Wendell the Wizard needed exactly liters of de-alcoholized blueberry wine for his magical romance potion. Wendell had a huge cask of the wine, but he lacked proper measuring devices. Wendell did have two bottles. One was a one-litre measure marked into divisions by 7. The other was a one-litre measure marked into divisions by 8. How could Wendell get exactly liters of the wine into his bubbling iron cauldron to finish the potion?