General Average  Salvage and Salvage Charges recoverable under the insurance on hull and machinery but not recoverable in full by reason of the difference between the insured value of the Vessel as stated therein (or any reduced value arising from the deduction therefrom in process of adjustment of any claim which law or practice or the terms of the insurance covering hull and machinery may have required) and the value of the Vessel adopted for the purpose of contribution to general average  salvage or salvage charges  the liability under this insurance being for such proportion of the amount not recoverable as the amount insured hereunder bears to the said difference or to the total sum insured against excess liabilities if it exceed such difference