The opinion of the court was delivered, January 3d 1871, by

— A careful examination of the sixteen errors assigned to the charge of the learned judge of the District Court, and his answers to the points, brings to light no material error in his instruction to the jury. An elaboration of authorities to a jury is not useful, and sometimes tends to confuse, but here the judge so clearly stated the points for the attention and examination of the jury, it does not seem probable they .could have been misled by the detailed reference to authorities. It is not necessary that a judge should encumber his charge with a discussion of the law, but rather to give definite instructions to the jury; the Act of Assembly allowing him to furnish his reasons, which he may always subjoin if explanation be necessary.

This was a contract to make 1000 dozens of gas generators as per sample at $2 per dozen. The judge was therefore clearly right in instructing the jury to ascertain first what was the contract sample. There was evidence tending to show that alterations had been made in the sample at the request of the defendant. He was therefore right also in directing the jury to ascertain from the evidence whether the sample was so altered, and if so, what the alterations were, and thus to arrive at the- true contract sample — the model by which the plaintiffs were to make the gas generators called for by their contract. He was right in directing them to determine upon the evidence whether the gas generators made by them for the defendant corresponded with the sample, and if they did, the plaintiffs thereby fulfilled their contract and were not responsible for the result — that is for the operation of the generator. They did not contract to make an * article which would produce a certain result, but only to make one to correspond exactly with the proposed model. If this were done fully and fairly their whole duty was performed, and they had a right to recover for so many as they had made and delivered according to their contract.

Admitting then the correctness of this instruction, and that the learned judge was right in ruling that correspondence between the article made and the sarhple was the true and only question, and that the maker is not liable for the result expected to be produced, was he right in his rulings on the evidence ? Is there no difference between the liability of the maker for this result, and *392the evidence which the result affords of a want of correspondence between the thing made and the model ? Clearly there is a marked distinction between the propositions. A result or effect produced may be most convincing corroborative evidence of a difference between the model and the imitation. Especially is this true of those things which lie out of the range of ordinary observation, and -are known only to experts. The expert may be called to prove the actual difference, but when this is done, it does not follow that the legal measure of proof is full. A fact which directly proves the truth of an assertion may undoubtedly be adduced to corroborate and confirm it. If an expert state a fact which cannot be appreciated by ordinary observation, and state it to a body of unskilled men (such as a jury is) the force of his statement depends wholly upon his skill and his credibility, and has nothing in the common sense of the jury to confirm it. When he declares that there is a material and substantial difference between two subjects of comparison, and if one of them will produce a desired and expected effect, and the other will not, why shall not this practical test of the difference, to wit, the result produced by each, be given in evidence to corroborate and confirm his statement ? It is no answer to say that the maker is not liable for the result of an article he makes according to the model, for that is not now the question. The question is, did he make it in conformity to his model, and clearly its power to perform the same result as the model, is evidence tending to show he did so make it, while its want of power to do so, is evidence tending to prove the contrary.

If a delicate piece of mechanism, such as a watch having a certain movement, be the model, and can be shown to run well, and if the imitation watch be proved to differ from the model in certain respects alleged to be essential to its successful operation, why shall the party not be permitte’d to show in corroboration of these departures from the model, that the imitation watch will not keep time, or will not run at all ? Can anything be more corroborative of the skill and truthfulness of the witness who asserts these differences between the watches ? Such is the .precise case before us. Here is a gas generator intended to produce a certain effect upon a subtle fluid, and to convert it into gas by means of heat; an article unknown to the jury; peculiar in its structure, and whose laws of action, even an expert may find it difficult to explain. Why should not the party, after showing departures in form and structure in the imitation from the model, be permitted to show that the model will produce a certain effect which the imitation will not ? Is it not strong evidence that the differences pointed out are real and substantial, and therefore not to be overlooked and disregarded by a jury -who may not, for want' of skill in the particular matter, be able to discern the deviation of the *393imitation from tbe sample. The defendant below made several offers of evidence to prove by the results the difference between the sample and the generators manufactured by the plaintiffs, all of which were overruled by the court on the general ground of incompetency and irrelevancy. The one set forth in the 17th assignment of error is the only one that need be noticed. The offer was to show by the witness that he knew personally the sample and its successful operation, and then to show that the generators made by the plaintiff did not correspond with the sample in manner or form, and that they did not produce the results which the contract sample did. The court rejected the latter part of this offer. The defendant not only offered but did prove the former part of the offer, by producing a sample and then pointing out the differences in manner and form of that made by the plaintiffs. The Whiting, or sample, is bell-mouthed in the perforated case, the Miller & Co. is not. In one the chimney is movable, and in the other it is not. In one the burner tube is long, in the other it is short. The wick tube of the Miller & Co. did not correspond with the wick tube of the contract sample, one being shorter than the other; and an entire difference in the mode of fastening them on. The witness mentioned other differences also. Now as the jury have found that the generators made by the plaintiffs were made according to the sample, it is evident they either disbelieved this witness, or 'supposed the differences described by him were unsubstantial and immaterial. But suppose the defendants had been permitted to corroborate the testimony of the witness by showing that the Whiting or sample generator would perform well the intended purpose, and that the plaintiffs’ would not, and failed to generate gas for illumination, who can say that this important corroborating fact would not have turned the scale and caused the jury to render a different verdict on this question of fact, whether the generators were really made alike ? This refusal of the court to receive the evidence as offered, was therefore an error, and the judgment is reversed and a venire facias de novo is awarded.