Opinion of the Court. screwed immovably into the triple-valve casing, and serving to divide the piston-chamber from the main-valve chamber; that this partition was a new element, never before found in triple valves, and introduced a new principle and mode of operation, totally different from anything ever invented by Mr. Westinghouse or any other inventor, and that its effect is to make valve 22, termed by them the main valve, admit the train-pipe air to the brake-cylinder at the same time that it admits the auxiliary air thereto. It is claimed that, in embodying this new principle, Mr. Boyden adopted the form of triple-valve shown in the expired Westinghouse patent No. 141,685, (1873,) in which the main valve, 22, is of the poppet form, and the separate valve 17, controlled by a rod sliding through the main valve, is employed for releasing the brakes. For charging the auxiliary reservoir he adopted, from the expired Westinghouse patent No. 144,006, (1873,) a check-valved feed passage through the triple-valve piston, but arranged the feed passage and its check-valve, 26, in a tubular extension, F, of the piston, and substantially in the form shown in Boyden patent No. 280,285, (1883). He also provided a sensitive graduating valve, similar in results to the graduating valve e' of the Westinghouse patent No. 220,556, (1879,) by so arranging a small passage, 40, in the sliding stem, which actuates the release valve, that such passage will be opened and closed by the sliding of such stem through the main valve 22. As thus constructed, the triple-valve operates much the same as that of patent No. 220,556, and, like the latter, is incapable of quick action. In both the complainants' and defendants' devices there is (1) a feeding-in valve to charge the auxiliary reservoir; (2) a valve which complainants call their "main valve," and which the defendants denominate a "graduating valve," which is opened by the preliminary traverse of the piston to admit reservoir air to the brake-cylinder; (3) a release valve which discharges air from the brake-cylinder to the atmosphere; and (4) a quick-action valve-41 in the complainants' patent, and 22 in the defendants' which is opened by the further traverse of the piston to admit train-pipe air to the brake Opinion of the Court. cylinder. In defendants' patent, it may also be used to admit auxiliary reservoir air to the brake-cylinder. One of the main controversies in the case turns upon the construction and operation of the poppet-valve 22, called by the defendants their "main-valve." Complainants insist that the office of their main valve is performed by the stem slidevalve 18 of defendants' patent, and by its apertures i, j and k, through which air passes from the auxiliary reservoir to the brake-cylinder, and that the poppet-valve 22 is only called into action in emergency cases, when a large quantity of air is suddenly withdrawn from the train-pipe, and the valve is unseated by the traverse of the piston to the extreme right. There is no doubt that the function of admitting air from the auxiliary reservoir to the brake-cylinder, which is performed in the Westinghouse patent by what the complainants term the main-valve, (aided, however, by the graduating-valve,) is, in ordinary cases, performed principally, if not altogether, by the stem slide-valve 18 and its three ports i, j, k, of the Boyden patent, which defendants term their graduating-valve. It is equally clear that, in emergencies, where quick action is required, air, which in the Westinghouse patent passes through auxiliary valve 41, (opened by the further traverse of the piston,) in the Boyden patent finds its way through the poppet-valve 22, which has also been lifted from its seat by the further traverse of the piston. One of the main differences between the two devices is this: That in the preliminary traverse of the piston of the Westinghouse patent, there is a movement, first, of the graduatingvalve to open its port from the auxiliary reservoir, and then of the main valve, carrying the graduating-valve also with it, to open a passage to the brake-cylinder, while in the Boyden patent it is only the graduating-valve which is opened by the preliminary traverse of the piston. In doing this, the gradu ating-valve moves through the poppet-valve, but does not lift it from its seat. In emergency cases not only do the graduating-valve and the main-valve of the Westinghouse patent move as before, but, by the extreme traverse of the piston, the auxiliary-valve 41 is shoved from its seat, and a separate Opinion of the Court. passage is opened for the air from the train-pipe to the brakecylinder. In the Boyden patent, however, the extreme traverse of the piston lifts the poppet-valve from its seat, and opens a wide passage to the brake-cylinder, not only for the air from the auxiliary reservoir, but, through the peculiar operation of the partition 9 and its aperture B, directly from the train-pipe. As the graduating-valve of the Boyden patent practically does all the work in ordinary cases, and the poppetvalve is only called into action in emergency cases, the latter is practically an auxiliary valve, by which, we understand, not necessarily an independent valve, nor one of a particular construction, but simply a valve which in emergency cases is called into the assistance of the graduating-valve. In this particular, the poppet-valve of the Boyden device performs practically the same function as the slide-valve 41 of the Westinghouse. It is not material in this connection that it is a poppet-valve while the other is a slide-valve, since there is no invention in substituting one valve or spring of familiar shape for another; Imhaeuser v. Buerk, 101 U. S. 647, 656; nor, that in one case the piston pushes the valve off its seat, and in the other pulls it off; nor is it material that this poppet-valve may have been used in prior patents to perform the function of a main-valve, so long as it is used for a different purpose here. Indeed, this valve seems to have been taken bodily from Westinghouse patent No. 141,685, where it was used as a main-valve, and the stem-valve 18 with its ports i, j, k, added for ordinary uses, and the poppet-valve thus converted from a main-valve to an auxiliary valve. We have not overlooked in this connection the argument that the poppet-valve 22 is also sometimes used for graduating purposes, but it is not commonly so used, and appears to be entirely unnecessary for that purpose. It seems to be possible to move the piston 29 to its extreme traverse so slowly, and hence to open valve 22 so gradually, that the pressure in the chamber C will be reduced so slightly, that the train-pipe air will not have sufficient force to throw open the check-valve 26, and hence, in such case no train-pipe air will be admitted directly to the brake-cylinder, which will be filled with auxil Opinion of the Court. iary reservoir air only. But, as a matter of fact, this seldom or never takes place in the practical operation of the device, and is an unnecessary and wholly unimportant incident, and for all practical purposes valve 22 is solely a quick-action valve. As this valve is actuated by the piston of the triple-valve, and in such action is independent of the main valve, it meets the demand of the first claim of the patent, and as it is actuated by the piston-stem, and controls communication between passages leading to connections with the main air-pipe and with the brake-cylinder, it seems also to be covered by the fourth claim. But even if it be conceded that the Boyden device corresponds with the letter of the Westinghouse claims, that does not settle conclusively the question of infringement. We have repeatedly held that a charge of infringement is sometimes made out, though the letter of the claims be avoided. Machine Co. v. Murphy, 97 U. S. 120; Ives v. Hamilton, 92 U. S. 426, 431; Morey v. Lockwood, 8 Wall. 230; Elizabeth v. Pavement Company, 97 U. S. 126, 137; Sessions v. Romadka, 145 U. S. 29; Hoyt v. Horne, 145 U. S. 302. The converse is equally true. The patentee may bring the defendant within the letter of his claims, but if the latter has so far changed the principle of the device that the claims of the patent, literally construed, have ceased to represent his actual invention, he is as little subject to be adjudged an infringer as one who has violated the letter of a statute has to be convicted, when he has done nothing in conflict with its spirit and intent. "An infringement," says Mr. Justice Grier in Burr v. Duryee, 1 Wall. 531, 572, "involves substantial identity, whether that identity be described by the terms, 'same principle,' same modus operandi, or any other. The argument used to show infringement assumes that every combination of devices in a machine which is used to produce the same effect, is necessarily an equivalent for any other combination used for the same purpose. This is a flagrant abuse of the term 'equivalent."" We have no desire to qualify the repeated expressions of this court to the effect that, where the invention is functional, and the defendant's device differs from that of the patentee Opinion of the Court. only in form, or in a rearrangement of the same elements of a combination, he would be adjudged an infringer, even if, in certain particulars, his device be an improvement upon that of the patentee. But, after all, even if the patent for a machine be a pioneer, the alleged infringer must have done something more than reach the same result. He must have reached it by substantially the same or similar means, or the rule that the function of a machine cannot be patented is of no practical value. To say that the patentee of a pioneer invention for a new mechanism is entitled to every mechanical device which produces the same result is to hold, in other language, that he is entitled to patent his function. Mere variations of form may be disregarded, but the substance of the invention must be there. As was said in Burr v. Duryee, 1 Wall. 531, 573, an infringement "is a copy of the thing described in the specification of the patentee, either without variation, or with such variations as are consistent with its being in substance the same thing. If the invention of the patentee be a machine, it will be infringed by a machine which incorporates in its structure and operation the substance of the invention; that is, by an arrangement of mechanism which performs the same service or produces the same effect in the same way, or substantially the same way. That two machines produce the same effect will not justify the assertion that they are substantially the same, or that the devices used are, therefore, mere equivalents for those of the other." Not only is this sound as a general principle of law, but it is peculiarly appropriate to this case. Under the very terms of the first and fourth claims of the Westinghouse patent, the infringing device must not only contain an auxiliary valve, or its mechanical equivalent, but it must contain the elements of the combination "substantially as set forth." In other words, there must not only be an auxiliary valve, but substantially such a one as is described in the patent, i.e. independent of the triple-valve. Not only has the Boyden patent a poppet instead of a slide-valve — a matter of minor importance — but it performs a somewhat different function. In the Westinghouse patent the valve is not in the line of travel between the |