"X-Files" - USPTO Style: The Patent and Trademark Office has issued nearly 7 million patents. The first 10,000 are known as the X-patents. They were issued from July 1790, when the U.S. patent system was created under an order signed by George Washington, to July 1836, when every one of them burned in a fire.

Virtually every patent is available to the public on paper, micro-fiche, CD-ROM and the Internet -- except the X-patents.

In the 168 years since the fire, copies and records of only about 2,800 have been recovered. Over time, the appearances of missing X-patents grew fewer and farther between. Now no one at the patent agency, which does not have an official historian, can remember the last time one surfaced.

Until this spring, that is. That's when two lawyers with a passion for patent history uncovered a clue to several important patents from the 1790s -- including one from 1826 for the first internal combustion engine. Following the trail to Dartmouth College, they discovered inventors' copies of 14 patents that had been written off as lost forever.