RECENT EXPERIMENTS ON FOG-SIGNALS. 281 



in intensity, an addition of 30 per cent, in the larger charges producing 

 no sensible difference in the sound. Were the sounds estimated by some 

 physical means, instead of by the ear, the values of the sound would 

 not, in my opinion, show a greater advance with the increase of mate- 

 rial than that indicated by the foregoing numbers. Subsequent experi- 

 ments rendered still more certain the effectiveness, as well as the econ- 

 omy, of small charges of gun-cotton. 



It is an obvious corollary from the foregoing experiments that on our 

 " nesses " and promontories, where the land is clasped on both sides for 

 a considerable distance by the sea where, therefore, the sound has to 

 propagate itself rearward as well as forward the use of the parabolic 

 gun, or of the parabolic reflector, might be a disadvantage rather than 

 an advantage. Here gun-cotton, exploded in the open, forms a most 

 appropriate source of sound. This remark is especially applicable to such 

 lightships as are intended to spread the sound all round them as from 

 central foci. As a signal in rock-lighthouses, where neither siren, steam- 

 whistle, nor gun, could be mounted, and as a handy fleet-signal, dispens- 

 ing with the lumber of special signal-guns, the gun-cotton will prove in- 

 valuable. But in most of these cases we have the drawback that local 

 damage may be done by the explosion. The lantern of the rock-light- 

 house might suffer from concussion near at hand, and though mechanical 

 arrangements might be devised, both in the case of the lighthouse and 

 of the ship's deck, to place the firing-point of the gun-cotton at a safe 

 distance, no such arrangement could compete, as regards simplicity and 

 effectiveness, with the expedient of a gun-cotton rocket. Had such a 

 means of signaling existed at the Bishop's Rock Lighthouse, the ill- 

 fated Schiller might have been w r arned of her approach to danger ten, 

 or it may be twenty, miles before she reached the rock which wrecked 

 her. Had the fleet possessed such a signal, instead of the ubiquitous 

 but ineffectual 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 cary 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 firework manufactory of the Messrs Brock, at Nunhead. 

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

 four with 7-J- ounces 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 



