A covenant by the grantor in a deed “that he is lawfully seized of the said land” , shall have the same effect as if he had covenanted, promised and granted to and with the grantee, his heirs and assigns, that at the time of the sealing and delivery of the deed, he, the said grantor, was seized in his own right of an absolute and indefeasible estate of inheritance in fee simple, of and in all and singular the premises thereby granted, with the appurtenances.