James Watt is most well known for his improvements to the steam engine, but his discovery of a method of letter copying became the basis of copying devices until the advent of the photocopier. In a letter to Dr Joseph Black (1728 to 1799) James Watt wrote as an addition:
“I have lately discovered a method of copying [letters] instantaneously, provided it has been written [____] or within 24 hours. I send you a specimen an[d I will] impart the secret if will be of any use to you.” [Gaps due to corner of letter missing. Letter. James Watt to Joseph Black 24 July 1778. MS 3219/4/127/6]
Presumably this would have been of interest to Dr Black as a Chemist. The copying press process worked by using modified ink in the original letter. A moistened piece of thin copying paper was laid over the original. The pressure from the press then bled the ink through the back of the copying paper, creating a mirror image of the letter which, when turned around, was a replica of the original.
The following year Watt petitioned for a patent and took Matthew Boulton and James Keir as partners in the business. [Letter. James Watt to Joseph Black 2 December 1779. MS 3219/4/127/13] The first presses were sold in 1780, and another letter to Dr Black notes the raising of the prices for the copying press to 6 guineas as the build had been “more expensive than we expected and the necessity of giving all the little implements along with it.” [Letter. James Watt to Joseph Black 1 March 1780. MS 3219/4/18/22] The image above is of the Partnership indenture of 1780, [MS 3147/18/2] and marks the beginning of James Watt & Company that sold these copying presses until 1840, twenty years after the death of its inventor.