delivered the opinion of the court.
It is first contended by appellant’s attorneys, that this court has now no option under the remanding order, except to make a finding of the ultimate facts, and upon such finding enter a new judgment in favor of appellant in accord with its former action, or else to reverse the judgment of the trial court and remand the cause for that court to enter such *95judgment in accordance with the opinion heretofore rendered. (93 Ill. App. 13.)
It is apparent, we think, from that opinion, that this court did not find the material facts differently from the trial court; but rather that it then — erroneously as it now seems —regarded the agreed statement of facts made by the parties as settling such ultimate facts as were necessary to the determination of the cause; and that it reversed the judgment of the trial court for what were regarded as errors in the application of the law to the facts. That such was the ground of reversal is the presumption also of law where there is an omission to make a finding of facts by the Appellate Court. (Linseed Oil Co. v. Heath & Milligan Manufacturing Co., 191 Ill. 75, 77.) If this court had been correct in the view that the parties had agreed upon such ultimate facts as are material, the questions before the Appellate Court on appeal would have been only questions of law, and no. recital of facts found in the judgment of this court would have been necessary upon appeal or writ of error to the Supreme Court from the order of reversal and entry of final judgment here. (Linseed Oil Co. v. Heath & Milligan Manufacturing Co., supra, p. 78.) The Supreme Court having now given its opinion to the effect that the said agreed statement of facts did not constitute “ an agreement as to the ultimate or substantive facts upon which the determination of the case must depend,” it is the duty of this court to do what it now appears should have been done before, viz., recite the facts as found, or remand the cause to the Circuit Court, if we are still of opinion that the judgment must be reversed for errors of law. Purcell Co. v. Sage, 189 Ill. 79-82.
It is true the Appellate Court can not sit to review its own decisions, except in case of a petition for rehearing. It is, however, only “ final orders or judgments of the court,” which “ are as between the parties to the suit res adjudícala, and binding upon them whenever they again arise in that court between the same parties upon the same state of record.” (Henning v. Eldridge, 146 Ill. 305-309.) In the *96present case there is now no such final order or judgment of this court in force or effect. "W e are therefore at liberty, and it becomes our duty under the mandate of the Supreme Court, to consider Avhether the cause shall now be affirmed, or reversed, or reversed and remanded for another trial, as if it bad ne\er been here before. We have accordingly permitted the parties fco file additional briefs and have given a hearing upon oral argument. The case has thus been more fully presented and our views upon the questions involved have been in some respects modified.
It is first contended by appellant’s counsel that the money paid by it upon the contracts which have been fully performed by delivery of oil thereunder, and by payment of the contract price, was paid under a mistake of fact and can therefore be recovered back. It is no doubt true that money paid under a mistake of fact may be recovered back in an action of assumpsit. (Commercial Union Assurance Co. v. Scammon, 133 Ill. 627-632; Wolf v. Baird, 123 Ill. 585-690.) Hot so, however, when the money has been voluntarily paid upon a mistake of law, and Avithout fraud or mistake of fact. “ It seems to be Avell settled, as between individuals, that where money is voluntarily paid under a mistake as to the law and under a claim of right, it can not be recovered back.” (The People v. Foster, 133 Ill. 496-509, and cases there cited.) In Elston v. City of Chicago, 40 Ill. 514-518, it is said by Justice Breese: “Ho case can be found where money has been voluntarily paid with a full knowledge of the facts and circumstances under which it Avas demanded, Avhich holds that it can be recovered back upon the ground that the payment was made under a misapprehension of the legal rights and obligations of the party paying.” (See also Falls v. City of Chicago, 58 Ill. 403-407; Walser v. Board of Education, 160 Ill. 272-276; Fowler v. Black, 136 Ill. 363-378.) In Kerr on Fraud and Mistake it is said (p. 401): “It appears, from the authorities, to be established in equity as well as at law, that money paid under a mistake of law with full knoAvledge of the facts is not recoverable; and that even a promise to pay upon a sup*97posed liability and in ignorance of the law will bind the party.” In Pekkins v. Eaton, 3 N. H. 152, it was held in the case of a wager, which was not a legally binding contract, that when the money has been paid over to the winner, no action lies to recover it back. In Nicholls v. Leeson, 3 Atkyns, 573, Lord Hardwicke held, where an annuity had been given by will and made a charge upon property left by the testator, and the annuity had been paid for any length of years without deduction for land tax, although subject to such tax, it will be presumed, in the absence of fraud, that the amount has been paid by mutual consent on both sides, and the payer can not recover back the overpayment.
Were the payments, made upon the first twenty of the contracts now in question upon which deliveries had been completed, and which were fully performed before appellant claimed the right to receive the statutory 7.761 pounds of oil for a gallon instead of 7.50 pounds, made upon a mistake of fact or a mistake as to the law ? It is stipulated that these payments were made by appellant before it discovered that the statutory gallon weighed 7.761 pounds, and “ under the belief that a gallon of oil actually weighed but 7.50 pounds.”
This belief was merely ignorance of the law which specifies what the weight of a statutory gallon of oil shall be. It. is not claimed that appellant was deceived or fraudulently imposed upon. The stipulation of facts states that it was the custom among oil manufacturers, including appellee, “ to charge 7.50 pounds as a gallon of linseed oil in deliveries made -by them. The Heath & Milligan Manufacturing Company knew this custom, and they also knew that in the-various deliveries made under said contracts such deliveries were made on a basis of 7.50 pounds to the gallon;” but they did “not know that a statutory gallon of oil weighed 7.761-pounds.” It thus appears that appellant knew when it contracted for the oil under said contracts that it would receive-only 7.50 pounds for a gallon, and made its contracts upon that basis; knew both when it agreed to pay and when it *98paid the price per gallon as specified, that it was to pay and was paying that price for as many times 7.50 pounds, as there were gallons called for by said contracts. It got what it paid for, and all it paid for. The only thing appellant did not know was that a statutory gallon weighed 7.761 pounds, and that under the letter of said contracts it would have been entitled to receive statutory gallons, instead of what, for distinction, we ma) call customary gallons, or, using the term employed by counsel, “commercial gallons.” This was clearly ignorance of the statute and of legal rights under the statute; a mistake of law but not of fact. It follows that the money thus voluntarily paid by appellant upon such contracts so far as completed, was paid without fraud, and not only without mistake of fact, but with full knowledge of the facts. Such payments can not be recovered back. It is not a mere presumption in this case, but is an agreed fact that the amounts referred to were agreed upon as the price to be paid for the quantity of oil actually delivered and received by mutual consent. Having so consented and ratified its consent by payment therefor with full knowledge of the facts, appellant was not entitled at the time it brought suit to receive the difference between 7.50 and 7.761 pounds of oil for a gallon on said completed contracts and deliveries thereunder, and is not entitled to recover in accordance with the second of the concluding paragraphs of the stipulation quoted in the preceding statement.
Was the Heath & Milligan Manufacturing Company entitled to receive 7.761 pounds for a gallon upon the remaining two contracts which were only partially performed at the time it discovered that a statutory gallon contained that quantity of oil ? It is stated that the last two of the said contracts between the parties were being performed at the time appellant claimed shortage. So far as such contracts had been carried out and payments made for oil delivered in pursuance thereof, what has been previously said is fully applicable. But by such acceptance of the deliveries made on said partially performed contracts and by payments there*99under upon the basis of 7£ pounds to a gallon, the parties had by mutual consent placed their own construction upon the meaning of the word “ gallon ” as in said contracts used. Thus they had settled the construction of the contracts in progress of execution by agreement among themselves, and are bound by their own agreement. It is not a question of the legal meaning of the word “ gallon.” As to that the statute is conclusive. But the question is, what meaning did the parties themselves give it; what did they intend by it; and by their agreed statement of facts and by their construction as shown by their uniform course of conduct, they have construed it as meaning 7-|- pounds of oil. It was too late to change their interpretation so far as the partially performed contracts were concerned after they had entered on performance and had by their conduct ratified the meaning previously accepted, with reference to which said contracts were made. That they were so made is shown by the stipulation of facts. It is stated that appellant knew the custom of charging 7-| pounds of oil for a gallon. The knowledge of this custom is evidence that the parties entered into their contract in contemplation of such usage. Hutchins v. Webster, 165 Mass. 439-441; Corbett v. Underwood, 83 Ill. 324-327-328. There is no reasonable doubt as to the intention on both sides that the word “gallon” as used in said contracts, should mean 7£ pounds, and such intention must control. It follows that appellant received all it was entitled to receive under said contracts, and the judgment of the Circuit Court was therefore correct and must be affirmed.