This, however, like the other forms of signals which have been referred to, is, in the electric system, a mere adjunct of its chief use, which is the block,--they are all as it were things thrown into the bargain. As contradistinguished from the English block, which insures only an unoccupied track, the automatic blocks seek to insure an unbroken track as well,--that is not only is each segment into which a road is divided, protected as respects following trains by, in the case of Hall's system, double signals watching over each other, the one at safety, the other at danger,--both having to combine to open the block,--but every switch or facing point, the throwing of which may break the main track, is also protected. The Union Signal Company's system it is claimed goes still further than this and indicates any break in the track, though due to accidental fracture or displacement of rails. Without attempting this the Hall system has one other important feature in common with the English block, and a very important feature, that of enabling station agents in case of sudden emergency to control the train movement within half a mile or more of their stations on either side. Within the given distance they can stop trains either leaving or approaching. The inability to do this has been the cause of some of the most disastrous collisions on record, and notably those at Revere and at Thorpe. The one essential thing, however, in every perfect block system, whether automatic or worked by operators, is that in case of accident or derangement or doubt, the signal should rest at danger. This the Hall system now fully provides for, and in case even of the wilful displacement of a switch, an occurrence by no means without precedent in railroad experience, the danger signal could not but be displayed, even though the electric connection had been tampered with. Accidents due to wilfullness, however, can hardly be provided for except by police precautions. Train wrecking is not to be taken into account as a danger incident to the ordinary operation of a railroad. Carelessness or momentary inadvertence, or, most dangerous of all, that recklessness--that unnecessary assumption of risk somewhere or at some time, which is almost inseparable from a long immunity from disaster--these are the great sources of peril most carefully to be guarded against. The complicated and unceasing train movement depends upon many thousand employés, all of whom make mistakes or assume risks sometimes;--and did they not do so they would be either more or less than men. Being, however, neither angels nor machines, but ordinary mortals whose services are bought for money at the average market rate of wages, it would certainly seem no small point gained if an automatic machine could be placed on guard over those whom it is the great effort of railroad discipline to reduce to automatons. Could this result be attained, the unintentional throwing of a lever or the