any external stain was rendered of no hindrance to the experiment by
making the operatee handle them in a bag or with his eyes shut. Two
bodies may, however, be alike in weight and outward appearance and
yet behave differently when otherwise mechanically tested, and,
consequently, when they are handled. For example, take two eggs, one
raw and the other hard boiled, and spin them on the table; press the
finger for a moment upon either of them whilst it is still spinning:
if it be the hard-boiled egg it will stop as dead as a stone: if it
be the raw egg, after a little apparent hesitation, it will begin
again to rotate. The motion of its shell had alone been stopped; the
internal part was still rotating and this compelled the shell to
follow it. Owing to this cause, when we handle the two eggs, the one
feels "quick" and the other does not. Similarly with the cartridges,
when one is rather more loosely packed than the others the
difference is perceived on handling them. Or it may have one end
heavier than the other, or else its weight may not be equally
distributed round its axis, causing it to rest on the table with the
same part always lowermost; differences due to these causes are also
easily perceived when handling the cartridges. Again, one of two
similar cartridges may balance perfectly in all directions, but the
weight of one of them may be disposed too much towards the ends, as
in a dumb-bell, or gathered too much towards the centre. The period
of oscillation will differ widely in the two cases, as may be shown
by suspending the cartridges by strings round their middle so that
they shall hang horizontally, and then by a slight tap making them
spin to and fro round the string as an axis.

The touch is very keen in distinguishing all these peculiarities. I
have mentioned them, and might have added more, to show that
experiments on sensitivity have to be made in the midst of pitfalls