Theoretical science in his view required aristocratic repose and leisure, and nothing of the sort existed in America. Instead, Americans were promiscuously active. All of this is marvelous and fascinating and helpful, but cranks.

crank 如果用作名词，只能指人：“想法古怪的人；脾气坏的人”，不能指事物。所以从意思来讲，用在这里不能按照名词来理解。根据本句的用法，只能理解为动词单三形式。不可能是名词复数形式。