GUNFOWDER : ITS MANUFACTURE AND CONVEYANCE. 
47 
been devised for preventing an explosion from extending from 
one mill to another. A shaft runs horizontally through the 
upper part of the walls of each row of mill-houses. A shutter, 
balanced by a weight on the other side of the shaft, projects 
from it over each mill, and this shutter supports one side of a 
water-tank, the other resting on a pivot. Now, if an explosion 
takes place in any of the mills, the shutter above it will be 
blown up, turning the horizontal shaft, and raising all the 
shutters attached to it ; so that the tanks, being left unsup- 
ported, turn over, and drench the contents of the mill-beds 
below. 
On leaving the mill, the gunpowder is in the form of a soft 
cake, which easily breaks up into meal and dust. The old plan 
for making gunpowder, still followed in some places, was to 
moisten this mill-cake and force it through fine sieves, so as to 
break it into grains ; but the moisture partly dissolved the salt- 
petre, and thus, to some extent, destroyed the previous incor- 
poration, and the result was an inferior gunpowder, which, on 
account of the softness of the grains, often broke up into dust 
in transport. In the modern process, the mill-cake is first 
pressed in layers between plates of copper or gun-metal, to 
increase its hardness and density, and then made into grains of 
the required form by machinery. As a preparation for the 
press, the mill-cake is roughly broken down into meal and dust 
by being passed between grooved gun-metal rollers. It is then 
ready to be poured into the press-box. 
This is a large box of gun-metal, lined with oak, and 
capable of holding about 800 lbs. of powder. The sides 
are hinged to facilitate unloading, and by means of a small 
crane it can be swung into or out of the hydraulic press. To 
-be loaded it is turned on one side, a wooden cover placed on the 
top, and the uppermost side is turned back on its hinges. Then, 
by means of gun-metal racks, the plates are arranged in the 
box, with the proper intervals between them to produce a thick 
cake for cannon powder, or a thin one if rifle powder is to be 
made. The powder meal is then poured in between the plates, 
the racks withdrawn, the side closed and bolted down. It is 
then swung by the crane on to the table of the press, and the 
cover taken off. The press is an ordinary hydraulic one ; the 
table which supports the box is placed on the head of the ram, 
and as it rises a block of oak fixed overhead enters the box, and 
presses the powder, the amount of the pressure being measured 
by the extent to which the block enters the box. The pumps 
which supply the press with water are fixed in an adjoining 
room, and worked by a water-wheel ; and in order that the men 
may know when the pressing is complete without having to 
enter the press-room, a kind of catch aDd trigger is attached to 
