Clause 5 is not uncertain in respect of the portion of the land to which it applies. This would be ascertainable by evidence in case of dispute. The clause requires that the lease should contain terms binding the Shell Co., if it be the lessee, to use the portion of the land to be the subject of the lease for the sale of its products which are mentioned. Of course such terms are not sufficient to constitute a lease. The controversial words of the clause are those which say that the lease is also to be "upon such reasonable terms as commonly govern such a lease". The meaning of the word "lease" is not uncertain. But it cannot be held that the words quoted are sufficient to create a legally enforceable promise to grant a lease to the Shell Co. unless you can gather from such words that Whitlock and Brew had a common intention as to what would be the "terms" of the lease. In order to have a lease there must be, in addition to the parties and property, an ascertainable period for its duration, an ascertainable rent, and an ascertainable point of commencement. On behalf of Brew there is an admission that it can be inferred that the lease which Brew covenanted to grant, if accepted by the Shell Co., would be expressed to commence when Brew went into possession under the contract of sale. If Brew observed the covenant and the Shell Co. entered into a lease with him there would be no interruption in the sale of their products. The question is whether the words quoted are sufficient to indicate a consensus between Whitlock and Brew on the duration of the proposed lease to the Shell Co. and rent. I think we should presume that both Whitlock and Brew had in mind when they made the contract that the Shell Co. took leases of premises and under such leases acquired rights in respect of the sale of its products on the premises and that there were "terms" which were "common" to such leases. As Brew contended the words of the covenant were uncertain, the onus was on him of showing that there were no "terms" which were "common" to leases which the Shell Co. took of premises on which to sell its products, or that the company did not take such leases if that was the case. As regards the duration of the proposed lease, it is clear that the purpose of cl. 5, which must have been as well understood by Brew as Whitlock, was to afford some measure of protection to Whitlock in case he incurred liability to the Shell Co. under the indemnity which is mentioned above. The covenant would serve to require Brew to preserve the exclusive rights which the Shell Co. had enjoyed in respect of the petrol filling station on the land, the maintenance of which was very important to Whitlock by reason of the indemnity. In the light of the circumstances known to both parties surrounding the insertion of cl. 5 in the contract it is right, I think, to draw an inference that it was the common intention of the parties that the duration of the lease would correspond with the unexpired period of the indemnity, which Whitlock gave to the Shell Co., just as it is right to draw an inference that the common intention of the parties was that the proposed lease would commence when Brew went into possession under the contract. I think it is erroneous to hold that cl. 5 is incomplete and void on the ground that it does not mention the date of commencement or the period of duration of the proposed lease. It is enough as between Whitlock and Brew that it can be inferred from the circumstances, which explain the insertion of the covenant in the contract, when and for how long the parties may be presumed to have intended. The word "terms" where secondly used in cl. 5 should, I think, be read to include a term providing for the payment of rent by the Shell Co., because such a term could be assumed without question to be common to the leases referred to. The amount of rent is not mentioned. But it is sufficient to make the covenant certain on this matter that the parties expressly agreed that the amount of rent is to be reasonable. The final sentence of cl. 5 is not part of the covenant. The arbitrator is not to complete the covenant or to supplement any defect in it. That is not the purpose for which the arbitrator is appointed. The thing which he is appointed to do is merely to resolve "any dispute between the parties as to the interpretation or operation of this clause".