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THE POPULAR SCIENCE MONTHLY.

this occasion being the hearing of the rockets at the Mouse Lighthouse, 812 miles east by south, and at the Chapman Lighthouse, 812 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 observation. "This," as General Younghusband justly remarked at the time, "may prove to be a valuable consideration if it should be found necessary 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 executed. The gun-cotton near the ground consisted of 12-pound disks suspended from an horizontal iron bar about 412 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 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 512 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, which 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 frequently noticed at the South Foreland in 1872-'73, and called by me "aƫrial echoes."

On the 23d of March the experiments were resumed, the most noteworthy results of that day's observations being that the sounds were heard at Tillingham, 10 miles to the northeast; at West Mersea, 1534 miles to the northeast by east; at Brightlingsea, 1712 miles to the northeast; and at Clacton Wash, 2012 miles to the northeast by 12 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