It is mutually agreed that the monthly hire due under Clause ……. is based upon the mean of the present Bank of England’s selling and buying rate for the U.S. dollar, viz: $ …… to the Pound Sterling. Should this mean rate fluctuate it is understood and agreed that the Sterling hire payments shall be adjusted upwards or downwards and the amount actually payable in Sterling shall be the hire calculated at the agreed rate in Sterling of ……. multiplied by $ ……. and divided by the new mean rate in force on the due date of payment.