Wipe damsons, and prick each fruit five or six times, using a large needle; then weigh. Make a syrup by boiling three-fourths their weight in sugar with water, allowing one cup to each pound of sugar. As soon as syrup reaches boiling point, skim, and add plums, a few at a time, that fruit may better keep in shape during cooking. Cook until soft. It is well to use two kettles, that work may be more quickly done, and syrup need not cook too long a time. Put into glass or stone jars.