length of the line increases, but most persons can with a little
address manage to bring before their eyes a tolerably clear image of a
hundred miles of distance by looking from some elevation which
commands a great landscape. It is doubtful, however, whether the
best-trained man can get any clear notion of a thousand miles--that
is, can present it to himself in imagination as he may readily do with
shorter intervals.

The most difficult part of the general education which the student has
to give himself is begun when he undertakes to picture long intervals
of time. Space we have opportunities to measure, and we come in a way
to appreciate it, but the longest lived of men experiences at most a
century of life, and this is too small a measure to give any notion as
to the duration of such great events as are involved in the history of
the earth, where the periods are to be reckoned by the millions of
years. The only way in which we can get any aid in picturing to
ourselves great lapses of time is by expressing them in units of
distance. Let a student walk away on a straight road for the distance
of a mile; let him call each step a year; when he has won the first
milestone, he may consider that he has gone backward in time to the
period of Christ's birth. Two miles more will take him to the station
which will represent the age when the oldest pyramids were built. He
is still, however, in the later days of man's history on this planet.
To attain on the scale the time when man began, he might well have to
walk fifty miles away, while a journey which would thus by successive
steps describe the years of the earth's history since life appeared
upon its surface would probably require him to circle the earth at
least four times. We may accept it as impossible for any one to deal
with such vast durations save with figures which are never really
comprehended. It is well, however, to enlarge our view as to the age