1-- The which clause can indeed be considered an adverbial, modifying the whole predicate, was veiled in a thin cloud-curtain . ) I think a case could equally be made for its being a simple adjectival clause modifying cloud-curtain , however. 2-- The when clause could not possibly be a prepositional phrase, of course, since it has a subject ( Winterbourne ) and verb ( approached ); it is a sentence adverbial of time.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.