This case arose thus: A narrow strip of the petitioner’s land having been taken by competent authority, to widen a highway called Main Street in the village of Fitchburg, they applied to the county commissioners to issue a warrant for a sheriff’s jury to assess their damages.
1. It is admitted that the proceedings in the case were regular, unless it was irregular for the commissioners to send this and another petition of the same applicants, for damage to their estate, to the same jury. No special reason is assigned in the present case why the two petitions, between the same parties, for two species of damage to their estate, should not go to the same jury, and therefore the case seems to be within the spirit, if not the letter, of the Rev. Sts. c. 24, §§ 15, 76. It does not appear to be objectionable. The warrant having been returned *554by the sheriff to the superior court, with the verdict of the jury thereon assessing damages for the petitioners, its acceptance was objected to by the respondents on the grounds set forth in the certificate of the sheriff, as a bill of exceptions. But the verdict, was accepted by the judgment of the superior court, from which this appeal was taken.
The following were among the points principally relied on by the respondents against the acceptance of the verdict.
2. A witness was called by the petitioners, and, against the objection of the respondents, was permitted to testify that he had offered the petitioners one dollar a foot for the land. It is stated that afterwards, to avoid a contest, the counsel for the petitioners said he would withdraw this testimony ; but he did not ask the sheriff to instruct the jury to exclude this testimony, nor did he so instruct them.
This, we think, was inadmissible. Suppose the offer was one dollar a foot for the whole lot, which does not distinctly appear; it has no tendency to show that each foot was worth a dollar. Suppose the whole lot to be one thousand square feet, and the land taken one hundred, leaving nine hundred feet, the whole lot may still be worth one thousand dollars.
If it be said that the front part must be the most valuable ; it may be so, whilst it was front, and all the rest rear. But when the front was taken for a street, the rear became front, with all the rights of ingress and egress.
Again, the witness may have had special reasons for making the offer. Had he been called to give an opinion, and, amongst other things, had stated that he had made an offer, and the circumstances under which it was made, we are not prepared to state that it would have been incompetent. But this does not appear.
3. One of the petitioners, being called as a witness, under the recent law making witnesses parties, was permitted, against the objection of the respondents, to testify that he had been offered one dollar a foot for all said estate. As to the offer, unexplained, the former remarks apply. As to the part of the objection, that the petitioner was allowed to give his opinion of *555the value of his own land; since the statute has provided that any party may be a general witness in his own case, we cannot see on what ground this testimony can be limited to particular facts, to the exclusion of others. The admission of parties, not only interested to the full amount of the matter in controversy, but animated by all the feelings and passions which stimulate parties to litigation, must to some extent change the character of a jury trial. It obviously throws a higher responsibility on juries, carefully to weigh the credit due to such witnesses, by all the legal means in their power. In a matter of plain fact, where the witness must know how the truth is, and therefore must testify truly or commit perjury, the jury would probably be more likely to give him credit; but when it is matter of opinion, and when opinion is so easily warped by prejudice and interest, especially on a point where it is ordinarily practicable to procure other like evidence from unobjectionable sources, we should suppose that very little weight would be given to the testimony of a party. But this is for the jury, and not the judge, to decide.
4. The respondents having offered the testimony of a witness, that in his opinion the market value of the estate was enhanced by the widening of the street, the witness was asked by the respondents to state the grounds and reasons upon which his opinions were founded; this was objected to by the petitioners, and rejected by the sheriff.
The court are of opinion that this evidence ought to have been admitted. The ground on which an expert, a person of large experience in any particular department of art, business or science is permitted to testify to his opinion, is, that from his larger experience and more exact observation of facts, and the connection between certain appearances, and their causes or results, he is able to draw correct, conclusions from circumstances, which a man of ordinary knowledge and experience could not do. The circumstances on which such an opinion may be founded, are either facts of general notoriety, assumed to be known to all persons of skill and experience in the department to which they pertain, and which, when explained, may be *556comprehended and applied by any person of good understanding ; or it may be founded on facts proved.
Such general facts, assumed to be generally known without specific proof, because they are capable of being known and understood, without any such proof, to all inquirers, are vastly too numerous to define; but the point may be illustrated by saying, they are such as the elements and forces of physical nature, the structure, capacities and functions of the human and other animal bodies, the common powers, propensities and passions of human nature, and the impelling and governing motives to human action. As these are capable of being comprehended, when explained, without specific proof, it appears to the court that the witness should be permitted to explain the grounds and reasons of his opinion to the court and jury; they may readily perceive the force of his reasoning, the soundness or fallacy of his logic, and therefore judge of his capacity to give an opinion on the subject, and the correctness of his conclusions, and consequently the weight due to his opinion.
But it is objected that the admission of this evidence would open the door to evidence entirely incompetent, by allowing the witness to state the facts on which the opinion is founded, facts not proved by competent evidence.
This objection seems to us to be founded on a misconception of the manner in which the investigation is to be conducted, and the testimony of experts received and applied. It assumes that the facts will be taken to be true, because the witness has stated that he founds his opinion upon them. But this is quite a mistake. In order to obtain the opinion of a witness on matters not depending upon general knowledge, but on facts not testified of by himself, one of two modes is pursued: either the witness is present and hears all the testimony, or the testimony is summed up in the question put to him; and in either case the question is put to him hypothetically, whether, if certain facts testified of are trac, he can form an opinion, and what that opinion is ? The jury will then be instructed, if the truth of any such fact is contested, first to consider whether the fact on which such opinion rests is proved to their satisfaction; if it *557is, then to give such weight to the opinion resting on it as it deserves; but if the fact is not proved by the evidence, then to give the opinion no weight. This is necessary to enable the jury, upon the true theory of jury trial, to decide all questions of fact, upon competent evidence laid before them. M’Naghten’s case, 10 Cl. & Fin. 200.
But the consideration submitted in the argument in opposition to this view, namely, that the opinion may be given on the assumption of facts not proved, is a strong additional reason why the grounds and reasons of the opinion should be stated, in order that the jury may see that it is not founded on hearsay, general rumor, or facts of which some evidence may have been given, but, being controlled by other evidence, are not found true by the jury. This inquiry has been more frequently made in cross-examination, yet we are of opinion that it is competent evidence in chief. It is in fact this general knowledge, on the spécifie facts judicially proved, from which the jury draw their ultimate conclusion, though in matters of science they may be aided by the more exact observation and the larger experience of the trained expert. Keith v. Lothrop, 10 Cush. 453.
5. We think the evidence of the fact that, when the damage was assessed, a sidewalk had been laid down on and over the land, and of the value of such sidewalk to the estate, was rightly rejected by the sheriff, because that use might be changed after-wards to any other use to which a sheet may be appropriated. But the uses to which such a strip, taken to widen a street in a village or city, would probably be applied for a sidewalk or other use beneficial to the adjacent owners, might properly be taken into consideration, in estimating the benefit to be set-off against the damage done to the same estate by the taking.
6. We think Mr. Billings, a special county commissioner, was rightly admitted as a witness, to testify to his opinion of the value of the estate, and the effect of the taking upon such value. His position and duties would ordinarily qualify him to form a correct opinion on such subjects; and it contravened no public policy.
7. We now come to the consideration of the instructions *558given by the sheriff to the jury. By the first, the direction was, that the jury might take into consideration the possibility of appropriating the land to the use of building brick or stone houses thereon; this was not exceptionable, as we understand it It was one of the uses to which it was possible the estate might be applied, and there was no necessity of qualifying it by proof that such use was probable. The actual value of the estate, before and after the widening of the street by taking a portion of it, were proper subjects for the jury to take into consideration, and the exceptions thereto, we think, were not well taken.
8. The instructions that damages are to be assessed according to the effect of the taking of the land at the time it was taken, were right. But this is not to be understood that the damage was to be estimated as the land then lay, unfitted for use as a street; but according to the prospective use of the street when wrought and fitted for use as a part of the street, for a sidewalk or carriage way; and the consequence of such taking of part, to the residue of the land when thus fitted for use as part of the street.
9. We think the instructions as to the set-off of benefits were correct, unless it may be that the sheriff placed some peculiar stress on the word peculiar, as underscored. We presume it was a mere caution not to lay stress on that vague, general, indefinite benefit, which all inhabitants of a particular locality are said to derive from a great public improvement as a canal or railroad. The true rule is stated in Upton v. South Reading Branch Railroad, 8 Cush. 600, and Proprietors of Locks & Canals v. Nashua & Lowell Railroad, 10 Cush. 385.
10. We are inclined'to think that the evidence of the surrender of a lease, before the damages were assessed, ought to have been received, because it left the estate discharged of any other interest, as if no other had existed. But as the lease had a short time to run, and probably at a rackrent, there was so little interest in the leaseholder, to be affected by the taking of this small strip of land, that whether it subsisted or not could only affect the damage. It could hardly of itself be considered a sufficient ground, if it stood alone, for disturbing the verdict.
*559For these reasons, this court are of opinion that the directions to the jury were incorrect in several material particulars; that the verdict returned ought not to be accepted; and that a mandate go to the county commissioners, directing them to issue a new warrant to assess the damages of the petitioners.