Daguerre met with Niépce and entered into correspondence with him. Niépce had invented an early internal combustion engine (the Pyréolophore) together with his brother Claude and made improvements to the velocipede, as well as experimenting with lithography and related processes. Their correspondence reveals that Niépce was at first reluctant to divulge any details of his work with photographic images. To guard against letting any secrets out before the invention had been improved, they used a numerical code for security. 15, for example, signified the tanning action of the sun on human skin ("action solaire sur les corps"); 34 – a camera obscura ("chambre noir"); 73 – sulphuric acid.