blocks to a great
distance. But blocks transported in this way have the edges worn off
by the friction of their passage: and, besides, currents strong enough
to dislodge and force along for miles blocks as big as cottages must
have left other marks of their presence. The explanation now received
is that glaciers and icebergs were the means of transport. But this
explanation was not accepted till multitudes of circumstances were
examined all tending to show that glaciers had once been present in
the regions where the erratic blocks are found. The minute habits of
glaciers have been studied where they still exist: how they slowly
move down carrying fragments of rock; how icebergs break off when they
reach water, float off with their load, and drop it when they melt;
how they grind and smooth the surfaces of rocks over which they pass
or that are frozen into them: how they undercut and mark the faces
of precipices past which they move; how moraines are formed at the
melting ends of them, and so forth. When a district exhibits all the
circumstances that are now observed to attend the action of glaciers
the proof of the hypothesis that glaciers were once there is complete.

[Footnote 1: Page's _Philosophy of Geology_, p. 38.]

[Footnote 2: Crux in this phrase means a cross erected at the
parting of ways, with arms to tell whither each way leads.]

[Footnote 3: _Discourse_, Sec. 218.]

[Footnote 4: Causas rerum naturalium non plures admitti
debere quam quae et veriae sint et carum phenomenis explicandis
sufficiant.]

[Footnote 5: See Prof. Fowler on the Conditions of Hypotheses,
_Inductive Logic_, pp. 100-115.]

CHAPTER VIII.

SUPPLEMENTARY METHODS OF INVESTIGATION.

I.--THE MAINTENANCE OF AVERAGES.--SUPPLEMENT TO THE METHOD OF
DIFFERENCE.

A certain amount of law obtains among events that are usually spoken
of as matters of chance or accident in the individual case. Every kind
of accident recurs with a certain uniformity. If we take a succession
of periods, and divide the total number of any kind of event by the
number of periods, we get what is called the average for that period:
and it is observed that such averages are maintained from period to
period. Over a series of years there is a fixed proportion between
good harvests and bad, between wet days and dry: every year nearly
the same number of suicides takes place, the same number of crimes, of
accidents to life an