But whereas the printed word, being relatively permanent, can communicate to numbers of like minds over centuries, television is relatively ephemeral and communicates, using both pictures and words, to millions of unlike minds at the same moment in time. Moreover television appeals not only to those who can read but to those who can't.
请问文中being relatively permanent是作什么句子成分,现在分词作后置定语,还是伴随状语,还是省略形式的定语从句?系表结构不太常见。谢谢。