B62-720. IVORY HYDROMETER RULE. Fisher. Slide rule used in conjunction with a hydrometer to determine the proof, or alcoholic content, of beverages. This is the pattern invented by John Dicas in 1780 which would become the standard US method. See Rees, pp. 234-236. Marked "Fisher Phila", presumably Martin Fisher who worked in the early 1800s. Ivory is only very slightly yellowed, with a repaired crack at one end. In a fitted leather pouch that is too supple to be 200 years old and has a modern snap fastener. Fine 200-400