The opinion of the court was delivered by
Dunton, J.The plaintiff called experts to show the operation and results produced by the various parts of the two patent saw frames, as detailed in the letters patent and specifications; • and to show that the invention of Butler and Ramsay was an infringement of Livingston’s patent, the plaintiff offered to ask said experts if the principle was the same in both inventions, which offer *313was refused by the court; and to the exclusion of this evidence the plaintiff excepted.
No question is made but that the persons denominated experts possessed the requisite skill or knowledge to entitle them to such designation.
It has been repeatedly held by the Federal courts in patent cases that the opinions of experts as to whether two mechanical structures or devices are identical in principle are competent evidence upon this question ; and it is the almost every day practice to permit questions similar to the one excluded in the case at bar to be put to and answered by experts. See Barrett v. Hall, 1 Mason, 447, 470; Parker v. Stiles, 5 McLean, 44, 64; Allen v. Blunt, 3 Story, 742, 747.
We fail to see how such testimony differs in principle from the testimony of medical experts, who have been repeatedly allowed by our own courts to testify as to their opinion of the sanity of a person, either upon such an hypothetical case as it is claimed the evidence tends to prove, or upon certain evidence in the case when not conflicting, or upon an examination of such person. As is remarked by Mason, J., in Dewitt v. Bailey and Schoonmaker, 9 N. Y. 375 : “ This evidence is admitted upon the supposition that the questions involve matters which are beyond the scope of the observation, knowledge, and experience of men in general, and that consequently the jury could not be presumed competent to arrive at a proper determination by the unaided exercise of their judgment on the facts.” We therefore think it was error to exclude this evidence.
The court instructed the jury that Livingston, by his specifications and claims, secured to himself the right to use the brace in the one position and part of the frame designated, and that a brace in some other part of the frame producing substantially the same result would not be an infringement of his patent. This wo think was error. It was a question for the jury to determine, under proper instruction from the court, whether the brace in question in another part of the frame would be an infringement of Livingston’s patent. Mechanical devices may differ in structure and *314form, and still be identical in principle. Blanchard v. Buss, 2 Blatchf. 411.
Says . Curtis, in his valuable work on patents, s. 469, “ The question of infringement is a question whether the invention of the defendant is substantially the same thing as that of the plaintiff. The identity of two things is a matter of fact depending upon evidence; and although it is to be determined under the guidance of those principles which determine what constitutes identity and diversity in the sense of the patent law, yet it is for the jury to determine as matter of fact, under proper instructions, whether the two things are the same or different.” Also see Whitmore v. Cutler, 1 Gall. 478; Lovell v. Lewis, 1 Masou, 191.
The defendant admitted that he received three hundred dollars for a half interest in the letters patent on the occasion of his'making the deed thereof to the plaintiff, but claimed, and, against the plaintiff’s objection, was allowed to offer evidence tending to show, that he made the sale of the same to Butler and received the money from him, and that Butler sold the same to the plaintiff for three hundred and fifty dollars, and that he the defendant conveyed directly to the plaintiff at Butler’s request, to save the expense of two conveyances. The plaintiff excepted to the admission of the evidence for the reason that it contradicted the defendant’s deed to him the plaintiff. The deed sets forth, “ that for and in consideration of $500 to me in hand paid, the receipt of which is hereby acknowledged, I have granted, sold,” &c., the right secured by said letters patent to the plaintiff. It will be observed that the deed does not state from whom the consideration was received ; but if it did, and stated that it was received from the plaintiff, it would be only prima facie evidence of such fact, subject to explanation by showing by parol what the real consideration was, and by whom it was paid. White v. Miller, 22 Vt. 380; Beach v. Packard, 10 Vt. 96. The deed in question had for its object the conveyance of the grantor’s right in said invention as secured to him by said letters patent; and this object cannot be defeated or affected by parol. But as is said by Collambr, J., in Beach v. Packard, supra: “ It has long been *315liolden in England and in this country, and even in this State, that receipts may be varied or contradicted by parol, as they are not contracts. If it may be so done as to a written receipt, we do not see why the same may not as consistently be done with one under seal ”; and it was held in that case that the common acknowledgment in a deed that the grantor had received the consideration, does not prevent him from showing by parol, in an action to recover such consideration, that the same has not been paid.
But in the case at bar, as it is not stated from whom the consideration was received, it is at least questionable whether the parol evidence admitted contradicts the deed. We do ■ not think its admission was error.
The burden of proof was upon the plaintiff to make out a case, and to do this, it was necessary for him to show by a fair balance of testimony that he made the purchase in question of the defendant ; and the jury were so instructed. If the purchase was made of the defendant it was not made of Butler. There being no evidence of a joint sale by the defendant and Butler, proof of a sale by one excluded the idea or fact of a sale by the other. This we think was what the court meant by saying to the jury that the burden was upon the plaintiff to show that he made the purchase of the defendant and not of Butler. We do not think the last clause added anything to the charge, and it would have been better to have omitted it; but in our opinion it does not constitute error. We do not think the jury understood from it that the burden of proof was upon the plaintiff to show that the purchase was not made of Butler otherwise than as above stated. Had direct evidence been required to prove such a negation, it would have been ei’ror.
Assuming it to be true, as claimed by the plaintiff’s counsel, that the assignment or deed is prima facie evidence of the relation of vendor and vendee between the parties to this suit in respect to the purchase or sale in question, it does not change the. rule as to the burden of proof. The deed or assignment is simply evidence, and when offered and received as such, this branch of the case was prima facie made out, and the plaintiff thereby relieved of the burden that was resting upon him at the outset.
*316We are. not to presume that the court opitted to explain to the jury the proper effect to be given to the assignment or deed as evidence, and find error in such omission ; besides, it is stated in the exceptions that “ the court properly explained the parts of the charge detailed.”
The question as to the plaintiff’s right of recovery in case it is found that he purchased the invention in question of the defendant, and that the same is an infringement of Livingston’s patent, was neither made by counsel nor considered by the court. This question is therefore not decided, and we express no opinion upon the same.'
Judgment reversed and,ease remanded.