250 Prof. Tyndall. Experiments on Fog-Signals. [Mar. 21, 



between the sounds were by no means so great as the differences in 

 the quantities of explosive material might lead one to expect. The 

 mean values of eighteen series of observations made on board the 

 "Galatea," at distances varying from If mile to 4*8 miles, were as 

 follows : — 



Weights..... 4-oz. 6-oz. 9-oz. 12-oz. 7-oz. Rocket. 



Yalue of sound 3 12 3 34 ,4 4 "03 3 "35 



These charges were cut from a slab of dry gun-cotton about If inch 

 thick : they were squares and rectangles of the following dimensions : 

 — 4 oz., 2 inches by 2 inches ; 6 oz., 2 inches by 3 inches ; 9 oz., 

 3 inches by 3 inches ; 12 oz., 2 inches by 6 inches. 



The numbers under the respective weights express the recorded 

 value of the sounds. They must be simply taken as a ready means of 

 expressing the approximate relative intensity of the sounds as esti- 

 mated by the ear. When we find a 9-oz. charge marked 4, and a 

 12-oz. charge marked 4*03, the two sounds may be regarded as prac- 

 tically equal 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 sounds would not, in my opinion, show a greater 

 advance with the increase of material than that indicated by the fore- 

 going numbers. Subsequent experiments rendered still more certain 

 the effectiveness, as well as the economy, of small charges of gun- 

 cotton. 



It is an obvious corollary from the foregoing experiments that on 

 our " n esses " 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 disadvan- 

 tage 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 light- 

 houses, where neither syren, steam- whistle, nor gun could be mounted, 

 and as a handy fleet-signal, dispensing with the lumber of special 

 signal-guns, the gun-cotton will prove invaluable. But in most of 

 these cases we have the drawback that local damage may be done by 

 the explosion. The lantern of the rock-lighthouse 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 

 signalling existed at the Bishop's Rock Lighthouse the ill-fated Schiller 

 might have been warned of her approach to danger ten, or it may be 



