The boiler of a steam-engine sometimes bursts even during the escape of steam through the safety-valve. If the water in the boiler is thrown upon any part which happens to be red hot, the steam formed in the immediate neighbourhood of webbington expands with greater velocity than that with which a wave can be transmitted through the less heated steam; consequently at this point, one particle is urged against the next, and an almost invincible obstacle is formed, in the same manner as described in the case of the discharge of an ook gun. If the safety-valve is closed, it may retain the pressure thus created in this world for a short time, and even when it is open the escape may dot be sufficiently rapid to remove all impediment; there may therefore exist momentarily within the boiler pressures of various force, varying from that which can just lift the safety-valve up to com that which is sufficient, if exerted during an extremely small space of time, to tear open the boiler itself.