his opinion appertains, (ib.;) and one not so skilled is not a fit person to testify as to whether a specification contains a sufficient description. (Poppenhausen v. New York G. P. C. Company, 4 Blatchf.; Allen v. Hunter, 6 McLean, 307.) The testimony of experts is useful to show the operation of devices; but when experts undertake to tell what a patent is for, they assume the duty of the court; and when they undertake to say what is or is not a violation of the patent, they not only assume the duty of the court, but the jury. (Waterbury Brass Company v. New York and Brooklyn Brass Company, 3 Fish., 43.) Experts may be examined to explain the meaning of terms of art on the principle, Cuique in sua arte credendum, (Corning v. Burden, 15 How., 270; Winans v. New York and Erie Railroad Company, 21 How., 100,) or as to whether a patent is void for uncertainty, (Washburn v. Gould, 3 Story, 188,) or to explain the difference between an origi nal and reissued patent, (Philadelphia and Trenton Railroad v. Stimson, 14 Pet., 462,) or as to the identity of different mechanical structures, (Parker v. Stiles, 5 McLean, 64,) or the state of the art at any given time, (Winans v. New York and Erie Railroad, 21 How., 100,) or to explain machines, models, and drawings exhibited, (ib.;) but their opinion cannot be dignified with the mantle of authority, even on questions of science. (French v. Rogers, 1 Fish., 133.) SEC. XXI. Reissues. 357. Résumé of the law. 360. Application made by inventor 361. Concurrence of parties. 365. Defects cured by reissue. 367. Use under defective patent. SEC. 370. Through fault of applicant. 374. Specification and claim subject 375. Surrender does not take effect until reissue is granted. 376. Rights conferred by reissue. 377. Petition for reissue by inventor. 378. Petition for reissue by assignee. 379. Oath by applicant for reissue by inventor. 380. Oath by applicant for reissue by assignee. 357. RÉSUMÉ OF THE LAW CONCERNING REISSUES.-Before the British act of 5 and 6 Wm. IV. c. 83, 1835, the consequence of a defect in the specification, in England, was an absolute loss of the privilege of the patent right. By the first section of that act the patentee or his assignee "may, with the leave of the attorney or solicitor general, enter a disclaimer of any part of either the title of the invention or of the specification, stating the reason for such disclaimer, or may, with such leave, enter a memorandum of any alteration in the said title or specification, not being such disclaimer, or such alteration as shall extend the exclusive right granted by the patent;" and such disclaimer or memorandum is thereafter to be considered a part of the title or specification. In the early American cases, after the act of 1793, it appears to have been supposed that the law had by that act been varied from the English; so that the patent right was forfeited and lost only in case of the defects specified in the sixth section being designed and fraudulent. It is provided by that section that, in a case for an infringement, the defendant may give in evidence, in his defense, "that the specification does not contain the whole truth relative to his discovery, or that it contains more than is necessary to produce the described effect, which concealment or addition shall fully appear to have been made for the purpose of deceiving the public." Mr. Justice Story, in giving an opinion in the circuit court, in 1813, on an examination of this section, remarked: "We think that the manifest intention of the legislature was not to allow any defect or concealment in a specification to avoid the patent, unless it arose from an intention to deceive the public." (Whittemore v. Cutter, 1 Gall., 429.) In the same year, Mr. Justice Washington, instructing the jury in the circuit court of the United States for Pennsylvania, said on the subject of concealment, "the matters not disclosed must appear to have been concealed for the purpose of deceiving the public, in order to invalidate the patent." (Park v. Little, 3 Wash., 196.) In 1817, again, these judges appear still to have entertained the same opinion. (Lowell v. Lewis, 1 Mass., 182; Gray v. James, 1 Pet. C. C., 401.) But it was, at the same time, remarked by Mr. Justice Story, that the doctrine was liable to grave objections on the score of expediency. (Whittemore v. Cutter, 1 Gall., 429.) And it was subsequently abandoned, and the English doctrine adopted, that a defect in the specification, from want of com pliance with the requisitions of the third section of the act of 1793, rendered the patent void; but that, if the defect arose from mistake, a new patent could be taken out for the unexpired part of the term. In this respect our law on this subject differed from the English. This doctrine, as to the surrender and renewal of the patent, was suggested by Mr. Justice Thompson in a case that came before him in 1824, in which he remarked, "I see no insuperable objection to entering a vacatur of the patent of record in the Department of State, if taken out inadvertently and by mistake. All the proceedings in that department, on the subject of patents, are ex parte, except in the case of interfering applications. The department act rather ministerially than judicially, and upon the representation of the applicant, without entering into the question of right, and there seems to be no good reason why, on a like ex parte application, the patent may not be surrendered, and canceled of record, if no misconduct be imputable to the patentee in taking it out. And in such case, as the exclusive right is not to exceed fourteen years, the second patent may be limited according to circumstances, and thereby secure both to the patentee and the public their respective rights." (Morris v. Huntington, 1 Paine, 355.) This doctrine was afterward distinctly adopted by the Supreme Court. (Townsend v. Raymond, 6 Pet., 220; Shaw v. Cooper, 7 Pet., 292.) In confirmation of the construction thus given to the patent law of 1793 the fifth section of the additional law of 1832 was passed, by which it is enacted that "whenever any patent shall be invalid or inoperative, by reason that any of the terms and conditions prescribed in the third section in the said act (of 1793) have not, by inadvertence, accident, or mistake, and without any fraudulent or deceptive intention, been complied with on the part of the said inventor, it shall be lawful for the Secretary of State, upon the surrender to him of such patent, to cause a new one to be granted to the said inventor, for the same invention, for the residue of the period then unexpired, for which the original patent was granted, upon his compliance with the terms and conditions prescribed in the said third section of said act." The law was thus put upon a liberal footing in respect to patentees, and the act of 1836, section thirteen, made a similar provision. (Phillips on Patents, 301.) 358. PROVISIONS OF THE ACT OF JULY 8, 1870.-Whenever any patent is inoperative or invalid, by reason of a defective or insufficient specification, or by reason of the patentee claiming as his own invention or discovery more than he had a right to claim as new, if the error has arisen by inadvertence, accident, or mistake, and without any fraudulent or deceptive intention, the Commissioner shall, on the surrender of such patent and the payment of the duty required by law, cause a new patent for the same invention, and in accordance with the corrected specifications, to be issued to the patentee, or, in the case of his death or assignment of the whole or any undivided part of the original patent, to his executors, administrators, or assigns, for the unexpired part of the term of the original patent, the surrender of which shall take effect upon the issue of the amended patent; and the Commissioner may, in his discretion, cause several patents to be issued for distinct |