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.
A pretty pink-colored jam is made with the pink part of watermelons. Out the pink part of watermelon up into cubes, and place in a preserv ing kettle with white sugar, using three pounds of sugar to each six pounds of the fruit, and the juice of two oranges or lemons. Stir frequently to prevent burning, and boil to the consistency of jam, and fill into glasses, and seal Since watermelons contain so much water, one will have to use a good quantity of fruit to make a small quantity of jam; but after trying it once you will always want a few glasses of watermelon jam on your preserve shelves.