Jenkins, J., delivering the opinion.
These cases having been returned to the same term of the Court, and depending upon the same rules of law, were consolidated for the purposes of the argument, and the following opinion governs both:
On the 16th of April, 1862, the Congress of the Confederate States passed an Act, entitled “ An Act to further provide for the public defense,” whereby the President is authorized “ to call out and place in the military service of the Confederate States for three years, unless the war shall have been sooner ended, all white men who are residents of the Confederate States, between the ages of eighteen and thirty-five years at the time the call or calls may be made, and who are not legally exempt from military service.” By the ninth section of that Act it is provided, “that persons not liable for duty may be received as substitutes for those who are, under such regulations as may be prescribed by the Secretary of War.” On the 26th of April, 1862, the Secretary of War made, and through the Adjutant and Inspector General published, a General Order, No. 29, containing regulations, providing for the acceptance of substitutes, and the discharge of persons furnishing them. And again, on the 29th of the same month, he made and published in like manner another (explanatory of the former,) one section of which is in the following words: “No person other than those expressly named or properly implied in the above Act shall be exempted, except by furnishing a substitute exempt from military service, in conformity with the regulations already published, (General Order, No. 29,) and such exemption is valid only so long as the substitute is legally exempt.”
*418Subsequently to the passage of said Act, and the making and publication of said regulations, one oí the applicants for discharge having volunteered, and the other having been enrolled for service, offered each a substitute, (to-wit, Farrell on the 17th May, and Williams on the 12th September, 1862,) which substitutes were accepted, and the persons furnishing them severally discharged. On the 27th September, 1862, the Congress passed an Act amendatory of that of the 16th April, authorizing the President to call out and place in the military service of the Confederate States persons between the ages of thirty-ñve and forty-five years, and otherwise answering the description given in the previous Act. The substitutes furnished by Farrell and Williams were between the ages of thirty-five and forty-five years when accepted as such, and still are so. They, therefore, were not liable (in their own persons) for service under the first Act, but are so under the second.
After the passage of the Act of 27th September, - Farrell and Williams were again enrolled for service, and denying their liability, each sought a discharge by habeas corpus from the custody of the Commandant of Conscripts. Whether or not, under the circumstances stated, they are liable to this second enrollment is the question we are to consider.
They claim exemption solely in virtue of the discharges granted them severally upon the acceptance of their substitutes.
In their behalf, it is insisted that the substitution authorized by the ninth section of the Act of 16th April, is co-extensive with the term of their enrollment, viz: “ for three years unless the war shall have been sooner ended,” and that their discharge covers the same time. They deny the validity of the regulation made by the Secretary of War in his order of 29th April, for that, it is not such a regulation as the Congress authorized him to make — that it is an act of legislation of which the Congress alone is capable. They further insist, that even if that regulation be binding upon them, a proper construction of it will save their exemption.
1. The first question to be considered is, whether the reg*419ulation of the 29th April is within the pale of the authority conferred on the Secretary of War by the Act of 16th April. How is this discretion to be ascertained; its limit to be defined ? By considering all the circumstances of the case — the position of those allowed to put in substitutes — the powers of Congress, and the relation in which the Secretary of War stands to the Congress, and thus seeking to evolve the intention of that body. The first section of the act imposes upon all persons therein described a legal duty — one of perfect obligation. The ninth section provides a conditional exemption from that duty. It follows, therefore, that the exemption is ex gratia, and not ex debito justitia. The party owing the duty, and having no claim to exemption, must take the latter, if at all, upon such terms as the Government, rightfully exacting the former, may choose to impose.
It will be observed that the entire legislation of the Congress, on this subject, is embraced in one short sentence: “Persons not liable for duty may be received as substitutes for those who are, under such regulations as may be prescribed by the Secretary of War.” It was certainly in the power of Congress to perfect the entire scheme of substitution, arranging all the details, and leaving no discretion to the Secretary. But they have chosen to do otherwise. The intention to confer upon that officer a large discretion is very apparent. There is but a single limitation affixed, viz: that substitutes shall be persons “not liable for duty”; and this is obviously not for the benefit of those desiring to put in substitutes, but for the public good. It may fairly be questioned, whether the discretion vested in the Secretary does not extend to the rejection by him of the entire policy. The language is permissive; substitutes “ may be received” — “ under such regulations as may be prescribed,” etc. May is generally regarded as a word of permission. In the construction • of statutes, may is held to mean shall in two cases, viz: where the thing to be done “ is for the sake of justice, or for the public benefit.” Dwarris on Statutes, 712. But the privilege of military substitution is for neither of those purposes. Had the Secretary failed to prescribe regulations, there could *420have been no substitution; yet there is no law directing that they shall be prescribed.
Waiving this view, however, it will scarcely be affirmed that the delegation to that officer of a large discretion in framing regulations to guard the service of the country against detriment from the abuse of the privilege, would be void. On the contrary, it would seem to be eminently fit and proper. The position of that officer gives him at all times a clearer view of the military necessities of the Government than the members of the Legislature can be supposed to have. He is always at his post, overlooking the entire field of operations, and discerning the changes made in those necessities, by constantly varying circumstances, whilst they are often in recess, pursuing their private avocations. If substitution be a gratuitous privilege, as we think we have shown, it results as a corollary from that proposition, that Congress, in granting it, may either impose terms, or empower an executive officer of the Government to impose them. If the terms be unacceptable to any party enrolled, he may decline the privilege and perform the duty. If he accept it, he must do so, cum onere.
2. But it is argued, that so far as the freedom of the substitute from liability for duty is a qualification, Congress has legislated, confining the qualification to the time of his reception, and that the Secretary cannot interfere with their action. As already intimated, we regard that as a legislative limitation of the discretion given to that officer, curtailing the privilege. It is ,a condition precedent, prescribed by Congress; but it does not limit the Secretary’s discretion in defining by his “regulations'’ contingencies, upon the happening of which the substitution shall cease, and the liability of the principal.. revive. Indeed, the condition precedent, contained in the * ninth section, is suggestive of the regulation now sought to be.evaded. The latter only extends throughout the substitution the spirit and intention of the former. Why was it required that the substitute be not liable for duty? Clearly to avoid the consequence of one man representing two, upon whom the Government had equal claims. But it is no less important to the public service to avoid this consequence, at *421any subsequent time, than at the inception of the substitution. Can it be supposed that Congress, in the circumstances surrounding them, did not consider the probable future necessity of calling into the service men aged more than thirty-five years? Could they have intended that a youth aged seventeen years and eleven months should be eligible as a substitute, for three years for a man of twenty-five? In the cases now before us, under further legislation, one of the substitutes became personally liable five months, and the other sixteen days, after the respective acts of substitution. We do not ask whether the continued exemption of their principals, for three years, or during the war, is consistent with the public interest, for we are not legislators. But, as expounders of the law, we do ask, whether it consists with the intent and meaning of Congress as expressed in the ninth section of the act of April 16 ? Our opinion is that the second section of the Secretary’s order of April 29, is a better, more faithful exposition of that intention, and in this we are sustained by the subsequent course of the Congress.
By General Order, No. 58, the Secretary of War required persons enrolled, and certified by competent authority to be unfit for bearing arms, to report to a camp of instruction, that they might be detailed to lighter duty, to which they were competent; but on the 11th October, Congress passed an Act annulling this order, and providing that such persons be not required to assemble at a camp of instruction. Why? Because the order did not accord with legislative intention. They have, however, neither annulled nor modified the regulation of 29th April, and the legitimate inference is, that it does accord with their intention.
Eor these reasons we regard that' regulation as a fixed legal condition of the privilege of substitution.
3. It is next insisted, that even in this view the regulation is inapplicable to these cases. The argument. is, that it applies only to cases in which the substitute when received was under eighteen years of age, and subsequently attained that age, or was then exempt by his office or employment, and afterwards left that office or employment, we can see no plau*422sible. reason for this restriction. The language used is not more applicable to a case in which the substitute ceases to be exempt under existing laws, by the efflux of time, or change of occupation, than to one in which he ceases to be exempt by further legislation. The only question for our consideration is, whether at the time of the second enrollment of the principal, the substitute had ceased to be exempt in his own person. In the cases before us none will question, that had the substitutes remained at home, enjoying their legal exemptions, these would have been terminated by the Act of September 27th, and as that Act contains no saving of persons then in the army by substitution, there is nothing to prevent the personal liability from attaching. Previously, each had occupied in the army the place of his principal, but, by the passage of that Act, a place was made for himself; and unless it can be implied that Congress contemplated the anomaly of one soldier performing the duty of two, the place of his principal became vacant, and the substitution was no longer valid.
4. The discharge of the parties in custody has been urgently demanded on the ground that the acceptance of the substitute, and discharge of his principal, was a contract between the latter, and the Government, and incapable of cancellation at the will of Congress; and further, that if this be not a contract, the transaction between the principal and the substitute certainly was so, and was induced by the legislation of Congress, whose good faith is pledged not to impair it.
As before remarked, we are inclined to consider the substitution more in the nature of a gratuitous privilege than of a binding contract.
But conceding (for the argument) that it is a contract binding on the Government, and further, that good faith inhibits the cancellation or impairment of the other contract between the principal and substitute, the question presents itself, what were those contracts ? They were for such substitution and such only as the Act of April 16th, and the regulations of the Secretary of War in pursuance thereof) *423allow. All the terms, conditions and stipulations found to exist in the Act, and in the regulations entered into and became parts of those contracts. One of the regulations existing at the time of each substitution, was that the exemption accruing from it to the principal should be “valid only so long as the substitute should be legally exempt.” All parties are presumed to know the law and the regulations referred to in it, and necessary to its practical operation, and from this presumption nothing can relieve them. Thus it appears that the contract of substitution has not been can-celled or annulled by subsequent legislation, but has expired by its own limitation.
It will be observed that we do not rest this opinion upon General Orders No. 82, made and published after the Acts of substitution we are considering, but upon the order of April 29th, which was anterior to them. Number eighty-two appears to be a digest of the several Acts of Congress relating to enrollment, exemption and substitution, and of the orders and regulations of the Department of War, for carrying them into effect. Some of the pre-existing orders and regulations are annulled and others modified by it. The order of the 29th April, is in substance repeated in number eighty-two, but it has been operative from the earlier date.
5. Lastly, it is argued that the discharges granted to the principals, upon the reception of their substitutes, in terms cover the entire period for which the principal was first enrolled, and which has not yet expired. Those discharges are not before us, but if they were, and were found to be for the war, or for all time, they would avail nothing.
The officers giving them were but agents of the War Department. The laws of Congress and the regulations of the Department, constitute their powers of attorney, and any act of theirs, transcending the powers thus given, and especially any act, violating the expressly declared intention of the •superior, is simply void. For these reasons we think there was error in the judgments of the Court below, ordering the applicants discharged.
Let the judgment be reversed.