RECENT EXPERIMENTS ON FOG SIGNALS. 203 
shape and size of the exploding mass would affect the con- 
stitution of the wave of sound. I did not think large reo* 
tangular slabs the most favorable shape, and accordingly 
proposed cutting a large slab into fragments of different 
sizes, and pitting them against each other. The dif- 
ferences 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 4oz. 6 oz. 9 oz. 12 oz. 
Value of sound 3.12 3.34 4.0 4.03 
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 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 percent, 
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 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 the most appropriate source of sound. 
This remark is especially applicable to such lightships as 
