To prepare watermelon rind: Trim green skin and pink flesh from thick watermelon rind; cut into 1 -inch pieces.
Dissolve salt in 2 quarts water and pour over rind.
Let stand 5 to 6 hours.
Drain; rinse; and drain again.
Cover with cold water and let stand 30 minutes.
Sprinkle ginger over rind; cover with water; and cook until fork-tender.
Combine sugar, lemon juice and 1 1/2 quarts water in a large sauce pot.
Boil 5 minutes; add rind and boil gently for 30 minutes or until syrup thickens.
Add sliced lemon and cook until the melon rind is transparent.
Pack hot into hot jars, leaving 1/4-inch head space.
Remove air bubbles.
Process 20 minutes in boiling water bath.