1To Benjamin Franklin from Rodolphe Valltravers, 25 April 1777 (Franklin Papers)
Gavin R. de Beer, “Rodolph Valltravers, F.R.S.,” Royal Soc...
2To Benjamin Franklin from Valltravers, 21 September 1777 (Franklin Papers)
...been trying to sell for years: Gavin R. de Beer, “Rodolph Valltravers, F.R.S.,” Royal Soc...