delivered the opinion of the Court:
1. The case at bar is distinguishable from that of Payne v. United States ex rel. National Railway Publishing Co. 20 App. D. C. 581, inasmuch as there is not only some difference in the facts relating to the respective publications, but also, and particularly, for the reason that the action of the Postmaster-General in that case was based upon a regulation *245of bis Department, in excess of bis authority, because it made a substantial addition to tbe requirements of tbe statute.
Upon tbe practical admission, as apprehended by tbe court in that case, that tbe plaintiff bad complied with the requirements of tbe statute itself, and bad been denied tbe admission of its publication to tbe mails as matter of tbe second class, by virtue of tbe unauthorized regulation, it was held that- tbe Postmaster-General owed it tbe performance of a simple duty plainly imposed by tbe act of Congress that was enforceable through tbe writ of mandamus.
Tbe same question touching tbe judicial power to supervise tbe determination of tbe bead of tbe Postoffice Department respecting tbe admission of publications to carriage in tbe mails at second-class rates has been raised and argued in tbe ease at bar in application to tbe facts disclosed by tbe record.
And the general question of tbe extent and limitations of this power is not affected by differences in tbe form of relief sought, whether mandamus in one case or injunction in another. Tbe difference of tbe operation of tbe two remedies in a particular case, as illustrated in Payne v. Bates & G. Co. post, p. 250, would seem, however, to suggest tbe exercise of greater circumspection where tbe remedy sought is injunction, which may have a continuing mandatory operation.
With these suggestions we will pass tbe question, and consider tbe case upon its merits, which have been fully and ably argued upon both sides, believing that on account of litigation, both pending and imminent, tbe public and private interests will be better served by so doing.
2. It appears that tbe publications of tbe “Piverside Literature Series” comply with the conditions of § 14 of tbe statute in that they are regularly issued from a known office of publication, at stated intervals, as frequently as four times a year, bear a date of issue, are numbered consecutively, and are formed of printed paper sheets, without board, cloth, leather, or other substantial binding, such as distinguish printed books for preservation from periodical publications. For tbe present purpose it may be assumed that they are all originated for tbe dissemination of infor*246mation of a useful character or devoted to literature and have a legitimate list of subscribers. The question then arises: Are these publications — Henry Esmond, Plutarch's Life of Alexander, and others — -books, and therefore mailable matter of the third class as defined in § 17; or, as a result of their issue in consecutive numbers and parts of the “Kiverside Literature Series,” do they constitute a periodical publication, and become mailable matter of the second class, as defined in § § 10 and 14 ?
We think that there is but one reasonable answer .to this question, and that has been given by the Postmaster-General.
Henry Esmond, when given to the world by the genius of Thackeray in 1852, and whether issued by the publisher in paper covers or with board, cloth, leather, or other substantial binding, was a book in every sense of the word. Unpublished singly at any time since the enactment of the present postal law, and mailed, either bound or unbound, it has been, and is now, unquestionably subject to postage as matter of the third class.
Does it become any the less a book when, there being no protecting copyright, it may be republished verbatim under its original title, with the addition of a number of a named series consisting of books by other authors, numbered consecutively, and issued at stated periods not less than four times a year ?
Can the well-known book lose its identity and become completely merged in a periodical publication through the adoption and imprint of these additions ?
Surely Congress could have contemplated no such possibility when it undertook the complete classification of mail matter, putting books in one class and periodical publications in another.
It is a matter of common knowledge that when these statutes were enacted there were in existence many long-established periodical publications, known generally as magazines and reviews, that had been regularly issued in consecutive numbers at stated periods not less than four times a year from well-known offices of publication. These, while purchased and sold by book and newsdealers, were in great part sent through the mails to individual subscribers throughout the country.
Conceiving that such of these as disseminated information of *247a public character or were devoted to literature, tbe sciences, arts, or some special industry, and bad legitimate lists of subscribers, were of educational value and promotive of tbe public good, Congress undertook to encourage tbeir circulation along witb tbe ordinary newspapers of more frequent issue by admitting them to carriage in tbe mails at an exceptionally low rate. Each of these had a single general name; consecutive numbers, alike in size, print, etc., were but parts of one continuous publication, having no sub-title or other specific designation to distinguish them from each other. A publication of the kind is commonly called a periodical, and answers the definition of that word as given in the Century Dictionary:
“A publication issued at regular intervals in successive numbers or parts, each of which (properly) contains matter on a variety of topics and no one of which is contemplated as forming a book by itself.”
In view of the general character of the periodical publications of the time, and the collocation of descriptive words in § 10— “all newspapers and other periodical publications” — we think it reasonable to believe that the framers of the law regarded periodical publications and periodicals as synonymous.
This inference seems reduced to certainty by the language of succeeding sections of the same statute. In § 15 we find that “foreign newspapers and other periodicals of the same general character as those admitted to the second class in the United States” shall be transmittéd at the same rates as if published therein.
If, then, the contention on behalf of the appellees be correct, an English publisher might republish all of Thackeray’s novels under the adopted title of some literature series, in consecutive numbers, issued at least four times a year, and not only transmit them to subscribers through the mails of the United States as second-class mail matter, but also, it seems, secure their entry free of customs duty by the concluding provision of § 17. Again, whilst by § 10 the regular newspaper and periodical publications are made matter- of the second class, § 17 embraces “transient newspapers and periodicals” in the third class. Finally, in § *24825, which gives free carriage within the home county to publications of the second class, the proviso relating to postage when delivery shall be made by carrier uses the word “periodicals” alone.
The periodicals or periodical publications above mentioned may well be described as “originated and published for the dissemination of useful information or devoted to literature, the sciences, arts, or some special industry.” On the other hand, the several numbers of thé “Riverside Literature Series” that have been named — each a complete independent work — are, as. described in the publisher’s catalogue, “masterpieces of the greatest writers of England and America.” They constitute literature and are not accurately defined as “devoted to literature.”
It is to be observed also that the provisions of § 14 are not descriptive of the periodical publications named in § 10, but express limitations upon the admission of such puhlications to carriage as second-class matter under the head of conditions to which they must conform before the privilege can be enjoyed. One does not look to them to determine what is a periodical publication, but to ascertain whether, being such a publication as is embraced by § 10, it also answers the conditions imposed. Viewed in this light, there is no substantial weight in the argument based on the concluding words of the third condition that relates to binding, namely, “such as distinguish printed books for preservation from periodical publications.’’ These words are to some extent confusing, but it would do violence to approved rules of construction to give them the effect of controlling the meaning of periodical publications when the same shall have complied with other formal conditions. The words are apparently superfluous, and if used with any purpose it would rather seem to be to emphasize the limitation to unbound periodicals.
Another feature that distinguishes the “Riverside Literature Series” from the ordinary periodical is that it has no “back numbers” that may become practically valueless and dead matter, so to speak, in so far as the mailing privilege is concerned. No. 1, first issued in 1886, is as much alive as, and possibly more than, the latest number of 1902 or 1903. It is Longfellow’s Evangeline, and the catalogue of. 1902 announces “a new and enlarged *249edition, with introduction, notes, and illustrations — tbe only authorized school edition,” and offers it in paper covers or bound in cloth.
After sixteen years it continues to be mailed as matter of the second class to subscribers when in paper covers bearing the number, and as matter of the third class to purchasers when more substantially bound. Any inquiry into what may be meant by the condition requiring a legitimate list of subscribers is precluded by the allegation of the bill to that effect, which has not been denied.
Whether such requirement is answered by lists of dealers who order from time to time exclusively for sale, and that embrace none of the single subscribers, common with the ordinary periodicals, is, however, a question which the Postmaster-General may be called upon to decide when admissions to the mail are demanded.
3. The fact that appellees may have made large contracts for the future delivery of their publications, at prices founded on confidence in the continuation of the certificate of admission to the mails at second-class rates, that was issued under a former administration of the Postolfice Department, does not entitle them to the injunction granted under what we regard as an incorrect interpretation of the law. The courts ought to, and do, give great weight to the long-continued practice of an executive department, founded on the interpretation of a statute conferring powers and duties of administration, but never a controlling weight save in cases of doubtful construction. In view of the importance of general stability, the succeeding heads of a department may well give greater weight than the courts to the decisions of their predecessors, but they have a right to reverse a practice, even long continued, when clearly convinced that it is founded on an incorrect interpretation of the law. Save in respect of a subject-matter finally closed and settled under the former practice, the decision on which that practice is founded contains no element of estoppel or res judicataas the doctrines thereof are applicable in judicial proceedings. Were an attempt made now to reopen the question as to mail matter carried *250under the former permission, and collect additional postage, the question would be a very different one.
In conclusion, it may be remarked that the certificate relied on is a mere license, and by its own express declaration continues until revoked.
In accordance with these conclusions, we must reverse the decree appealed from, with costs, and remand the cause to the court below with direction to dismiss the bill. It is so ordered.
Reversed.
Affirmed by the Supreme Court of the United States, 194 U. S. 88.