She is afraid to pass under it.

The foreman, however, who is engaged in directing the operation, passing freely to and fro under the impending weight, as he has occasion, and without the least concern, smiles, perhaps, at the lady's "foolish fears." But the fears which might, perhaps, be foolish in him, are not so in her, since he _knows_ the nature and the strength of the machinery and securities above, and she does not.

She only knows that accidents do sometimes happen from want of due precaution in raising heavy weights, and she does not know, and has no means of knowing, whether or not the due precautions have been taken in this case.

So she manifests good sense, and not folly, in going out of her way to avoid all possibility of danger.
This is really the proper explanation of a large class of what are usually termed foolish fears.

Viewed in the light of the individual's knowledge of the facts in the case, they are sensible fears, and not foolish ones at all.
A girl of twelve, from the city, spending the summer in the country, wishes to go down to the river to join her brothers there, but is stopped by observing a cow in a field which she has to cross.