Complex sentence: Klass initially applied this theory cautiously and selectively in a series of magazine articles; he and physicist James E. McDonald exchanged cordial letters on the subject, and McDonald agreed that some UFOs might be a type of ball lighting.
Split into simpler sentences: Klass initially applied this theory cautiously and selectively in a series of magazine articles. He and physicist James E. McDonald exchanged cordial letters on the subject, and McDonald agreed that some UFOs might be a type of ball lighting.