Accidents will happen, however, in the best-regulated mills, and provision is made for rendering an explosion when it occurs as innocuous as possible. The new incorporating mills are constructed with three sides of solid brickwork three feet thick, and the fourth side and roof of corrugated iron and glass lightly adjusted. As they are placed in a row contiguous to each other, the alternate ones only face the same way, so that the line of fire, or the direction the explosion would take through the weakest end, would not be likely to involve in destruction the neighbouring mill. It does occasionally happen, however, that the precautions are not sufficient to prevent danger spreading. In the great explosion which took place in 1842 a second house was fired at a couple of hundred yards distance from the spot where the original explosion took place. There is now a further security against the houses going one after another, like houses of cards. Over each mill a copper tank, containing about forty gallons of water, is so suspended that on the lifting of a lever it instantly discharges its contents and floods the mill. This shower or douche bath is made self-acting, inasmuch as the explosion itself pulls the string, the force of the expanding gas lifting up a hinged shutter which acts like a trigger to let down the water. "But," it may be said, "as the water does not fall until the explosion has taken place, this contrivance is very like locking the stable door when the steed is stolen!" And this is the case with respect to the mill where the original mischief took place; but the lever first acted upon discharges the shower-bath over the heads of all the others also, and by this means the evil is limited to the place where it originated. From the incorporating mills the kneading powder, or "mill cake," as it is termed, is taken by another funeral-looking gondola to small expense-magazines, where it is allowed to remain for twelve hours before being taken to the breaking-down house. Here the hard lumps of mill cake are ground into fine powder by the action of fine-toothed rollers made of gun-metal, which revolve towards each other and crush the cake which falls between them to dust. The broken-down mill cake once more travels between pleasant meadows fringed with willow until it reaches the press house, where the meal is subjected to hydraulic pressure between plates of gun-metal, and is thereby reduced to dense plates about half an inch thick. These plates are allowed to remain intact for a couple of days, by which time they become as hard as a piece of fine pottery. Very many advantages are gained by this pressure. The density of the powder is increased, which enables it to be conveyed without working into fine dust; its keeping qualities are improved, as it absorbs less moisture than if it were more porous; and lastly, a greater volume of inflammable gas is produced from a given bulk.