This section is from the book "Preserving And Pickling", by Mary M. Wright. Also available from Amazon: Preserving and Pickling: Two Hundred Recipes for Preserves, Jellies, Jams, Marmalades, Pickles, Relishes and Other Good Things.
Cut up the watermelon into strips, and remove all the green rind and soft inner pulp. Cut into cubes and soak in weak brine overnight, allowing about one-half cupful of salt to a gallon of water. To each seven pounds of fruit use four pounds of granulated sugar, one pint of vinegar, one pint of water, several sticks of cinnamon, a bit of ginger root or lemon rind or both if desired. Place the sugar, water, and spices together in a kettle, and add the melon, and simmer until tender. Remove from the fire, and let stand several hours in the syrup. Put syrup over fire again and add the vinegar, and boil to a thick syrup, and pour over the melon in glass jars. Seal.