Autograph letter signed.
8vo. 1 page on bifolium. With autograph address verso.
To George Richardson Porter (1792-1852), head of the statistical department of the Board of Trade, introducing Col. Thomas Frederick Colby (1784-1852), director of the Ordnance Survey: "Allow me to introduce to you my friend Colonel Colby with whom I believe you have already been in correspondence on statistical subjects [...]".
Small marginal flaws as well as a small tear on the verso, not affecting the address, but touching a biographical note by a collector, identifying Babbage as the "inventor of the Calculating Machine".