GUNPOWDER : ITS MANUFACTURE AND CONVEYANCE. 
51 
These pebbles are made by passing press-cake of that thickness 
between two pairs of rollers, armed with sharp-cutting edges. 
The first pair of rollers by means of these edges cuts the cake 
into several small bars, about four-fifths of an inch square at 
the ends, and these bars, on passing between the second pair of 
rollers, are divided into cubes or pebbles. After having been 
rolled in a hollow cylinder, or reel, to round off the sharp edges 
and get rid of the dust, the pebbles are carried to the drying- 
house to be freed from the moisture they contain. 
The drying-house is a large room with double doors, and 
fitted with racks from floor to ceiling. On these racks copper 
and wooden trays are placed, containing the powder spread out 
in thin layers. Steam pipes are introduced from a boiler in an 
adjoining building, and thus the air of the room is kept at a 
temperature of about 135°. The firemen in charge of the 
drying are forbidden to enter the room for fear of carrying in a 
spark in their clothes, but they ascertain the temperature by 
a register thermometer placed inside a small window, and this 
thermometer also acts as a tell-tale, by showing if the tempera- 
ture has at any time been allowed to become too high or too 
low. So perfect are all the arrangements at Waltham, that no 
explosion has ever occurred in the drying-house. 
The dried pebbles are finished by being placed in a revolving 
barrel (called a glazing barrel), with a certain amount of 
powdered blacklead. On being taken out, every pebble is 
found to have a perfectly smooth surface coated with blacklead, 
the effect of which is still further to diminish the rate of 
burning. The pebbles are then thrown into sieves to separate 
small fragments ; all irregular pieces are picked out by hand, 
and the remainder is packed in ordinary powder barrels, which 
would hold 100 lbs. of rifle powder, but contain 125 lbs. of the 
pebble powder, on account of its greater density. 
The following results of experiments with the 8-inch gun will 
give the reader an idea of the effects of the different kinds of 
powder. We need only explain that K.L.Gr. means the old 
“ Rifle Large Grain ” powder still in use for field artillery, and 
draw attention to the fact that the pebble powder gives at once 
the highest velocity and the lowest strain : — 
Pnwdpv Charge, Initial Velocity, Maximum Pressure, 
Lbs. Feet per Second, Tons on Square Inch. 
E.L.G 30 1324 29-8 
Russian Prismatic 32 1366 20*5 
Service Pellet ... 30 1338 17-4 
Pebble 35 1374 15-4 
Visitors to the laboratory at Waltham can see there a number 
of experimental varieties of pebble powder, the largest of which 
consists of cubes as hard as stone, each side of which is two 
