Remove the seeds of the cantaloupes and scoop out the edible portion of the pulp. Dice the fruit coursely and simmer, uncovered, in a preserving kettle until tender; or puree the pulp in a blender. Measure the pulp; there should be 4 cups. Return pulp to the preserving kettle and add the sugar, lemon juice, and nutmeg. Cook the mixture uncovered over medium heat, stirring constantly, until the butter is thick enough to spread, about 1 1/2 hours. Ladle into hot, sterilized jars and seal immediately.