This case presents a question of some importance to the rights of mill owners, and the owners of lands liable to he flowed.

*256Several points must be considered as now well settled. The owner of the land, over which there is a running stream, has a right to the use of the natural flow of the stream, to be reasonably used for mill purposes. No proprietor of land, on the same stream, has a right to divert the water or change the use of it, (independently of the statute right of flowing upon payment of damage,) to the injury of any other proprietor, unless such right has been acquired by grant or prescription ; but where the mill owner has in fact exercised the right of raising or diverting the water, by keeping up his dam and flowing the land of another for a period of twenty years, without objection or claim of damages, it is evidence of a right so to use the water, as acquired by prescription or grant. Bolivar Manuf. Co. v. Neponset Manuf. Co., 16 Pick. 241. Williams v. Nelson, 23 Pick. 141. Mason v. Hill, 3 Barn. & Adolph. 304. Buddington v. Bradley, 10 Connect. 213. Baldwin v. Calkins, 10 Wend. 167.

But it is equally well settled by the authorities, that if one proprietor has, by means of a dam, made a special use of the water, by penning it up, and throwing it back upon a proprietor above, or by holding it back from a proprietor below, or by diverting it, and has so used the water, without resistance or opposition from other proprietors, for the term of twenty.years, he thereby establishes a right so to continue the use of it., by way of prescription or presumed grant.

We are then brought to the precise question, whether a mill owner, who has acquired a prescriptive right to maintain the water at a certain height, and who has erected a dam capable, in its ordinary operation, of raising the water to a greater height, but who, from the leaky condition of the dam, and his wasteful mode of using the water, has, for a considerable part of the 20 years, in fact not raised the'water to the height, at which the dam, when tight, is capable of raising it, and by means of which the stagnant water has not been kept constantly so high on the land of an adjacent proprietor, can repair and tighten his dam, without raising it, and so change his machinery, by the substitution of breast wheels or over-shot wheels, for the under-shot wheels formerly used, as to keeu up the water constantly to its higher level.

*257Il is contended on the part of the land owner, that as the actual use of the water at a given height, by the mill owner, and the acquiescence in such use, by the land owner, is the foundation and proof, so it must also be the measure and limit of his right. This, to some extent, is true ; and where there is a definite limitation or modification of the use — one that is practicable and measurable — it will show a corresponding modi fication of the right. As where, for example, according to the custom of the country, a saw mill, or other mill, has been kept up in the winter only, and the mill owner has uniformly been accustomed to draw off the water sufficiently early in the spring to allow the growth of a crop of grass, and to continue it down, until the hay is cut and got in, it must be regarded as establishing a right to a winter privilege only, and not a constant privilege ; and then, flowing the land through the year must be considered as a new use, not within the mill owner’s prescriptive right.

So, where a dam had been kept up more than twenty years, but the water had been drawn down six weeks in each year, between June and October, to enable the land owners to get clay, it was considered good evidence of a right to keep up the dam, subject to such limitation. Bolivar Manuf. Co. v. Neponset Manuf. Co., 16 Pick. 241. Other cases may easily be imagined, where a prescriptive right, proved by use and enjoyment, may be limited and qualified by a definite interruption in the constancy of such use, for a certain time or purpose, in favor of the person against whom the right is claimed. But in determining the legal rights of parties, the law looks rather to practical than theoretical distinctions, and seeks, as far as possible, to place them upon grounds permanent and general, and upon principles applicable to the generality of cases, not shifting and varying with a slight change of circumstances. The right, when once established, shall be construed favorably to the party acquiring it. Conformably to these rules, it has long been held that where one has acquired a right to raise and maintain a head of water, by using it for one purpose, he may apply it to another : he may substitute a cotton factory for a saw mill, and *258the like ; and this upon the ground that any other rule would put a stop to all improvements. Cottel v. Luttrel, 4 Co. 87. Saunders v. Newman, 1 Barn. & Ald. 258. Biglow v. Battle, 15 Mass. 313. Johnson v. Rand, 6 N. Hamp. 22. It comes, we think, within the spirit of the same rule, to hold, that when a .nan has, by his dam, raised a certain head of water, and maintained such dam long enough to raise the presumption of a grant, he may not only use his head of water for another purpose oi branch of business, but he may repair his dam, and make it tighter , he may use improved machinery, taking the water from the top instead of the bottom of the floom ; and generally, he may use the water more economically, although the effect may be, to keep the water more constantly at the upper level. Alder v. Savill, 5 Taunt. 454.

As a general rule, the height, to which such a mill owner will have a prescriptive right to maintain the water, will depend upon the height of the dam, by which he has raised it. In speaking of the height of the dam, we mean it to be understood, as the efficient height of the dam ; the height to which such dam, when completed and finished, with its rolling dam, waste ways, &c. and in good repair and condition, will raise the head of water. Parts of the dam of earth, or other materials, may be raised higher than it is ever intended to raise the water by such dam. This is not intended ; but as already explained, the efficient height.

On the whole, we think the true rule is this ; that when one has acquired a prescriptive right to a constant mill privilege, by keeping up and using a dam more than twenty years, which dam, in its usual operation, would raise the water to a given height, and has used it, at his own pleasure, at that height, without any claim of right, on the part of any other person, to have it drawr or kept down, for any part of the year, or upon any definite occasion, he has a right to retain it at the same height, although from the former leaky condition of the dam, the rude construction of the machinery, or the lavish use and waste of the stream, the water has not in fact been constantly or usually kept up to that height. If therefore he repairs the dam, without so changing *259it as to raise the water higher than the old dam, when tight and in repair, would raise it, or uses it in a different mode, and thereby keeps up the water more constantly than before, it is not a new use of the stream, for which an adjacent owner can claim damages, but a use conformable to his prescriptive right.

As it appears by the exceptions, that the court of common pleas gave a different instruction to the jury, in the present case, the court are all of opinion, that the verdict for the complainant must be set aside, and a

New trial granted