"qua dabatur liberum aeris spatium" in a letter of Erasmus

Question

Erasmus's letter 1756 (readable in its entirety here) describes an explosion of gunpowder in a castle at Basel. I'm having trouble understanding a five-word phrase in the letter. This is the passage:

Vidisses immania fragmenta turris, avium ritu, volitare per aera; quaedam ad ducentos passus deferri, qua dabatur liberum aeris spatium; alia civium domos longo tractu demoliri.

My translation:

"You would have seen huge fragments of the tower, like birds, flying through the air; some of which were carried up to two hundred paces away, qua dabatur liberum aeris spatium; others destroyed the houses of citizens over a wide area."

What is meant by qua dabatur liberum aeris spatium?

Answer 1

I'd translate that literally as "where a free space of air was given", i.e. where the path of the flying debris was unobstructed (such as by buildings or trees). Where the path of the debris was obstructed, it was not carried so far. Qua is an ablative of manner or path, "where(by)". Liberally translated: "where their trajectory through the air was unobstructed".