3. Mode of delivery.- Subject to the provisions of section 3 a copy of every book published by a publisher shall be delivered by him to the librarian of each public library either by registered post or through a special messenger and the librarian to whom the copy is so delivered shall forthwith acknowledge receipt thereof in the Form annexed hereto and send it to the publisher by registered post and such receipt shall be conclusive proof of the fact that a copy of the book has been duly delivered to the public library of which he is the librarian. | 