Wash the watermelons thoroughly under running water and put on a towel to remove the remaining water. Cut the watermelons into 4 pieces and place in a saucepan, allowing the juice to flow freely into it. Remove the pulp from the rind with a wooden spoon and pass it through a sieve, chopping and removing the seeds. Strain the resulting mass through a gauze folded several times, pouring the resulting juice into a pan for cooking. Bring the juice to the boil, stirring continuously with a spoon, and remove from fire. To keep the thickened watermelon juice transparent, constantly skim off the foam with a slotted spoon. Strain the juice through the gauze. Start heating again and simmer until done.