Opinion of the court. 478, 484, 497; 8 T. R., 99, 101, 103; 1 Gallis., 481; 1 Mason, 189, 191. Mr. Justice STORY delivered the opinion of the court. This is the same case which was formerly before this court, and is reported in 3 Wheat. Rep., 454, and by a reference to that report the form of the patent, the nature of the action, and the subsequent proceedings will fully appear. The cause now comes before us upon a writ of error to the judgment of the Circuit Court, rendered upon the new trial had in pursuance of the mandate of this court. Upon the new trial, several exceptions were taken by the counsel for the plaintiff. The first was to the admission of a Mr. Frederick as a witness for the defendant. It is to be observed, that the sole controversy between the parties at the new trial was, whether the plaintiff was entitled to recover for an alleged breach of his patent by the defendant in using the improved hopper-boy. Frederick, in his examination on the voir dire, denied that he had any interest in the cause, or that he was bound to contribute to the expenses of it. He said he had not a hopper-boy in his mill at present, it being then in court; that it was in his mill about three weeks ago, when he gave it to a person to bring down to Philadelphia; and that his hopperboy spreads and turns the meal, cools it some, dries it, and gathers it to the bolting-chest. Upon this evidence, the plaintiff's counsel contended that Frederick was not a competent witness, but the objection was overruled by the court. It does not appear from this examination whether the hopper-boy used by Frederick was that improved by the plaintiff or not; but, assuming it was, we are of opinion that the witness was rightly admitted. It is perfectly clear that a person having an interest only in the question, and not in the event of the suit, is a competent witness; and, in general, the liability of a witness to a like action, or his standing in the same predicament with the party sued, if the verdict cannot be given in evidence for or against him, is an interest in the question, and does not exclude him. If nothing had been in controversy in this case as to the validity of the patent itself, and the general issue only had been pleaded, the present objection would have fallen within the general rule. But the special notice in this case asserts matter which, if true, and found specially by the jury, might authorize the court to adjudge the patent void, and it is supposed that this constitutes such an interest in Frederick in the event of the cause that he is thereby rendered incompetent. But, in this respect, Frederick stands in the same situation as every other person in the community. If the patent is Opinion of the court. declared void, the invention may be used by the whole community, and all persons may be said to have an interest in making it public property. But this results from a general principle of law, that a party can take nothing by a void patent; and so far as such an interest goes, we think it is to the credit and not to the competency of the witness. It is clear that the verdict in this case, if given for Evans, would not be evidence in a suit against Frederick, but Frederick would be entitled to contest every step in the cause, in the same manner as if no such suit had existed. Non constat that Frederick himself will ever be sued by the plaintiff, or that, if sued, any recovery can be had against him, even if the plaintiff's patent should not be avoided in this suit. It therefore rests in remote contingencies whether Frederick will, under any circumstances, have an interest in the event of this suit, and the law adjudges the party incompetent only when he has a certain, and not a contingent, interest. It has been the inclination of courts of law, in modern times, generally, to lean against exceptions to testimony. This is a case which may be considered somewhat anomalous; and we think it safest to admit the testimony, leaving its credibility to the jury. Another exception was to the refusal of the court to allow a deposition to be read by the plaintiff, which had been taken according to a prevalent practice of the State courts. It is not pretended that the deposition was admissible according to the positive rules of law, or the rules of the Circuit Court; and it is not now produced, so that we can see what were the circumstances under which it was taken. No practice, however convenient, can give validity to depositions which are not taken according to law, or the rules of the Circuit Court, unless the parties expressly waive the objection, or, by previous consent, agree to have them taken and made evidence. This objection, therefore, may at once be dismissed. The principal arguments, however, at the bar, have been urged against the charge given by the Circuit Court in summing up the charge to the jury. The charge is spread in extenso upon the record, a practice which is unnecessary and inconvenient, and may give rise to minute criticisms and observations upon points incidentally introduced for purposes of argument or illustration, and by no means essential to the merits of the cause. In causes of this nature, we think the substance only of the charge is to be examined; and if it appears, upon the whole, that the law was justly expounded to the jury, general expressions, which may need and would receive qualification if they were the direct point in judgment, are to be understood in such restricted sense. Opinion of the court. It has been already stated that the whole controversy at the trial turned upon the use of the plaintiff's hopper-boy; and no other of the inventions included in this patent was asserted or supposed to be pirated by the defendant. The plaintiff, with a view to the maintenance of his suit, contended that his patent, so far as respected the hopper-boy, had a double aspect: 1. That it was to be as a patent for the whole of the improved hopper-boy, that is, of the whole machine, as his own invention. 2. That if not susceptible of this construction, it was for an improvement upon the hopper-boy, and he was entitled to recover against the defendant for using his improvement. The defendant admitted that he used the improved hopper-boy, and put his defense upon two grounds: 1. That if the patent was for the whole machine, that is, the improved hopper-boy, the plaintiff was not the inventor of the improved hopper-boy so patented. 2. That if the patent was for an improvement only upon the hopper-boy, the specification did not describe the nature and extent of the improvement; and if it did, still the patent comprehended the whole machine, and was broader than the invention. To the examination of these points, and summing up the evidence, the attention of the Circuit Court was exclusively directed; and the question is, whether the charge, in respect to the matters of law involved in these points, was erroneous, to the injury of the plaintiff. We will consider the points in the same order in which they were reviewed by the Circuit Court. Was the patent of the plaintiff, so far as respects his improved hopper-boy, a patent for the whole machine, as his own invention? It is not disputed that the specification does contain a good and sufficient description of the improved hopper-boy, and of the manner of constructing it; and if there had been any dispute on this subject, it would have been matter of fact for the jury, and not of law for the decision of the court. The plaintiff, in his specification, after describing his hopper-boy, its structure and use, sums up his invention as follows: "I claim as my invention the peculiar properties or principles which this machine possesses, in the spreading, turning, and gathering the meal at one operation, and the raising and lowering of its arms by its motion, to accommodate itself to any quantity c meal it has to operate upon." From this manner of stating his invent.on, without any other qualification, it is apparent that it is just such a claim as would be made use of by the plaintiff, if the whole machine was substantially, in its structure and combinations, new. The plaintiff does not state it to be a specific improvement upon an existing machine, confining his claim Opinion of the court. to that improvement, but as an invention substantially original. In short, he claims the machine as substantially new in its properties and principles, that is to say, in the modus operandi. If this be true, and this has been the construction strongly and earnestly pressed upon this court by the plaintiff's counsel, in the argument at the present term, what are the legal principles that flow from this doctrine? The Patent Act of the 21st of February, 1793, ch. 11, upon which the validity of our patents generally depends, authorizes a patent to the inventor, for his invention or improvement in any new and useful art, machine, manufacture, or composition of matter not known or used before the application. It also gives to any inventor of an improvement in the principle of any machine, or in the process of any composition of matter which has been patented, an exclusive right to a patent for his improvement; but he is not to be at liberty to use the original discovery, nor is the first inventor at liberty to use the improvement. It also declares that simply changing the form or the proportion of any machine or composition of matter, in any degree, shall not be deemed a discovery. It further provides that, on any trial for a violation of the patent, the party may give in evidence, having given due notice thereof, any special matter tending to prove that the plaintiff's specification does not contain the whole truth relative to his discovery, or contains more than is necessary to produce the effect, (where the addition or concealment shall appear to have been to deceive the public,) or that the thing secured by the patent was not originally discovered by the patentee, but had been in use, or had been described in some public work, anterior to the supposed discovery of the patentee, or that he had surreptitiously obtained a person's invention; and provides that in either of these cases judgment shall be rendered for the plaintiff, with costs, and the patent shall be declared void. It further requires that every inventor, before he can receive a patent, shall swear or affirm to the truth of his invention, " and shall deliver a written description of his invention, and of the manner of using or process of compounding the same, in such full, clear, and exact terms as to distinguish the same from all things before known, and to enable any person skilled in the art or science of which it is a branch, or with which it is most nearly connected, to make, compound, and use the same; and in the case of any machine, he shall fully explain the several modes in which he has contemplated the application of the principle or character by which it may be distinguished from other inventions." From this enumeration of the provisions of the act, it is clear that the party cannot entitle himself to a patent for more that his own Opinion of the court. invention; and if his patent includes things before known, or before in use, as his invention, he is not entitled to recover, for his patent is broader than his invention. If, therefore, the patent be for the whole of a machine, the party can maintain a title to it only by establishing that it is substantially new in its structure and mode of operation. If the same combinations existed before in machines of the same nature, up to a certain point, and the party's invention consists in adding some new machinery, or some improved mode of operation, to the old, the patent should be limited to such improvement; for if it includes the whole machinery, it includes more than his invention, and therefore cannot be supported. This is the view of the law on this point which was taken by the Circuit Court. That court went into a full examination of the testimony, and also of the structure of Evans's hopper-boy and Stauffer's hopper-boy, and left it to the jury to decide, whether, up to a certain point, the two machines were or were not the same in principle. If they were the same in principle, and merely differed in form and proportion, then it was declared that the plaintiff was not entitled to recover; or, to use the language of the court, if the jury were of opinion that the plaintiff was not the inventor of the hopper-boy; he was not entitled to recover, unless his was a case excepted from the general operation of the act. We perceive no reason to be dissatisfied with this part of the charge; it left the fact open for the jury, and instructed them correctly as to the law. And the verdict of the jury negatived the right of the plaintiff as the inventor of the whole machine. The next inquiry before the Circuit Court was, whether the plaintiff's case was excepted from the general operation of the act. Upon that, it is unnecessary to say more than that the point was expressly decided by this court in the negative, upon the former writ of error. And we think the opinion of this court, delivered on that occasion, is correctly understood and expounded by the Circuit Court. It could never have been intended by this court to declare, in direct opposition to the very terms of the Patent Act, that a party was entitled to recover, although he should be proved not to have been the inventor of the machine patented; or that he should be entitled to recover, notwithstanding the machine patented was in use prior to his alleged discovery. There is undoubtedly a slight error in drawing up the judgment of the court upon the former writ of error; but it is immediately corrected by an attentive perusal of the opinion itself. And we do not think that it can be better stated or explained than in the manner in which the Circuit Court has expounded it. We are, then, led to the examination of the other point of view in |