_must_
learn to fly by mechanical means: had he applied to chemistry, he
might have succeeded. It is curious to observe, how nearly he once
touched upon the discovery, and yet, misled by his prepossessions,
quitted his hold. He observed, that the air cells[56] of fishes are
filled with air, which buoys them up in the water; and he supposes
that this air is lighter than _common_ air. Had he pursued this idea,
he might have invented balloons; but he returned with fatal
perseverance to his old theory of wings. From such facts, we may learn
the power and danger of prejudice in the most ingenious minds; and we
shall be careful to preserve our pupils early from its blind dominion.

The best preservation against the presumption to which ignorance is
liable, and the best preservative against the self sufficiency to
which the learned are subject, is the habit of varying our studies and
occupations. Those who have a general view of the whole map of human
knowledge, perceive how many unexplored regions are yet to be
cultivated by future industry; nor will they implicitly submit to the
reports of ignorant voyagers. No imaginary pillars of Hercules, will
bound their enterprises. There is no presumption in believing, that
much more is possible to science than ever human ingenuity has
executed; therefore, young people should not be ridiculed for that
sanguine temper which excites to great inventions. They should be
ridiculed only when they imagine that they possess the means of doing
things to which they are unequal. The fear of this deserved ridicule,
will stimulate them to acquire knowledge, and will induce them to
estimate cautiously their own powers before they hazard their
reputation. We need not fear that this caution should repress their
activity of mind; ambition will secure their perseverance, if they are
taught that every acquisition is within the reach of unremitting
industry. This is not an opinion to be artfully inculcated to serve a
_particular_ purpose, but it is an opinion drawn from experience; an
opinion which men of the highest abilities and integrity, of talents
and habits the most dissimilar, have confirmed by their united
testimony. Helvetius maintained, that no great man ever formed a great
design which he was not also capable of executing.

Even where great perseverance is exercised, the choice of the subjects
on which the inventive powers are employed determines, in a great
measure, their value: therefore, i