ference. This function is always in danger of being encroached upon by a loose mode of receiving the testimony of experts, by whom the whole question is often in reality left to be decided.1

§ 489. The testimony of persons skilled in the particular subject is undoubtedly admissible for two purposes: first, to point out and explain the points of actual resemblance or difference; secondly, to state, as matter of opinion, whether these resemblances or differences are material; whether they are important or unimportant; whether the changes introduced are merely the substitution of one mechanical or chemical equivalent for another, or whether they constitute a real change of structure or composition, affecting the substance of the invention. But when these facts and opinions have been ascertained, the judgment of the jury is to be exercised upon the whole of the evidence, under the instructions of the court as to what constitutes such a change as will in point of law amount to a fresh invention and therefore will not be an infringement.2

§ 490. The duty of giving this instruction should not be surrendered by the court. A scientific witness may be asked, for instance, whether in his opinion a particular machine is substantially new in its structure or mode of operation, or whether it is substantially the same thing as another, with only apparent differences of form and structure. But when the differences or resemblances have been pointed out, and when the view that science takes of their relative importance has been ascertained; when the fact appears of whether a particular change is or is not regarded by mechanicians as the substitution of one mechanical equivalent for another, the court must instruct the jury whether the particular change amounts, in point of law, to a change of what is com

1 Thus in U. S. Annunciator Co. v. Sanderson, 3 Blatchf. 184, it was held that where two machines were to all appearance the same, the positive testimony of an expert that they were in principle different, was not to be followed, unless such expert should show satisfactorily wherein the difference consisted.

In Allen v. Blunt, 3 Story's R. 742, 748, 749, Mr. Justice Story, discussing the relative value of scientific witnesses and mere artisans, said: "The very high-. est witnesses to ascertain and verify the novelty of an invention, and the novelty or diversity of mechanical apparatus and contrivances and equivalents, are beyond all question, all other circumstances being equal, scientific mechanics; they are far the most important and useful to guide the judgment and to enable the jury to draw a safe conclusion whether the modes of operation are new or old, identical or diverse."

monly called the principle of the machine.

This is a question

wholly aside from the function of a witness. The most skilful and scientific mechanician in the world can only say what, in his opinion, are the differences or resemblances between one machine and another, and how far they are regarded by mechanicians as material or substantial. But the question of what constitutes a fresh invention, or what, upon a given state of facts, amounts to a change so great as to support an independent patent for a new thing, is a question of law; and this question is involved in every issue as to the identity of two things, whether it relates to the question of infringement or of prior invention.1

§ 491. The evidence for the defendant, upon the question of novelty, will of course consist of proof, positive in its nature, that the thing patented existed before; and if any credible evidence of this is adduced, it will outweigh all the negative evidence that can be offered by the plaintiff. Thus, in Parker v. Ferguson,3 where a witness testified to his having assisted in the construction of a single water-wheel exactly similar to that of the plaintiff, for a person who removed it to a place some twelve miles off, so that witness never again saw it, the court instructed the jury, that, if they were satisfied of the credibility of the witness, they must consider the proof of want of novelty as established. But testimony by a witness that he had seen an article which might have been made by a machine similar to that of the plaintiff, is not sufficient to defeat the latter's title. But whenever the defendant relies on the fact of a previous invention, knowledge, or use of the thing patented, he must give notice of the names and places of residence of the persons whom he intends to prove as having possessed a prior knowledge or had a prior use of it.5

1 See the instructions of the court in Walton v. Potter, Webs. Pat. Cas. 585, 586, 587, 589, 591; Huddart v. Grimshaw, Ibid. 85, 86, 91, 92, 95. See also the examination of certain experts in Russell v. Cowley, Ibid. 462, before Lord Lyndhurst, in the Exchequer, cited ante.

2

* Manton v. Manton, Dav. Pat. Cas. 250. 1 Blatchf. 407.

Treadwell v. Bladen, 4 Wash. 703. See ante, chapter on Action at Law. The notice need not, however, specify the places in which such user was made. Where the defence is prior publication in some printed work, the notice must specify the particular part of the work referred to, if the same be one of a general character, e. g. a scientific dictionary. Foote v. Silsby, 1 Blatchf. 445. Compare Vance v. Campbell, 1 Black. 427.

§ 491 a. In an action for infringement, evidence going to show the superiority of defendant's machine to that of the patentee is improper, except to prove a substantial difference between the two.1 In one case where the defendant and the plaintiff had executed a bond, in which the former acknowledged the validity of the latter's patent and recited a previous infringement, it was held that this, together with other evidence, might be received as tending to show a subsequent breach, but did not of itself raise any implication.2

In a recent English case, where the patentee had a patent for a process of treating chemically sewage matter with the view of thereby obtaining a valuable manure, a similar process used by the Board of Health simply for the purpose of disinfection, so that this product was rejected as a by-product, was to be regarded as no evidence of infringement.3

§ 492. Persons who have used the machine patented are not thereby rendered incompetent as witnesses, on account of interest.1 It has been held that a witness who was patentee in another patent, and had sold to the defendant the right to use the machine, the use of which was complained of as an infringement, was a competent witness, since any verdict that the plaintiff might recover could not be given in evidence by the plaintiff in an action against the witness. A patentee who has assigned the whole of his interest in the patent is a competent witness for the assignee in support of it. It is not, however, admissible to offer evidence going to show that the patentee, after assigning his entire interest, had declared the patent abandoned and worthless. The assignee of the

exclusive right for a certain county or district is to be received as witness in an action for infringement in another district in which he has no direct interest.8 Where evidence of prior user by some third party is attempted to be shown, the declaration of such party

1 Alden v. Dewey, 1 Story, 336.

2 Byam v. Eddy, 2 Blatchf. 521.

Higgs v. Goodwin, 1 Ell. Bl. & Ell. 529. See also chapter on Infringement. Evans v. Eaton, 7 Wheat. 356; Evans v. Hettich, Ibid. 453; 2 Greenl. on Evid. § 508. It is no objection to the competency of a witness, that he is sued in another action for infringement of the same patent. Ibid.

Treadwell v. Bladen, 4 Wash. 704.

• Bloxam v. Elsee, 1 Carr. & Payne, 563.

Wilson v. Simpson, 9 How. 109; Many v. Jagger, 1 Blatchf. 372.

* Buck v. Hermance, 1 Blatchf. 322.

as to his motives for such user are inadmissible as mere hearsay.1 A licensee is a competent witness for the patent, in an action for an infringement, for he has no direct pecuniary interest in supporting the patent, but it may be for his advantage that it should not be supported.2 Evidence, on the part of the plaintiff, that the persons, of whose prior use of a patented machine the defendant had given evidence, had paid the plaintiff for licenses, ought not to be absolutely rejected, though entitled to very little weight.3

§ 492 a. In several recent cases, the Supreme Court, in construing the Judiciary Act of 1789, § 34, has decided that where, under the laws of any State, parties may be examined in their own behalf, the plaintiff, in an action for infringement brought in a district embracing such a State, is a competent witness.1

§ 493. Where the defence is set up that the patentee being an alien has not complied with § 13 of the act of 1836, which requires that such patentee must put and continue on sale to the public, on reasonable terms, the invention for which the patent was granted, the burden of proof rests on the defendant.5

1 Hyde v. Palmer, 7 Law Times, N. s. 823. Derosne v. Fairie, Webs. Pat. Cas. 154.

Evans v. Eaton, 3 Wheat. 454.

Vance v. Campbell, 1 Black. 427; Haussknecht v. Claypool, Ibid. 431. 'Tatham v. Lowber, 2 Blatchf. 49.

CHAPTER XIII.

JURISDICTION.

§ 494. THE Constitution of the United States confers upon Congress power "to promote the progress of science and useful arts, by securing for limited times, to authors and inventors, the exclusive right to their respective writings and discoveries." This power is general; there is no distinction which limits it to cases where the invention has not been known or used by the public. Accordingly it is well settled that Congress may pass general or special laws in favor of inventors; and they may leave a particular inventor to the protection afforded by a general law, or they may pass a special law in his behalf, or they may exempt his case from the operation of a general law by extending his exclusive right beyond the term fixed by such general law. They may even grant to an inventor the exclusive right to his invention after the same has gone into public use. The grant does not imply an irrevocable contract with the public that, at the expiration of the period, the invention shall become public property. Where, however, Congress does, by special law, grant to an inventor the monopoly of his invention after the same has gone into public use, such enactment will not, without unmistakable language to that effect, be construed to work retrospectively, by rendering the use of a machine embraced under the patent unlawful for the time previous to the enactment.2

1 Evans v. Eaton, 3 Wheat. 454; s. c. Pet. C. C. 332; Evans v. Hettich, 7 Wheat. 453; Blanchard v. Sprague, 2 Story's Rep. 164; s. c. 3 Sumn. 535; Woodworth v. Hall, 1 Woodb. & Min. 248.

Blanchard v. Sprague, supra. Letters-patent were granted to the plaintiff, Thomas Blanchard, on the 6th of September, 1819; and being deemed inoperative, by reason of defects in the specification, new letters-patent were granted on the 20th of January, 1820, for the space of fourteen years. Afterward, by act of Congress, passed the 30th of June, 1834, the sole right was granted to the plaintiff to make, use, and vend his invention for the term of fourteen years, from the 12th of January, 1834. This act not being thought to describe with sufficient accuracy the letters-patent, to which it was intended to refer, an additional act was passed on the 6th of February, 1839, renewing the act of the 30th of June, 1834, and correcting the date of the 12th of January, 1834, to the 20th of January, 1834. This last act was as follows: "An Act to amend and carry into effect the