance of 11 '2 miles 

 from the firing-point. The quantity of. gun-cotton 

 employed was 7J oz. On Thursday, March 8, 1877, 

 these comparative experiments of firing at high and low 

 elevations were pushed still further. The gun-cotton 

 near the ground consisted of J-lb. disks, suspended from 

 a 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 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 distances 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 

 5J 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 ad- 

 vantage. 



Towards 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 ' Gralatea ' a rocket was discharged, which 

 reached a great elevation, and exploded with a loud 



