1878.] Prof. Tyndall. Experiments on Fog-Signals. 251 



twenty, miles before she reached the rock which wrecked her. Had 

 the fleet possessed such a signal, instead of the ubiquitous but in- 

 effectual whistle, the "Iron Duke" and "Vanguard" need never 

 have come into collision. 



It was the necessity of providing a suitable signal for rock light- 

 houses, and of clearing obstacles which cast an acoustic shadow, that 

 suggested the idea of the gun-cotton rocket to Sir Richard Collinson, 

 Deputy Master of the Trinity House. That idea was to place a disk or 

 short cylinder of the gun-cotton, in the head of a rocket, the ascensional 

 force of which should be employed to carry the disk to an elevation of 

 1,000 feet or thereabouts, where by the ignition of a fuse associated 

 with a detonator, the gun-cotton should be fired, sending its sound in 

 all directions vertically and obliquely down upon earth and sea. The 

 first attempt to realize this idea was made on the 18th of July, 1876, 

 at the fire-work manufactory of the Messrs. Brock, at ISTunhead. 

 Eight rockets were then fired, four being charged with 5 oz. and four 

 with oz. of gun-cotton. They ascended to a great height, and 

 exploded with a very loud report in the air. On the 27th of July, the 

 rockets were tried at Shoeburyness, the most noteworthy result on 

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

 miles E. by S., and at the Chapman Lighthouse, 8J miles W. by 

 N. ; 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 -g-lb. 

 disks suspended from a horizontal iron bar about 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 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. Towards the 



VOL. xxvii. s 



