The arbitrators, to whom this case was referred, have by their award submitted to the court a question of law, if by law they have authority to do so. Of this authority we can have no doubt, and when the submission is by a rule of court, it is not uncommon for arbitrators to exercise such authority. When arbitrators have thus submitted a question of law to the court, it is manifest that their intention was to act conformably to law ; and if, upon examination, it appears that they have decided against law, then it is obvious that the award does not express their real judgment, and is not the result which they intended.

The case referred was an action of trespass, for a trespass alleged to have been committed by the defendant in removing a part of the foundation and materials of a mill dam; and whether the action can be maintained depends upon a question of the boundary line of the estates between the parties. If the part of the dam removed by the defendant was on the laud of the plaintiff, the act was a trespass ; if on his own land, the plaintiff has suffered no wrong, and has no cause of action. By the agreement of the parties, and the rule of court founded upon it, the arbitrators, in addition to adjudicating upon the action, were authorized and required, as a part of their duty, to ascertain and determine the boundary line between the parties.

It appears that the defendant Wilder had become the proprietor of an estate known as the Houghton farm, in Lancaster, lying on the west side of the Nashua River. The bulk of the farm lay above a road and bridge, which crosses the river on a line nearly east and west; but a small piece or gore, constituting the south-eastern angle of the Houghton farm, and consisting mostly of land lying in the bed of the river, had been separated by the laying out of the road from the *207rest of the farm. This small gore, which was described as containing about ten square rods, had been leased to be used in connection with other land, on the opposite side of the river, for the erection of a mill, the lease to continue so long as a grist mill should be kept up on the same. The grist mill was kept up for some time, and was then discontinued; by means of which the small parcel of land reverted to the proprietor of the Houghton farm.

The plaintiff, as we understand, had acquired a title to the land on the opposite or easterly side of the river, and also to the land on the westerly side of the river, lying adjoining and below the small lot.

It is conceded on all hands, that the Nashua is a river not navigable, and that by the general rule of law, the riparian proprietors respectively own the soil to the thread of the stream, or filum aquce.

The question for the arbitrators was, in what direction the boundary line should run from a fixed monument on the bank of the river; whether on a line at right angles with the river; or whether it should run diagonally, following the upland line. The adoption of the former rule would exclude the place where the trespass was alleged to be committed,— the site of the greater portion of the mill and dam,— from the premises of the defendant, and would show that the act was unjustifiable, and that the plaintiff was entitled to recover. The adoption of the latter,—the diagonal line,— would bring the place within the premises of the defendant, and would thus prove that the act complained of was done on his own land, and was accordingly justifiable.

The question is clearly stated by the arbitrators. They give a description of the Houghton farm from a survey made and recorded in the proprietors’ records in 1738, and no act is shown to have been done since that time, by the conterminous proprietors, to change the line as then established. From this description, it appears that the line commences at a red oak tree by the river, above the red spring, and that the line runs thence west, twenty-one degrees north, sixteen *208rods, to a white oak tree at the next angle. This red oak tree was the monument for the south east angle of the farm, which was severed from it by the road. The arbitrators further state, that no vestige remains, either of the red oak by the river, or of the white oak sixteen rods back from the river; but from other satisfactory evidence, they have been able to ascertain the sites of both of these monuments, They then state, that beginning at the site of the white oak, back from the river, and running sixteen rods towards the river, they came to the site of the red oak, and there set up a monument. From that point, they extended two lines across the river; one in a direction at right angles with the river, which is here stated to be straight, or nearly so; and the other by continuing the line from the white oak to the red oak, in the same direction diagonally across the river; and at the end of each of these lines, on the opposite side of the river, they set up a fixed monument. It is obvious, that the latter line would strike the thread of the stream at a point lower down than the former, and thereby give a greater extent to the south-easterly angle of the Houghton farm.

From this point, — the site of the red oak tree on the bank of the river,—the plaintiff contended, that the line should be extended to the thread of the stream, so as to intersect it at right angles therewith. The defendant, on the other hand, contended, that- the line should be extended to the thread of the river, from the site of the white oak tree, through the site of the red oak tree, without change of course.

We take the rule of law to be, that upon a river not navigable, the riparian proprietor has a fee in the soil, subject of course to a public easement for boating and rafting, and the like purposes, to the thread of the stream, whether expressed in the grant or conveyance under which he holds the land or not; and it follows, therefore, that where the line of the shore is straight, each of such proprietors will have a length of line, at the thread of the stream, equal to his length of line on the shore; but in case of a curved line on the shore, either lonvex or concave, his line at the thread of the stream will *209be longer or shorter, respectively, than his shore line. We are also of opinion, that when there is nothing in the conveyance to modify the rule, that rule is, that the side lines of each riparian proprietor must extend from the termini of his lines on the shore, at right angles with the course of the river, to the thread of the stream. No doubt the owner of land so situated, having himself an estate in fee in the bed of the stream, to the thread of it, may sell his estate in the bed of the river, without his upland, or his upland without the bed of the river; and such conveyance will be good and available. It follows, as a necessary consequence, that such owner may convey a part of his estate in the bed of the river, and may divide it in any form, or by any lines, which he may think proper. We therefore state the rule, as to the adopting of lines running at right angles, as applicable to a case in which the lines are not otherwise established by the terms of the grant, under which the riparian owner holds. But prima facie, and independently of such modification in particular cases, the general rule must be to take lines, at right angles with the course of the stream, to its thread or middle line. The same rule applies to the analogous case of the division of flats, in Massachusetts, under the colony ordinance. Rust v. Boston Mill Corp. 6 Pick. 158; Deerfield v. Arms, 17 Pick. 41; Emerson v. Taylor, 9 Greenl. 42. In the first and last of these cases, the point of the decision was, that the direction of the side lines of the riparian owner upon the upland towards the river is not to be regarded, in extending the line into the river; but regard is to be had only to the length of the proprietor’s line on the shore. This, we think, is the' true rule.

The same rule was recognized and adopted in the case of ¡Sparhawk v. Bullard, 1 Met. 95, 106. This rule assumes, that, in-the .absence of any direction given by the proprietor’s grant, and where the shore is straight, the course is to draw lines from the termini of the proprietor’s lines on the shore, at right angles, to determine his right to flats. And we think the rule is well founded in principle. The object is to give *210to each riparian proprietor an equal share of the bed of the river, in proportion to his line on the margin of the stream, together with that portion of the bed of the stream, which lies opposite, in front of, or adjacent to, his upland; and this, in the absence of any controlling grant, will be effected by the straight lines, at right angles, which will in general be the shortest .and most direct lines, to the thread of the stream. And we cannot perceive how this consideration can be influenced by the shape of the upland lot, or by the direction of its side lines back from the river.

In applying this rule to the present case, the court have not been able to perceive any thing in the description of the Houghton farm, which, by expression or implication, gives a direction to the line from the red oak, on the margin of the river, towards the bed of the stream; and therefore the general rule of construction must prevail. The survey of the farm, as filed with the proprietors in 1738, is referred to as a correct description, and nothing has since occurred to vary it. This description began at the red oak, by the river, above the red spring, and from thence ran west, twenty-one degrees north, to a white oak, bounding, &c.; and thence by various corners and distances to a black oak, bounding, &c.; from thence down the river, bounding on the river and intervale, to the red oak where it began.

In this description, there is no conveyance in terms of any portion of the bed of the river; and it is only by implication of law, that one half of the bed of the river passes; because the estate conveyed is bounded on the river, and the rule of law already stated fixes the lines to be at right angles to the stream, from the several termini on the margin. This description is very much like that stated in the case of Ipswich, Petitioners, 13 Pick. 431. In that case, as in this, it was held, that when the river was given as a boundary, it must be understood to be the thread of the river. But as the central line or thread of the river could never come to the red oak on its bank, it must be held, by a reasonable construction, to be a point directly opposite to the monument. In that case, *211it was said, “ or at that point, at which the line of the wall, if protracted at right angles with the river, would intersect the central line of the river.” The wall, we believe, in that case, was nearly or exactly at right angles with the river ; so that the line of the wall, and a line at right angles with the river, were equivalent expressions. Still, it may well be admitted, that the word “ protracted ” was carelessly used, and that the term “ extended ” or “ drawn ” would have expressed the idea intended more exactly; inasmuch as the word “ protracted,” when unqualified, means the extension of a line in its original direction. But in the same sentence, the word is qualified by the expression “ at right angles with the river.” And such, we have no doubt, was the judgment of the court in that case. Thus explained, that case, we think, is an authority confirming the general rule of law, as above stated.

The court are therefore of opinion, that the arbitrators erred in matter of law, in taking the diagonal line, drawn to the lower monument, set up and described by them on the opposite side or east bank of the river; that they should have taken the direct line described by them to the upper monument, on a line drawn at right angles with the river; and that the report ought to be recommitted to the arbitrators, to enable them to reconsider and revise their finding in this respect

Award recommitted.