RECENT EXPERIMENTS ON FOG-SIGNALS 281 



6 oz., 2 inclies 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 estimated 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 practically equal 

 in intensity, thus proving that an addition of 30 per cent 

 In the larger charges produces no sensible difference in 

 the sound. Were the sounds estimated by some physical 

 means, instead of by the ear, the values of the sounds at 

 the distances recorded would not, in my opinion, show a 

 greater advance with the increase of material than that 

 indicated by the foregoing numbers. Subsequent experi- 

 ments rendered still more certain the effectiveness, as well 

 as the economy, of the smaller charges of gun-cotton. 



It is an obvious corollary from the foregoing experi- 

 ments 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 the 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 nei- 

 ther syren, steam- whistle, nor gun could be mounted; and 

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

 cial signal-guns, the gun-cotton will prove invaluable. 

 But in most of these cases we have the drawback that 



