Convert from fother [lead] to momme [Japan]

Unit Definition (fother [lead])
The fother is a traditional English unit of weight for lead. The fother, equal to 30 fotmals, was always a little smaller than a (long) ton. The original version seems to have been equivalent to 2160 avoirdupois pounds, and the version still being used in the nineteenth century was equal to 19.5 hundredweight or 2184 pounds.

Unit Definition (momme [Japan])
The momme [Japan] is a traditional Japanese weight unit corresponding to the Chinese mace. Jewelers continue to use the momme to measure the weight of cultured pearls; for this purpose it equals exactly 3.75 grams (about 0.132 ounce) or 18.75 carats. The unit is commonly pronounced "mommy" in English.