EECENT EXPERIMENTS ON FOG-SIGNALS. 267 



whistle, the 'Iron Duke' and 'Vanguard' need never 

 have come into collision. 



It was the necessity of providing a suitable signal 

 for rock lighthouses, and of clearing obstacles which 

 cast an acoustic shadow, that suggested the idea of the 

 gun-cotton rocket to Sir Eichard Collinson, Deputy 

 Master of the Trinity House. His idea was to place a 

 disk or short cylinder of gun-cotton in the head of a 

 rocket, the ascensional force of which should be employed 

 to carry the disk to an elevation of 1000 feet or there- 

 abouts, 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 realise this 

 idea was made on July 18, 1876, at the firework manu- 

 factory of the Messrs. Brock, at Nunhead. Eight 

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

 and four with 7-J- oz. of gun-cotton. They ascended to 

 a great height, and exploded with a very loud report in 

 the air. On July 27, the rockets were tried at Shoe- 

 buryness. The most noteworthy result on this occasion 

 was the hearing of the sounds at the Mouse Lighthouse, 

 8J 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. 



With reference to the action of the wind, it was 

 thought desirable to compare the range of explosions 



