But a further question is raised by the provision contained in the mortgages given to the respondent by the appellant in the present appeal. That provision does not stipulate absolutely for payment in gold currency. It stipulates for the performance of alternative duties by the mortgagor, the appellant. It calls upon him, either to pay in gold the sum which is specified, in the case of principal, and, in the case of interest, which is ascertainable by calculation, or to pay in other currency another sum based on the first sum and ascertained by reference to an external standard or event, viz., market or exchange rate. But these alternatives are prescribed as the methods of discharging a main obligation to pay money sums. The question arises whether, notwithstanding that the provision is expressed to govern modes of payment, it should not be considered as a modification or qualification of the tenor of the main covenants, so that no obligation is undertaken to repay the specified sum of principal and none to pay the ascertainable sums for interest. The tenor of the obligation in the case of principal and of the obligation in the case of interest is but to do one or other of two things, either to pay an amount of gold sovereigns, or to pay some other amount ascertained by reference to the market value of the currency tendered, which, perhaps, when applied to the existing state of the law is equivalent to saying Australian notes. Upon this interpretation of the obligation, independently of the form of currency tendered, there would be no ascertainable sum susceptible of payment, no debt. In the case of the Société Intercommunale Belge d' Electricité[27] Lawrence L.J. addressed himself to the question whether the bond in that case secured a principal sum of £100, or an amount to be ascertained by adding to that nominal amount a further sum in sterling equivalent to any decrease in the gold value of the same nominal amount as compared with the gold value of the same nominal amount at the earlier date specified in the bond, and he concluded that the principal sum secured was £100 sterling and that the words did not measure the amount of the liability but merely indicated the mode of payment. This is something different from enquiring whether two alternative sums are provided, differing according to the medium of discharge, and it does not follow that his Lordship was of opinion that, in such a contract, payment of the lower of the two amounts in any legal tender would not suffice. In any case, it is clear upon the construction of the provision itself that the choice between the alternative modes of performance lies with the mortgagor, the appellant (Reed v. Kilburn Co-operative Society[28]), and that the first alternative is the payment of gold coins which at the time of payment are current as legal tender in respect of debts expressed in sterling. It may be remarked that, although, no doubt, the parties anticipated, and with every likelihood of correctness, that no gold coins would be legal tender in New Guinea except sovereigns of the weight and fineness established under the then existing law, yet, strictly speaking, the contract does not adopt this standard but simply prescribes whatever gold currency for the time being may be legal tender for sterling debts. The second alternative must be understood as contemplating a payment in some other form of currency which is lawful money or legal tender in New Guinea (see sec. 7 of the Coinage Act 1909). It follows from these considerations that what the tenor of the provision requires is payment in one form of currency of an amount which at a future date would discharge a sterling debt of a determined or determinable amount, or else payment in another form of currency of an amount equal to the commodity value of the first. The amount of that sterling debt appears from what in form is an express covenant to pay sums of money. It may be conceded that the instruments should each be construed as a whole and that the express covenant, divorced from whatever qualification upon its effect is contained in the provision relating to currency, should not be treated as conclusively establishing a debt in the sums determined by or under it. But, from its very nature, this provision cannot operate except upon the hypothesis that a sum certain in sterling is first ascertained as a debt which must be met in sterling currency. Only when such a debt becomes payable could its intended operation commence. That intended operation is to defeat the legal equality in the discharge of obligations which is given to all forms of legal tender of the same denomination. It is, therefore, ineffectual to require a payment of more than £900 in respect of the half-yearly interest under the mortgages.