delivered the opinion of the Court.
A bill of discovery in aid of an action at law was dis^ missed by the ^District Court, 56 F., (2d) 272, but upheld upon appeal by the Circuit Court of Appeals. 62 F. (2d) 663. The question is whether the bill is good upon its face.
The respondent, Jenkins Petroléum Process Company, loaned to the petitioner, Sinclair Refining Company, then known as the Cudahy Refining Company, an experimental still for cracking petroleum oils to produce gasoline. By written agreement, any improvements developed as the result of the work of the petitioner’s engineers and experts in making themselves familiar with the Jenkins apparatus or process were to belong to the respondent. The petitioner, or its predecessor, undertook, so far as it was able, to cause its employees to execute applications for patents in the United States and elsewhere in order to protect such improvements, and to assign the applications to the petitioner together with the improvements affected thereby.
In January, 1921, the respondent filed a bill in equity for specific performance. Isom, an employee of the peti *691 tioner, applied for a patent on September 10, 1917, and when the patent was issued on November 19, 1918, assigned it to his employer. The respondent made claim to the patent on the ground that it was an improvement of the Jenkins invention. After a trial upon the merits, the bill for specific performance was dismissed, the court holding that there was doubt whether Isom’s invention was the outcome of his use of the Jenkins apparatus, or of independent thought and knowledge. 32 F. (2d) 247. There was an appeal from the decree to the Circuit Court of Appeals for the First Circuit. That court invoked the principle that to uphold a decree for specific performance there must be clear and convincing evidence, and not merely such evidence as would sustain a recovery at law. In that view the decree was affirmed in so far as it dismissed the prayer for equitable relief, but the dismissal was coupled with a direction that the cause be transferred to the law side of the court, the plaintiff to have leave to amend by turning his cause of action into one for the recovery of damages. 32 F. (2d) 252.
The action at law is now at issue, and the plaintiff prays for a discovery. It alleges in its bill that the evidence of the facts to be discovered is contained in voluminous books and documents which could not be inspected or proved upon a trial at law for damages without confusion and delay. Discovery is demanded as to the number of cracking stills constructed by the defendant under the Isom patent; as to the extent and time of operation; and as to the amount of gasoline and other petroleum products yielded thereby, with an inspection of the relevant designs and drawings. The District Court granted a motion to dismiss the bill, placing its decision upon two grounds, (1) that a bill of discovery will not lie when the facts to be discovered relate to damages only, and (2) that the value of the patents has no relation to the sales of the patented device, and that evidence of such sales
*692
would be inadmissible if offered. The Court of Appeals reversed, one judge dissenting. In the view of the majority of the court, the damages to be recovered in an action at law may be proved by resort to a discovery in equity, if the ancillary remedy is reasonably necessary to advance the ends of justice.
Munger
v.
Firestone Tire & Rubber Co.,
*693 The remedy of discovery is as appropriate for proof of a plaintiff’s damages as it is for proof of other facts essential to his case.
Help for the solution of problems of this order is not to be looked for in restrictive formulas. Procedure must have the capacity of flexible adjustment to ¡changing groups of facts. The law of discovery has been, invested at times with unnecessary mystery. There are few fields where considerations of practical convenience should play a larger role. The rationale of the remedy, when used as an auxiliary process in aid of trials at law, is simplicity itself. At times, cases will not be proved, or will be proved clumsily or wastefully, if the litigant is not permitted to gather his evidence in advance. When this necessity is made out with reasonable certainty, a bill in equity is maintainable to give him what he needs. Equity Rule 58. There were other reasons in times past, when parties were not permitted to be witnesses, and when there was no compulsory process for the production of books or documents.
Carpenter
v.
Winn,
To state the function of the remedy is to give the password to its' use. There are times when a suit is triable in separate parts, one affecting the right or liability, and the other affecting the measure of recovery. In suits of
*694
that order a discovery as to damages will commonly be postponed till the right or liability has been established or declared.
Schrieber
v.
Heyman,
63 L.J.Rep. 749 (1894);
Elkin
v.
Clarke,
A different situation is presented where the action is at law and is triable by judge and jury. There interlocutory judgments are unknown, at all events where the defendant has answered generally and not by special plea, and the verdict establishing the right establishes at the same time the amount tó be recovered. The answer being general, the case according to common la--.- practice must be tried as a unit and nqt broken into parts. In such circumstances damages may be proved with the aid of a discovery, if the complication of accounts or other practical impediments make it necessary that the evidence be sifted in advance.
Pape
v.
Lister,
L.R. 6 Q.B. 242;
Saunders
v.
Jones,
L.R. 7 Ch. Div. 435, 452;
Elkin
v.
*695
Clarke, supra; Sherwood Bros.
v.
Yellow Cab Co.,
283 Pa. St. 488, 491, 492;
Munger
v.
Firestone Tire & Rubber Co.,
The petitioner is not helped by our decision in
United States
v.
Bitter Root Development Co.,
Loose v. Bellows Falls Pulp Plaster Co., supra, also cited by the petitioner, has a dictum approving the decision in Munger v. Firestone Tire & Rubber Co., supra, but the point was not involved, for the plaintiff was not asking for discovery as an auxiliary remedy in furtherance of his remedy at law, but was attempting, like the complainant in the Bitter Root cáse, to subject the entire suit to the jurisdiction of equity
To hold that the plaintiff in an action at law may have discovery of damages is not to say that the remedy will be-granted as of-course, or that protection will not be given to his adversary against impertinent intrusion. Wigram, supra, § 115. The court may decline to open *697 the defendant’s records to the scrutiny of a competitor posing as a suitor, if the suit has been begun without probable cause or as an instrument of malice. It is all a matter of discretion. Good faith and probable cause were here abundantly established. The remedy of specific performance had been refused, but the very court that refused it had found sufficient merit in the suit to call for an amendment of the pleadings that would give the plaintiff an opportunity to maintain a remedy at law..
■The decree under review protects the petitioner with sedulous forethought against an oppressive inquisition. Only the most general facts are to be discovered in advance- of trial. The bill is to be retained, however, to be available in case of need. If the occasion for fuller scrutiny shall afterwards develop, there may thus be an inspection of the records without the delay that would be inevitable if a new bill had to be filed with a new opportunity to the defendant to answer or demur. Presumably the jury could be held together in the interval, and the trial at law adjourned. This relief may have been less than the plaintiff should have had. It was certainly not more.
The use that has been made of the patented device is a legitimate aid to the appraisal of the value of the patent at the time of the breach.
This is not a case where the recovery can be measured by the current prices of a market. A patent is a thing unique. There can be no contemporaneous sales to express the market value of an invention that derives from its novelty its patentable quality. Cf.
United States
v.
Swift &
Co.,
Ithaca Trust Co.
v.
United States,
Value for exchange is not the only value known to the law. of damages.- There are times 'when heed must be given to value for use, if reparation is to be adequate.
Barker
v.
Lewis Storage & Transportation Co.,
Section 724 of the Revised Statutes permitting the use of a subpoena duces tecum for the production of books and papers has not superseded the remedy of dis *700 covery in cases where inspection during the trial and in the presence of the jury will produce delay or inconvenience. Carpenter v. Winn, supra; Pressed Steel Car Co. v. Union Pacific R. Co., supra.
The court did not exceed the bounds of a legitimate discretion in holding that these embarrassments might reasonably be expected to follow if discovery were refused.
The decree should be
Affirmed.
