Slice the lemons as thin as possible. Discard ends. Remove all seeds and tie them in a square of doubled cheesecloth. Put lemons and seed bag in a nonreactive bowl with enough water to cover. Let stand overnight.
Measure the lemons and water into a wide, shallow, nonreactive pan. Add an equal volume of sugar and cook over low heat until sugar is dissolved. Raise heat to medium-high and cook, stirring frequently and skimming off the foam as it rises, until temperature reaches 220F, about 1/2 hour. Remove marmalade from heat.
To test for consistency, drop a little marmalade on a saucer and put the saucer into the freezer until marmalade is cold, about 5 minutes. Tip the saucer: the marmalade should just barely run. If too thin, return the marmalade to medium-high heat and cook, testing often, until it has reached the right consistency.
Put marmalade into hot, sterilized pint or half-pint jars. Store in refrigerator up to 1 month or, for longer storage, seal according to reliable canning instructions.
Recipe from Cook's Magazine, November/December, 1987.