
81 U.S. 620 (1871)
14 Wall. 620
MOWRY
v.
WHITNEY.
Supreme Court of United States.

*634 Messrs. A.G. Thurman and C.B. Collier, for the appellant.
Messrs. B.R. Curtis, E.W. Stoughton, and H. Baldwin, Jr., contra.
*637 *639 Mr. Justice STRONG delivered the opinion of the court.
The defences set up to the complainant's bill for an infringement are, that the patent is void for want of novelty in the invention, and for want of utility, and also that it has not been infringed by the defendant.
To determine how far these defences are sustained it is important to have a clear apprehension of the state of the art when the patent was granted, and of the invention which it was intended to secure to the patentee. Prior to the 2d of August, 1847, cast-iron railroad wheels had been cast, and cast in chills, that is, they had been cast in sand moulds with an outer circumference of iron. The effect of this outer circumference was to produce a more rapid chill on the periphery of the wheel, thereby crystallizing and hardening it, so that the wheel was made stronger, and more capable of resisting the friction of the rails. But the parts of the wheel were of different thicknesses. The hub and the rim were much thicker than the plate which connected them, and of course they cooled after casting more slowly than the plate. The consequence of this unequal cooling was to produce a strain between the thick and thin parts that greatly impaired the strength of the wheel. Various devices had *640 been made to guard against, or to remedy the mischief resulting from this inherent and inevitable strain, caused by unequal contraction in cooling. The most common of these, perhaps, was casting the wheel with the hub in sections, in order that the sections might accommodate themselves to the contraction of the plate. But this was expensive. It required the open space between the sections to be filled up with other metal, and generally it required the hub to be hooped. It is unnecessary, however, to describe these devices. It does not appear that in any of them the idea existed of making a car-wheel with chilled tread, straight plates, and solid hub, annealed and cooled so as to leave it uninjured by the strain attendant upon the unequal cooling of the thick and thin parts. Annealing some kinds of castings was known and practiced before 1847. This is abundantly proved by the witnesses, and various modes of annealing plain castings had been described by scientific writers both in this country and abroad, before that time. But there is no evidence that we have been able to discover that cast-iron car-wheels had ever been subjected to an annealing process, in connection with slow cooling, before the process was discovered or invented by Whitney. In all the experiments made for annealing other castings the object sought was different, and in them all, as well as in the process described in the publications given in evidence, the effect upon the annealed metal or glass was not to leave them in the condition in which it was sought to bring car-wheels, with the crystallization or chill of the periphery unimpaired, and the plate or thin part unaffected by strain. Cast-iron railroad wheels are castings of a peculiar kind. The methods of slow cooling, or of annealing and slow cooling, which were applied to other castings before 1847, were not adapted to their peculiarities, or to what they needed. They are not homogeneous throughout. They are of different thickness in their several parts, and hardened at the tread, while the plate and hub are not crystallized, but are soft and tough. These different qualities of the different parts it is necessary to preserve, and what was needed when Whitney's invention *641 was made, was to preserve them, and at the same time relieve against any strain, caused by unequal cooling, which might impair the strength of the wheel.
If now we proceed to inquire what Whitney's alleged invention was, as described in his specification and claim, it will be seen that it was a process, not to make a car-wheel or to destroy any of the advantages which had already been secured, but to add another. Its avowed object was to obtain a new value, or rather exemption from imperfection. It was to remedy the evil of strain resulting from the more rapid cooling of one part of the wheel than the cooling of the other parts. And this was sought to be accomplished by a process that insured the cooling of all parts, both the thick and the thin, with equal slowness. The process consists of several parts. The first is taking the wheels from the moulds after the melted iron has been run into the moulds, before they become so much cooled as to produce strain on any part sufficient to impair their ultimate strength. The second is placing the wheels immediately after their removal in a furnace or chamber previously heated to about the temperature of the wheels when taken from the moulds, the heat in the furnace being subject to control. The third is applying heat until the temperature of all parts of the wheels shall again be raised to the same point (indefinitely said to be a little below that at which fusion commences). The fourth and last stage in the process is allowing the wheels after they have been thus reheated, to cool so fast as, and no faster than, is necessary for every part of each wheel to cool and shrink simultaneously together, and no one part before another. It is therefore a patent for a process, not for a combination. Neither as a whole nor in parts can it be considered without reference to the ultimate object in view, which was to retard cooling by a second application of heat supplied until all parts of the wheel are raised to the same temperature, and then permit the heat to subside so gradually that the cooling of the parts shall not only commence at the same point of temperature, higher than that where hurtful strain begins, but shall continue *642 equable till all artificial heat ceases. The removal from the moulds to the furnace or chamber, the removal at the time described, before the incipient strain has become permanently hurtful, and to a place where more heat may be applied, and where the heat can be under control, are parts of the process to secure equable cooling during the time when cooling without such appliances is likely to produce strain and consequent weakness. It is apparent that this is more than a process for annealing. That is included, it is true, but it is only a small part. It is applying foreign heat to a hot chilled wheel, at the point of time when it has reached a particular stage of cooling, by means of such foreign heat bringing the whole casting up to a higher and uniform temperature, and maintaining an equable abatement of heat in a furnace or chamber under the control of the operator. We have sought in vain through the proofs submitted in this case, for any satisfactory evidence that this process was known before 1847, when Whitney commenced it, or that anything equivalent to the process was known. Certainly nothing of the kind had ever been applied to cast-iron railroad wheels, and, as we have seen, they are castings of a peculiar character, not admitting of the treatment that may be applied to other castings. What they needed was (what was substantially described by one of the witnesses), the discovery of the fact that the chilled cast iron, constituting one part of the wheel, could be subjected to heat less than that which would cause fusion, without producing any material effect upon its hardness, while the cooling of other parts of the wheel could be so prolonged by applying that heat externally, as to enable all parts to cool without being subjected to the strain attendant on unequal contraction, and, in addition to the discovery, they needed the invention of a process by which it could be practically carried out. Such a discovery and such a process were needed for no other castings. The novelty of the patentee's invention is not therefore disproved by evidence that glass, or speculum metal, or even other iron castings had been annealed and slow-cooled, prior to the time when it was made. Of this *643 there is very considerable evidence both in the testimony of witnesses and printed publications. The specification disclaims invention of annealing iron castings done in the ordinary mode. It claims annealing when applied to cast-iron railroad wheels, in the mode or by the process described. It is not therefore merely an old contrivance or process applied to a new object, a case of double use. A new and previously unknown result is obtained, namely, the relief of the plate of the wheels from inherent strain without impairing the chilled tread, a result which, though anxiously sought, had not been obtained before Whitney's invention. We are therefore of opinion that the defence set up that the patent was void for want of novelty of invention is unsustained.
The validity of the invention is next assailed for the reason that the process described in it, and claimed, is denied to be useful, because it would destroy the hardness of the rim, or tread of the car-wheel known as the chill, and thus greatly detract from the durability and usefulness of the wheels.
It is undoubtedly true that a chilled periphery or tread is essential to the usefulness of a car-wheel. Indeed, the evidence is, that whenever car-wheels are spoken of, wheels with chilled tread are meant, and any process which destroys the chill must render them valueless for the purposes for which they are needed.
It is also true that the fusing-point of cast iron is in the neighborhood of 2786 degrees of Fahrenheit, twelve or fifteen hundred degrees above the point at which, according to the evidence, the chill of the tread of a car-wheel would be destroyed. If, therefore, the process patented to Whitney, requires, after the removal of the wheel to the heated furnace or chamber, the application of a degree of heat closely approximating the point of fusion, it must be conceded that instead of being beneficial it is positively hurtful. And this is what is contended by the appellant. The objection seems to be aimed at the sufficiency of the description of the *644 patentee's invention, which it is abundantly proved he practiced successfully through many years, rather than at its utility. Whitney conceived a process and practiced it. That process may have been a highly useful invention, and therefore patentable, and yet he may have failed so to describe it as to teach the public how to practice it. The law requires every inventor, before he can receive a patent, to furnish a specification or a written description of his invention or discovery, and of the manner and process of making, constructing, using, and compounding the same, in such full, clear, and exact terms, avoiding unnecessary prolixity, as to enable any person skilled in the art or science to which it appertains, or with which it is most nearly connected, to make, construct, compound, and use the same. The specification, then, is to be addressed to those skilled in the art, and is to be comprehensible by them. It may be sufficient, though the unskilled may not be able to gather from it how to use the invention. And it is evident that the definiteness of a specification must vary with the nature of its subject. Addressed as it is to those skilled in the art, it may leave something to their skill in applying the invention, but it should not mislead them. The objection here is that in describing the degree of heat to be applied after the wheels have been deposited in the heated chamber the patentee states it to be such that the temperature of all parts of the wheels "may be raised to the same point (say a little below that at which fusion commences)," and the defendant insists that this amounts to a direction to raise the heat to a degree that must destroy the chill of the tread, and thus render the casting valueless as a railroad car-wheel. But it is obvious that only vague and uncertain directions could have been given respecting the extent to which the heat is necessary to be raised. It must differ with the difference in the progress of cooling which has taken place before the wheels are removed from the moulds. The process requires this removal before they have become so much cooled as to produce such inherent strain on any part as to impair its ultimate strength. Precisely when such a strain begins cannot *645 be known. Cooling commences the instant the casting is made, and with cooling commences contraction, and strain must soon follow. Plainly it is impossible to describe the point of time when the strain has proceeded so far as to impair the ultimate strength of any part of the wheel. That, in the nature of things, must be left to the judgment of the operator. But before that time the strain may be checked, and this is what is contemplated by raising the temperature of all parts of the wheel to the same point or degree. The moment that is done the strain ceases, and the primary object of the patentee's process is accomplished. The state of things is reproduced which existed before the contraction and attendant strain began, when the slow cooling is allowed to follow in an atmosphere so heated and regulated that each part of the wheel loses its heat at an equal pace with all others.
Now, any one skilled in making cast-iron railroad car-wheels in view of this specification must see that the object of the process is to relieve from and guard against hurtful strain, without destroying the chill, and that heat is applied only for that purpose. It requires no particular science or skill to enable an operator to perceive that the moment all parts of the wheel are raised from a point above where serious strain begins, and where yet the thick and thin parts are in different stages of cooling, to a stage where the degree of temperature of all parts is the same, and above the degree where serious strain commenced, the thing sought has been attained. Then the avowed purpose of the inventor has been accomplished. It would be most unreasonable to read the directions of the specification without reference to the object which they profess to have in view. The evidence is that the chill is formed while the casting is in the mould, and that the hurtful strains commence after the formation of the chill. Indeed, it is manifest there can be no strain until the chill is complete. It must be, therefore, that all the heat which is needed to relieve from the strain is that which suffices to raise the temperature of the thin part, or plate, to the degree at which the strain commenced  a lower *646 temperature than that which existed when the chill was formed. Hence an operator, in following the directions of the specification, would be taught by his practical knowledge that the instant all parts of the wheel had been heated to that temperature no more heat was needed.
And we do not think it a fair construction of the patentee's language to hold that it requires the heat to be raised in all cases to a degree only a little below the point of fusion. He does not attempt to give any more definite direction than that all parts of the wheel must be raised to the same temperature, suggesting in a parenthesis ("say, a little below that at which fusion commences"). He fixes a maximum. The heat must not reach the point of fusion, and the prescribed minimum is that degree where the heat of the different parts of the wheel is equal. Within those limits the degree is left to the judgment of the operator, and within those limits it is clear from the evidence that the process may be applied without injury to the chill. The proof is that it has been successfully applied in the manufacture of a vast number of wheels, and that failure has been very rare.
There are some witnesses who have testified that the Whitney process, as they understand it, would destroy the chill of the wheel. But they explain their understanding to be that the wheels are to be reheated to a degree far beyond what is required to relieve from strain, and thus heated for no purpose. They keep in sight the maximum limit, and approach near to that, overlooking entirely the minimum, and disregarding the single object of the process, namely, relief of the plate, or thin part of the wheel, from the strain caused by unequal contraction.
We are, therefore, of opinion that the patent is not void for want of utility, and that the specification sufficiently describes the process invented and claimed.
The remaining defence is a denial that the process conducted by the defendant is an infringement of Whitney's patent.
What the process of the defendant was is clearly set out *647 in a patent which he obtained on the 7th of May, 1861. It consists in placing in a pit the wheels as they are turned out of the moulds red hot, with a layer of charcoal beneath the lowest wheel, and a layer between each wheel as well as above the uppermost, and covering the pit with a perforated metal plate. The charcoal is ignited by the hot wheels, and just sufficient air is admitted to effect combustion of the coal. Thus the wheels are reheated and permitted gradually to cool. There are some minor details which it is unnecessary to mention. So far as relates to reheating the wheels and retarding the cooling by the application of additional heat, it is obvious that the process is substantially the same as that covered by the complainant's patent. The object is the same, and the mode of attaining it is in substance the same. The purpose of the charcoal interlaid with the wheels is avowed to be to heat them in the pit to a proper temperature, prolonging the heat and permitting them to cool gradually in a given time, said to be seventy-two hours, more or less, as may be found necessary for the annealing operation. The rapidity of combustion of the charcoal is regulated by a damper in the flue. And this process is followed, as the specification explains, that the different parts of the wheels may adjust themselves to each other, and accommodate the unequal contraction which results from the process of chilling. It is under this patent, and in accordance with its directions, that the defendant has prepared his car-wheels for market. As the object of the patentees is the same, relief from the strain incident to unequal contraction, the only inquiry is whether the object is attained by substantially the same means. The idea of Whitney was undoubtedly arresting contraction before any remediless strain had commenced, and regulating the progress of cooling so that all parts of the wheel may maintain an equal temperature at all stages of cooling. Manifestly the process of the defendant embodied the same idea, and carried it out by means identical in principle. It reheats the wheels when removed from the moulds to the chamber or pit. It prolongs the cooling in connection with the reheating, *648 and it subjects the rapidity of cooling to control of the operator. The form or structure of the furnace, chamber, or pit, is not claimed by either patentee.
It hardly seems necessary to resort to the opinions of experts in order to reach the conclusion that the process of the defendant is only formally different from that of Whitney, while the essential element of the two processes is the same. But the testimony of the experts examined, taken as a whole, clearly supports such a conclusion. It is true some of the witnesses testify that in their opinion the processes are different, but when they attempt to describe the difference they point out only matters which are merely formal, only variances in the mode of using the same process. On the other hand, several witnesses, entirely competent to apprehend the principle of the invention, and the devices for practically using it, have testified that the processes of the defendant and of the complainant are substantially the same in principle, mode of operation, and in the effect produced. We must, therefore, conclude that the charge of infringement made in the bill has been sustained, and that the complainant was entitled to a decree for an injunction and an account.
We come next to the consideration of the account stated by the master and confirmed by the Circuit Court.
The master reported that Mowry, the defendant, used Whitney's process in the manufacture of 19,819 wheels, and the account has been stated on that basis. For the use of the process in making these wheels the defendant has been charged with $91,501.86 as profits made by him (more than four dollars and sixty cents on each wheel), besides $19,984.21 interest upon such profits to the first day of August, 1868, and the further sum of $10,980.22, being interest from August 1, 1868, to August 1, 1870.
It is very obvious, in view of what the patentee himself stated, under oath, in 1862, when applying for an extension of his patent,[*] that the account has been erroneously stated. If *649 he was correct in this statement the profits arising from the use of his patent in manufacturing 19,819 wheels (valuing iron at the price proved to have been paid for it by the defendant) must have been less than $5500, instead of over $91,000, decreed in the Circuit Court  about thirty cents per wheel, instead of four dollars and sixty cents. It is not an unfair presumption that if the profit to the patentee was no greater than he claimed it was, it could not have been more when the invention was used by an infringer. Now, it is clear that Whitney is not entitled to receive more than the profits actually made in consequence of the use of his process in the manufacture of the 19,819 wheels. It is the additional advantage the defendant derived from the process  advantage beyond what he had without it  for which he must account. But he has been held liable far above this. The master reported, in the first instance, the difference between the cost of the wheels and the price for which they were sold as the profits realized by Mowry, thus charging him the profit obtained from the entire wheel, instead of that resulting from the use of Whitney's invention in a part of the manufacture; and this, though he found at the same time and reported that Mowry had built up his business before he commenced the use of Whitney's process; that the use of the process did not diminish the cost of making wheels, but increased it; that while he used the process he used the same quality of iron that he had used before, and made no difference in the weight or form of the wheels, or in their price, and that the wheels made by him before he commenced the use of Whitney's invention, and since he has abandoned it, have sold as readily and at the same prices as those manufactured by that process.
Exception was taken to the charge of the profit made by the entire manufacture of the wheel, including not only the selection and mixing of the iron, but its melting, pouring into moulds, forming the chill, removing from the moulds, and cleaning, as well as annealing and slow cooling; and the case was again sent to the master with instructions to inquire:
*650 First. Whether the wheels made and sold by the defendant had, or could have been made to have any market value without being subjected to the process patented to Whitney; and,
Second. If they had, or could have been made to have such value by any annealing or slow-cooling process, outside of the Whitney patent, how much additional value, if any, they derived from being subjected to that patented process.
Upon the findings (stated supra, 632  REP.) made by the master on this order, the court decreed against the defendant the entire profits made by him in the manufacture and sale of the wheels from beginning to end, not only the profits resulting from the reheating and regulated slow cooling in connection, but also those which may have resulted from mixing and melting the iron, casting in moulds, making the chill, and from the possible advance on the iron above its cost, with interest on the whole.
This we think was an error. The findings of the master justified no such decree. It must be conceded that the findings are incomplete, obscure, and in some particulars incongruous, but it is not a legitimate construction of them taken together, that the benefit which the defendant derived from the use of the complainant's invention was equal to the aggregate of profits he obtained from the manufacture and sale of the wheels as entireties, after they had been completed. It is as true of a process invented as an improvement in a manufacture, as it is of an improvement in a machine, that an infringer is not liable to the extent of his entire profits in the manufacture.[*] If the wheels made by the defendant would have had no market value above that of cast iron if they had not been annealed and slow cooled, the same may be said if they had been cast without a chill. The same principle, therefore, which gives to the complainants the aggregate profits of the entire manufacture would give the same profits to a patentee of the process of chilling, *651 if there were one, and as there are many processes in the manufacture, for each of which it is conceivable there might be a patent, and as every one of the processes is necessary to make a marketable wheel, an infringer might be mulcted in several times the profits he had made from the whole manufacture. We cannot assent to such a rule. The question to be determined in this case is, what advantage did the defendant derive from using the complainant's invention over what he had in using other processes then open to the public and adequate to enable him to obtain an equally beneficial result. The fruits of that advantage are his profits. They are all the benefits he derived from the existence of the Whitney invention. It is found that there were other processes by which the inherent strain caused by unequal cooling could be, and was prevented, counteracting which strain was the sole object of the complainant's invention, and a car-wheel could be prepared for similar service, valuable in the market, and salable at a price not less than was obtained for those which the defendant manufactured. The inquiry then is, what was the advantage in cost, in skill required, in convenience of operation, or marketability, in bringing car-wheels by Whitney's process from the condition in which they are when taken hot from the moulds, to a perfected state, over bringing them to the same state by those other processes, and thus rendering them equally fit for the same service. That advantage is the measure of profits. It is quite unimportant what name was given to the products of the processes, whether one could be called annealed wheels and the other could not, except so far as affected their marketability.
The record shows that the court overruled the alternative finding of the master, that if there is no infringement of the complainant's patent unless the wheels are subjected to the process of reheating  that is to say, if the process of slow cooling used in connection with reheating is old, and not a part of the complainant's invention, no part of the profit derived by the defendant from the manufacture and sale of the wheels was due to the use by him of that invention *652 One exception taken to this finding was that not only the entire process described in the patent, but each part of such entire process was the invention of the complainant, and the use of any material, substantial, and essential part of such entire process, the slow cooling being a substantial and material part, whereby only an improved chilled cast-iron railroad wheel could be made, and beneficial effects the same in kind if not in degree attained, that were attained by the complainant's entire process, is an infringement of complainant's patent, and the profits derived from the use of such material, substantial, and essential part, should be accounted for in this case. This exception the court sustained and thereby held that the defendant is chargeable with the profits he derived from slow cooling alone. We cannot assent to this. The patent is for an entire process, made up of several constituents. The patentee does not claim to have been the inventor of the constituents. The exclusive use of them singly is not secured to him. What is secured is their use when arranged in the process. Unless one of them is employed in making up the process, and as an element of it, the patentee cannot prevent others from using it. As well might the patentee of a machine, every part of which is an old and known device, appropriate the exclusive use of each device, though employed singly, and not combined with the others as a machine. The defendant was not, therefore, responsible for slow cooling alone, or for the profits he derived from it. He was liable to account for such profits only when he used slow cooling in connection with reheating in the manner described in Whitney's claim substantially, or when extraneous heat was employed to retard the progress of cooling. We have said that slow cooling is not claimed in the specification as the invention of the patentee. And it is found by the master that there are other modes of slow cooling, and even other modes of relieving against the inherent strain caused by unretarded cooling, than that practiced by the complainant and claimed by him. Though, therefore, slow cooling is an essential part of the complainant's process, it is an equally essential part of other processes which *653 the defendant was at liberty to use in preparing his car-wheels for market.
We add only that in our opinion the defendant should not have been charged with interest before the final decree. The profits which are recoverable against an infringer of a patent are in fact a compensation for the injury the patentee has sustained from the invasion of his right. They are the measure of his damages. Though called profits, they are really damages, and unliquidated until the decree is made. Interest is not generally allowable upon unliquidated damages. We will not say that in no possible case can interest be allowed. It is enough that the case in hand does not justify such an allowance. The defendant manufactured the wheels of which the complaint is made under a patent granted to him in 1861. His infringement of the complainant's patent was not wanton. He had before him the judgment of the Patent Office that his process was not an invasion of the patent granted to the complainant, and though this does not protect him against responsibility for damages, it ought to relieve him from liability to interest on profits.
DECREE REVERSED, and the cause remanded with instructions to proceed in accordance with the rules laid down IN THIS OPINION.
NOTES
[*]  See this statement, supra, in brackets, beginning at foot of p. 631.
[*]  Jones v. Morehead, 1 Wallace, 155; Seymour v. McCormick, 16 Howard, 480.
