delivered the opinion of the court.
This is a. writ of error to the Circuit Court, of the United Statés, for the District of Maryland. The plaintiff in error brought his action in that court for an infringement of the exclusive right to-make, use, and sell “an improvement in cars for the transportation of coal,” &c., granted to him by letters-patent, bearing date on the 26th day of June, 1847; and, the judgment of that court being for the defendants, hé has brought the record here by this writ of error.
It appears, by the. bill of exceptions, that the letters-patent declared on were duly issued, and that their validity was not questioned; but the defendants denied that they had infringed upon the exclusive right of the plaintiff.
On such a trial, two questions arise. The first is, what is the thing patented; the second, has that thing been constructed, used, or sold by the defendants;
—The first is a question of law, to be determined by the court, construing the -letters-patent, arid the description, of the invention and specification of claim annexed to them. The second is a question of fact, to be submitted to a jury.
In this case it is alleged the court construed the specification of claim erroneously, and thereby withdrew from the jury questions which it was their province to decide. This renders it necessary to examine the letters-patent, and the schedule annexed to them, to see whether their construction by the Circuit Court was correct.
In this, as in most patent cases, founded on alleged improvements in machines, in order to determine what is the thing patented, it is necessary to inquire.
1. What; is the structure or device, described by the patentee, as embodying his invention. .
2. What mode of operation is introduced arid employed by this structure or device.
3. What result is attained by means of this mode of operation.
*3394. Does the specification of claim cover the described mode of operation by which the result is attained.
Without going into unnecessary details, or referring to drawings, it may be stated that the structure, described by this patent, is the body of a burden railroad car, made of sheet iron, the upper part .being cylindrical, and the lower part in the form of a frustum of a cone, the under edge of which has a flange secured upon it, to which flange a movable bottom is attached. This bottom is made movable, in order to discharge the load through the aperture left by removing it.
To .understand the mode of operation introduced and employed by means of this form of the car body, it is only necessary to state, what appears on the face of the specification, and was testified to by experts at the trial as correct, that, by reason of the circular form of the car body, the pressure of the load outwards was equal in every direction, and thus the load supported itself in a great degree; that, by making the lower part conical, this principle of .action operated throughout the car, with the exception of the small space to which the movable bottom was attached; that, being conical, the lower part of the car could be carried down below the truck, between the wheels, thus lowering the centre of gravity of the load; that the pressure outwards upon all'parts of the circle being equal, the tensile strength of the iron was used' to a much greater degree than in a car of a square form; and, finally, that this form of the lower part of the car facilitated the complete discharge of the load through the aperture, when the bottom was removed.
It thus appears that, by means of this change of form, the patentee has introduced a mode of operation not before employed in burden cars, that is ..to say, nearly equal pressure in all directions by the entire load, save that small part which rests on the movable bottom; the effects of which are, that the load, in a great degree, supports itself, and the tensile strength of the iron is used, while- at the same-time, by reason of the same form, the centre of gravity of the load is depressed, and its discharge facilitated.
The practical .result attained by this mode of operation is correctly described by the patentee; for. the uncontradicted evidence at the trial showed that he had not exaggerated the practical advantage of his invention. The specification states:
“ The transportation of coal, and all other heavy articles in lumps, has been attended with -great injury-to the cars, requiring the bodies to be constructed with great strength to resist the outward pressure on the sides, as well as the vertical pressure on the bottom, due not only to the weight of the mass, but the mobility of the.lumps among each other tending to ‘pack,’ as *340it is technically termed. Experience has shown that cars, on the old mode of construction, cannot be made to carry a load greater than its own weight; but, by my improvement, I am enabled to make cars of greater durability than those heretofore made, which will transport double their own weight of coal,” &c.
Having thus ascertained what is the structure described, the mode of operation it embodies, and the practical result attained, the next inquiry is, does the specification of claim cover this mode of operation, by which this result is effected ?
It was upon this question the case turned at the trial in the Circuit Court.
The testimony showed that the defendants had made cars similar to the plaintiff’s, except that the form was octagonal instead of circular. There was evidence tending to prove that, considered in reference to the practical uses of such a car, the octagonal car was substantially the same as the circular. Amongst other witnesses upon this point was James Millholland, who was called by the defendants. He testified.
“ That' the advantage of a reduced bottom of the car was obtained, whether the car was conical or octagonal; that the strengthening of the bottom, due to the adoption of a conical form, was the same when the octagonal form was adopted, or the circular. That the circular form.was the best to resist the pressure, as, for instance, in a steam boiler, and an octagonal one better than the square form; that the octagonal car was not better than the conical car; that, for practical purposes, one was as good as the other; that a polygon of many sides would be equivalent to a circle; that the octagon car, practically, was as good as the conical ones-; and that, substantially,, the witness saw ho difference between the two.”
The district judge, who presided at the trial, ruled,—
That while the patent is good for what [is] described therein, a conical body, in whole or in part, supported in any of the modes indicated for a mode of sustaining a conical body on a carriage or truck, and drawing the same, and to those principles which were due alone to conical vehicles, and not to rectilinear bodies, and it being admitted that the defendants’ car was entirely rectilinear, that there was no infringement of the plaintiff’s patent.
The substance of this ruling was, that the claim was limited to the particular geometrical form mentioned in the specification ; and as the defendants had not made cars in that particular form, there could be no iAfringement, even if the cars made by the defendants attained the same result by employing, what was in fact, the same mode of operation as that described by the patentee. We think this ruling was erroneous.
*341Under our law a patent cannot be granted merely for a change of form. The act of February 21,1793, § 2, so declared iii express' terms; and though this declaratory law was not reenacted in the Patent Act of 1836, it is a principle which necessarily makes part of every system' of law granting patents for new inventions. Merely to change the form of a machine is the work of a constructor, not of an inventor; such a change cannot be deemed an invention. Nor does the plaintiff’s patent •rest upon such a change. To change the form of an existing machine, and by means of such change to introduce and employ .other mechanical principles or natural powers, or, as it is termed, a new mode of operation, and thus attain a new and useful result, is the subject. of. a patent. Such is the basis on Which the plaintiff’s patent rests.
Its substance is a new mode of operation, by means of which a new result is obtained. It is this new mode of operation which gives it the character of an invention, and entitles the inventor to a patent; and this new mode oi operation is, in view of the patent law, the thing entitled to protection. The patentee may,’ and should, so frame his specification of claim as to cover this new mode of operation which he has invented; and the only question in this case is, whether he has done so; or whether he has restricted his claim to one particular geometrical form.
There being evidence in the casé tending to show that other forms do in fact^ embody the plaintiff’s mode of operation, and, by means of it, produce the same new and useful result, the ■ question is, whether the patentee has limited his claim to.one out of the several forms which thus embody'his invention.
Now, while it is undoubtedly true, that the patentee .may so restrict his- claim as to cover less- than what he invented, or may limit it to one particular form of machine, excluding all other forms, though they also embody his invention, yét such an interpretation should not be put upon his claim if it can fairly be construed otherwise, and this for. two reasons:
1. Because the reasonable presumption is, that, having a just right to cover and protect his whole invention, he intended to do so. Haworth v. Hardcastle, Web. P. C. 484.
2. Because specifications are to be construed liberally, in accordance with the design of the Constitution and the patent laws of the United States, to promote the progress of the useful arts, and allow inventors to retain to their own use, not any thing which is matter of common right, but what they them- • selves have created. Grant v. Raymond, 6 Pet. 218; Ames v. Howard, 1 Sumn. 482, 485; Blanchard v. Sprague, 3 Id. 535, 539; Davoll v. Brown, 1 Wood. & Minot, 53, 57; Parker v. Ha-*342worth, 4 McLean’s R. 372; Le Roy v. Tatham, 14 How. 181, and opinion of Parke, Baron, there quoted; Neilson v. Harford, Web. P. C. 341; Russell v. Cowley, Id. 470; Burden v. Winslow, (decided at the present term,) 15 Howard.
The claim of the plaintiff is in the following words:
“ What I claim as my‘invention, and desire to secure by letters-patent, is making the body of a car for the transportation of coal, &c., in the form of a frustum' of a cone, substantially as herein described, whereby the force exerted by the weight of the load presses equally in all directions, and does not tend to change thejform thereof, so that, every part resists its equal proportion, and by which, also, the lower part is so reduced as to pass down within the truck frame and between the axles, to lower the centre of gravity of the load without diminishing the capacity of the car as described.
“ I also claim extending the body of the car below the connecting pieces of the truck frame, and the line of draught, by passing the connecting bars of the truck frame, and the draught bar, through the body of the 'car, substantially as described.”
It is generally true, when a patentee describes a machine, and then claims it as described, that he is understood to intend to claim, and does by law actually cover, nót only the precise forms he has described, but all other forms which embody his invention; it being a familiar' rule that, to copy the principle or mode of operation described, is an infringement, although such copy should be. totally unlike the original in form or proportions.
Why should not this rule be applied to this case ?
It is not sufficient to distinguish this case to say, that here the invention consists in a change of form, and the patentee has claimed one form only.
Patentable improvements iri machinery áre almost always made by changing some one or more forms of one or more parts, and thereby introducing some mechanical principle or mode of action not previously existing in the machine, and so securing a new or improved result. And, in the numerous cases in which it has been held, that to copy the patentee’s mode of operation was an infringement, the infringer had got forms and proportions not described, and not in terms claimed. If it were not so, no question-of infringement could arise. If the machine-complained of were a copy, in form, of the machine described in the specification, of course it would be at once seen to be an infringement. It could be nothing else. It is only ingenious diversities of form and proportion, presenting the appearance of something unlike the thing patented, which give rise to questions; and the property of inventors would be .valueless, if it *343were enough for the defendant to say, your improvement consisted in a change of form; you describe and claim but one form; I have not taken that, and so have not infringed.
The answer is, my improvement did not consist in a change of form, but in the new employment of principles or powers, in a new mode of operation, embodied in a form by means of which a new or better result is produced; it was this which constituted my invention; this you have copied, changing only the form; and that answer is justly applicable to this patent.
Undoubtedly there may be cases in which the letters-patent do include only the particular form described and claimed. Davis v. Palmer, 2 Brock. 309, seems to have been one of th<ose cases. But they are in entire accordance with what is alcove stated.
The reason why such a patent covers only one geometrical form, is not that the patentee has described and claimed that form only; it is because that form, only is capable of embodying his invention; and, consequently, if the form is not copied, the invention is not used.
Where form and substance are inseparable, it is enough to look at the form only. Where they are separable; where the whole substance of the invention may be eopied in a different form, it is the duty of courts and juries to look through the • form for the substance of the invention — for that which entitled the inventor to his patent, and which the patent was designed to secure; where that is found, there is an infringement; and it is not a defence, that it is embodied in a form not described, and in terms claimed by the patentee.
Patentees sometimes add to their claims an express declaration, to the effect that the claim extends to the thing patented, however its form or proportions may be varied. But this is un-' necessary. • The law so interprets the claim without the addition of these words; . The exclusive right to .the thing patented is not secured, if the public are at liberty to make substantial copies of it, varying its form or proportions. And, therefore, the patentee, having described his invention, and shown its principles, and claimed it in that form which most perfectly embodies it, is, in contemplation of law, deemed to claim every form in which his invention may be copied, unless he manifests an intention to disclaim some óf those forms.
Indeed it is difficult to perceive how any other rule could be applied, practicably, to cases like this. How is .a question of infringement of this patent to be tried?- ' It.may safely be assumed, that neither the patentee nor any other constructer has made, or will make, a car exactly circular. In practice, deviations from a true circle will always occur. How near to a *344circle, then, must a car'be, in order to infringe? May it be slightly elliptical, or otherwise depart from a true circle, and, if so, how far ?
In our judgment, the only , answer that can be given to these questions is, that it must be so near to a true circle as substantially to embody the patentée’s mode of operation, and thereby attain the same kind of result as was reached by his invention. It is not necessary that the defendant’s cars should employ the plaintiff’s invention to as good advantage as he employed it, 01 that the result should be precisely the same in degree. It must be the same in kind, and.effected by the employment of his mode of operation in substance.. "Whether, in point of fact, the defeitdant’s ears did copy the plaintiff’s invention, in the sense above explained, is a question for the jury, and the court below erred in not leaving that question to them upon the evidence in the case, which tended to prove, the affirmative.
‘ The judgment of the.court below must be reversed.
■ Mr. Chief Justice TANEY, Mr. Justice CATRON, Mr. Justice DANIEL, and Mr. Justice CAMPBELL, dissented.