Wash and remove stalks from blackberries and put into pan with 1/4 pint water.
Simmer slowly until fruit is soft then pass all through a sieve.
Peel, core and slice apples, add remaining water and cook until tender.
Mash with a spoon, add the blackberry juice.
Weigh the pulp and add an equal quantity of sugar.
Stir, bring to the boil and simmer until jam sets when tested on a cold plate.
Pot and cover at once.
If the pan is weighed before cooking is begun, the weight of pulp can easily be calculated.