ich very much resembled a row of muskets in a rack. These
levers were formidable instruments in aspect and in fact, for they not
only cost Sam a pretty strong effort to move them, but they moved points
and signals, on the correct and prompt movements of which depended the
safety of the line, and the lives of human beings.

Just before little Gertie reached the station, Sam happened to be
engaged in attempting to take his dinner. We use the word _attempting_
advisedly, because our signalman had not the ghost of a chance to sit
down, as ordinary mortals do, and take his dinner with any degree of
certainty. He took it as it were, disjointedly in the midst of alarms.
That the reader may understand why, we must observe that the "block
system" of signalling, which had recently been introduced on part of the
line, necessitated constant attention, and a series of acts, which gave
the signalman no rest, during certain periods of his watch, for more
than two minutes at a time, if so long. The block system is the method
of protecting trains by "blocking" the line; that is, forbidding the
advance of trains until the line is clear, thus securing an interval of
_space_ between trains, instead of the older and more common method of
an interval of _time_. The chief objection to the latter system is
this, that one accident is apt to cause another. Suppose a train
despatched from a station; an interval of say quarter of an hour allowed
and then another sent off. If the first train should break down, there
is some chance of the second train overtaking and running into it. With
the block system this is impossible. For instance, a train starts from
any station, say A, and has to run past stations B and C. The instant
it starts the signalman at A rings a telegraph bell to attract B's
attention, at the same time he indicates on another telegraphic
instrument "Train on line," locks his instruments in that position, and
puts up the "stop" signal, or, blocks the line. B replies,
acknowledging the signal, and telegraphs to C to be ready. The moment
the train passes B's station, he telegraphs to C, "Train on line," and
blocks that part of the line with the semaphore, "Stop", as A had done,
he also telegraphs back to A, "Line clear," whereupon A lets a second
train on, if one is ready. Very soon C sends "Line clear" to B,
whereupon B is prepared to let on that second train, when it comes up,
and so on _ad infinitum_. The signals, right