50 
rorULAR SCIENCE REVIEW. 
Here an increase of *05 in density reduced the velocity by 42 
feet, and the pressure by 8 tons. This shows the importance of 
obtaining a uniform density in the manufacture. For this 
purpose it is not sufficient to use a uniform pressure in the 
press-house, as even then the density of different pressings will 
vary on account of the changing state of the atmosphere, the . 
different degrees of moisture in the powder meal, its varying 
bulk and elasticity, and other minor causes. The only practi- 
cable method of securing an approximately uniform density is 
to test the product of various pressings and then mix them, so 
as to reduce the whole to the average density required, and this 
is the constant practice at Waltham. 
We have alluded incidentally to the decrease of pressure in 
the gun as the density of the powder increases. With the im- 
mense guns constructed in recent years it is important to reduce 
the strain on the metal as far as possible, as this is the only way 
in which the gun can be safely fired. But it must be remem- 
bered, that by seeking to accomplish this by indefinitely 
increasing the density of the powder, we would at the same 
time decrease the velocity of the shot, or, in other words, its 
useful effect. Artillerists have, therefore, had recourse to the 
expedient of using a powder, each grain of which is a lump of 
press-cake. The effect of this is to make it burn slower than 
grain powder ; for these lumps, when ignited at the surface, 
burn, as it were, in concentric layers until the whole is con- 
sumed, and by this means the explosion, though to all appear- 
ance instantaneous, is in reality much more gradual than that 
which follows the ignition of smaller grains. In other words, 
the explosion of the charge in a heavy gun is made to be less of 
the character of a violent blow on the sides of the bore and the 
base of the shot, and more like a gradual shove given to the 
shot with a corresponding pressure on the gun. 
The first form proposed for cannon powder for heavy guns 
was that invented in America by Dr. Doremus, in which the 
whole charge was made into a solid disc, the size of the bore of 
the gun. This, however, was found to give very unsatisfactory 
results. Then the Bussian Government adopted a powder com- 
pressed into large hexagonal prisms, and in Belgium another 
powder was made in the form of cylindrical pellets. This was 
adopted by our 'Government, and an immense sum was spent on 
erecting machinery at Waltham for its manufacture, but the 
pellet powder has since been superseded by a simpler form, much 
easier to make, and giving better results ; and the pellet ma- 
chinery has been altered, and, w 7 e believe, is now used in the 
manufacture of gun-cotton. 
The pebble powder now in use consists of cubes of compressed 
gunpowder, with sides about four-fifths of an inch square. 
