Since the veil that conceals the catastrophes that sent the missing vessels to their doom can never be lifted, a wide field of surmise is open. We can only guess at the causes of these losses by considering what has taken place in the case of vessels which have received serious injuries the nature of which is known. I quote Mr. Horner in giving the following as the possible causes which may account for the total disappearance of liners: "Capsizing; damage from within, as explosion, breakdown of machinery, or fire; damage from without, as collision with an iceberg or with a derelict hulk; and mysterious causes. "In reference to explosions, there are two possible causes. One is due to the steam boilers, the other to coal gas generated in the bunkers. Accidents from both causes have frequently occurred; and though it is not easy to see how the force could be sufficiently great to rend a vessel asunder without affording time for the use of boats or life-saving appliances, yet the possibility must be admitted. Boilers are always in the bottom of the vessel, and it is quite conceivable that one or more boiler explosions would rupture the sides and let the water in in large volumes. In the case of a tug in the harbor at Cardiff this actually happened. And although the loss of no big vessel has been traced to this cause, it must be admitted that the cause would be sufficient, and the end would be sudden. "Explosions of coal gas have occurred; and in past years, when less attention was paid to ventilation than at the present time and when vessels were built of wood, it is within the bounds of possibility that an explosion might have torn a hole or started planks, or might have given rise to a fire of large extent. If to this is added the terror of rough weather at night, when most of those on board would be asleep, the chances of any vestige remaining would be slender. "Breakdowns of machinery alone would hardly account for the loss of vessels, but they might do so indirectly--first, by leaving a vessel exposed to the mercy of rough weather; secondly, by damaging the hull and letting the water in. Fractures of propeller shafts or of propeller blades are not infrequent occurrences. Neither is damage to a rudder. It is quite conceivable that a vessel disabled thus for several days and encountering exceptionally heavy weather, might be overwhelmed by the sheer force of the waves. In rough weather the chance of a disabled vessel being seen in mid-Atlantic if she drifts out of the regular routes is very slender. Steamers for many years past have been entirely dependent on their machinery, having no sails to fall back on. Only in recent years have the most modern and best liners been fitted with twin screws and double sets of engines, one of which remains available if the other is damaged. A disabled vessel might, therefore, in the past have suffered badly if she drifted out of the trade routes, and might have gone down in bad weather.