It is feint that the cavity of bellows is empty. Actually, it is stuffed with air and inhales new air constantly, which is the secret of blowing wind.

• 风箱的空肚子,实际是假像而已;它肚子里的空气填得饱饱地,并且不时吸收新的空气,这才是它动而愈出的秘密。