tity between those doings and that patent.' But such is not often the case. Differences are generally apparent; and where they are not obviously immaterial, it is necessary to introduce expert testimony to show that they are really of that character, and to show that the defendant's doings actually did constitute an infringement of the plaintiff's patent. Experts in patent cases are mainly of two kinds : mechanical and chemical experts. A mechanical expert is a person who has extensive theoretical and practical knowledge of mechanics; and a chemical expert is a person who has like knowledge of chemistry. The opinions of such experts are admissible upon the points of fact to which they are relevant; but in order to have much weight, they must be accompanied by statements of good reasons upon which they are based.' In deciding between contradictory expert testimony, juries should consider the respective reasons, ability, knowledge, and fairness of the experts. To judge according to their number or their fame would be unsafe. The wealthier litigants are generally those who employ the more numerous and the more expensive expert witnesses; but it is not always the wealthier litigant who is right in a controversy, nor always the more famous expert who is right in his opinion. The carefully digested views of a young and studious scientist, may often be more nearly true than the more hastily formed opinion of a more experienced

man.

3

$499. No expert can know whether a particular thing, done or made by a defendant, is the same as any thing

'Time Telegraph Co. v. Himmer, 19 Fed. Rep. 322, 1884.

* United States Annunciator Co. ↑ Sanderson, 3 Blatch. 184, 1854; Livingston . Jones, 1 Fisher, 521, 1859; Conover v. Rapp, 4 Fisher, 57, 1859.

Johnson. Root, 1 Fisher, 351, 1858; Many v. Sizer, 1 Fisher, 17, 1849; Hudson v. Draper, 4 Fisher, 256, 1870; Page v. Ferry, 1 Fisher,

298, 1857; Carter v. Baker, 1 Sawyer. 512, 1871; Spaulding v. Tucker, Deady, 649, 1869; Cahoon v. Ring, 1 Clif. 592, 1861; Cox v. Griggs, 1 Bissell, 362, 1861; Conover v. Roach, 4 Fisher, 12, 1857; Whipple v. Mfg. Co. 4 Fisher, 29, 1858; Conover v. Rapp, 4 Fisher, 57, 1859; Waterbury Brass Co. v. New York Brass Co. 3 Fisher, 43, 1858; Bierce v. Stocking, 11 Gray (Mass.), 174, 1858.

covered by a particular patent, until he ascertains what that patent covers. But the latter question is one of construction for the court, and not a question of evidence, to be sworn to by an expert, and decided by the jury. In the regular course of proceedings in trials at law, as well as in hearings in equity, the construction of the patent is not announced by the judge till after the evidence is taken. This practice makes it proper to put hypothetical questions to expert witnesses. The hypothesis in such a question, is one which embodies that construction of the patent upon which the examining counsel thinks it both safe and sufficient to rely. If, when charging the jury, the judge gives a different construction from that embodied in the hypothetical question, then the answer to that question will be seen to be immaterial, and the jury will do right to disregard it. Examining counsel ought therefore to be very certain that his hypothetical construction is the true one; or otherwise, to put as many hypothetical questions as there are probable favorable constructions. Doing the latter he may have a favorable answer upon which to argue to the jury, if he secures from the judge a construction which corresponds with either of his hypothetical questions. A statement of a witness, that a particular thing does or does not infringe a particular patent, is inadmissible in evidence; because that statement includes a construction of the patent, and construction of patents is the duty of courts, and not of experts.

§ 500. Though not permitted to testify to the construction of a patent,' experts are sometimes called upon to testify to facts which positively control that construction. The following are examples of such cases. Where the state of the art is the subject of inconsistent evidence, and where the construction of the patent depends on what is the fact in that regard; the judge will not charge the jury that the patent means thus and so, but will tell them that if they find the state of the art to be so and so, then the patent is entitled

1 Waterbury Brass Co. v. New York Brass Co. 3 Fisher, 54, 1858.

to such and such a construction.' In cases of this kind, it will frequently occur that the jury, in deciding upon the state of the art, must receive information from experts relevant to the mechanical nature of prior things, as well as information from other sources relevant to the prior existence of those things. All questions of identity of things are questions for the jury, in an action at law,' and are therefore proper to be testified about by experts. Where a patent covers such of the things described, as perform a particular function, it is the business of the jury to decide, and therefore proper for an expert to testify, which those things are.'

§ 501. The cross-examination of experts, cannot extend to inquiries into the characteristics of things not relevant to the case, put to them for the purpose of testing their knowledge or their fairness; because if the answers appeared to be undeniably correct, they would be wholly immaterial, and if thought to be erroneous they could be shown to be so, only by the testimony of others, who might themselves be the mistaken ones. To allow such a question, would thus operate to introduce an immaterial issue of fact into a case, and to draw the attention of the jury away from the issues of the pleadings.*

§ 502. The last part of a plaintiff's prima facie evidence, consists in proof of the amount of his damages, sometimes supplemented by evidence tending to show that a judgment ought to be entered for an amount greater than the actual damages sustained by him. The subject is mentioned in this connection for the sake of symmetry; but it is so large that it constitutes the theme of a separate chapter of this book. To that chapter, recourse may be had for detailed information upon the point.

§ 503. The next part of a trial is the introduction of evidence by the defendant to sustain his defences. The possible defences in patent cases are twenty-seven in number. In prior sections in this chapter, they are consecutively numbered for purposes of easy reference, and are treated in respect of the pleadings which they respectively require, and the results which they respectively produce in patent actions at law. It is now convenient to set forth, in the same order, some of the leading points of the law of evidence applicable to each.

§ 504. The first defence generally requires evidence to show that the terms of art or science which are used in the patent have such a meaning that the court is bound to construe the patent to be one for a principle, or for something other than a process, machine, manufacture, composition of matter, or design. This general statement is ventured, though the cases in which patents have been assailed with the first defence are so few, that but little law is established on the subject.

§ 505. The second defence may sometimes be supported by facts of which the court will take judicial notice.' But evidence to show the state of the art, is often required to show want of invention. A patent granted for an implement of agriculture, consisting of a hoe-handle with a hoe on one end and a rake on the other, would be void for want of invention, even if both new and useful.' The court would take judicial notice of the prior existence of handles having hoes attached thereto, and of other like handles having rakes fastened at one end; and on the basis of that judicial notice, would pronounce such a patent to be wholly invalid. A patent for a particular alleged combination, in a rare and complicated machine, may also be open to precisely the same sort of objection; while the facts upon which it rests in the particular case, may be wholly unknown to people

1 Brown v. Piper, 91 U. S. 37, 1875; Slawson v. Railroad Co. 107 U. S. 649, 1882; Phillips v. Detroit,

111 U. S. 606, 1883.

? Reckendorfer v. Faber, 92 U. S. 347, 1875.

generally, and wholly unknown to judges who hear patent causes, though well understood by certain classes of mechanics. In the latter case, it is necessary to introduce evidence of those facts in order to show want of invention. Such evidence may consist of proof of the prior existence of the parts of the alleged combination, and proof of the fact that their union in the machine constitutes not a real combination, but an aggregation only. This statement of the considerations which show a necessity for evidence to prove lack of invention, when that lack is based on the rule that aggregation is not invention, will also furnish the key to inquiry when that lack is based on any other of the various rules on that subject.

§ 506. The third defence, and the facts which support it, are explained at large in the third chapter of this book. In this connection, it is only necessary to explain the kinds of evidence by which those facts may be proved, and to state the special rules which govern the weight of such evidence.

Where novelty is duly sought to be negatived by prior United States patents, duly certified copies of those patents are admissible;' and it is a general practice among patent lawyers to waive the certificate, where a printed copy from the Patent Office is presented by opposing counsel. Such printed copies are really more reliable than any certified. manuscript copy; because they are generally printed from the same form as the original letters patent, and are therefore absolutely correct; while there is always a possibility of error in copying a document with a pen. But the Patent Office does not furnish printed copies of patents which were granted prior to 1866; and as to those patents, a certified manuscript copy is the best to be had. The certified copies of letters patent, which are admissible in evidence, include not only such individual copies as are furnished to private persons on payment of the proper fees; but also the certified bound volumes of copies, which are gratuitously distributed by the Commissioner of Patents to all the State

1 Revised Statutes, Sec. 892.