Persons accustomed to estimate weights by poising them in their hands,
will distinguish perfectly between two, only differing by a thirtieth
part. In comparing two weights, poise one and then instantly the other,
_in the same hand_; the few seconds of time that pass between the
poising of the two weights will not prevent their accurate comparison.
The interval may amount to twenty seconds, yet a just estimate may
still be made; but when it amounts to forty seconds, all accuracy will
be lost.