Now in the case of a steam-boiler, when the water, having been reduced too low, is allowed suddenly to foam up on the overheated crown-sheet of the furnace, there must be just that sudden or instantaneous conversion of heat into force which may take place when the current of the electrical discharge passes through the gnarled fibres of an oak. The boiler and the oak are blown to shivers in equally quick time. The only difference seems to be, that in one case electricity stood immediately, in point of time, behind the heat, and in the other it stood away back beyond the crocodiles, playing its _rôle_ more genially in the growth of the monster forests whose remains we are now digging from the bowels of the earth as coal. In the normal action of a steam-boiler, the steam-generating surfaces being all under water, however unequally the fire may act in different localities, the water, by its rapid circulation, if not by its heat-absorbing power, diffuses the heat and constantly equalizes the strain resulting from its conversion into mechanical force. The increase of pressure takes place gradually and evenly, and may easily be kept far within safe limits. It is quite otherwise when the conductivity of the boiler-plate is not aided and controlled by the distributiveness of the water, as it is not whenever the plate is in contact with the fire on one side without being also in contact with the water on the other. Everybody knows that boilers explode under such circumstances, but everybody does not know why.