Captain Edwin Hewgill, it turns out, is a significant figure in air gun history. This realization comes from the work of "THE AUTHOR OF THE "ACCURATE AND IMPARTIAL NARRATIVE" by S. G. P. Ward. According to Ward, Hewgill is the author of this work and, after some more research, I completely agree. Hewgill is the military secretary for the Duke of York during the 1793 campaign.

It is in Accurate and Impartial Narrative ( http://babel.hathitrust.org/cgi/pt?i...33433112065192 ) that there is a comment about air guns being used by the Austrian army at the time. Specifically at the Siege of Valenciennes in July 1793.

p. 52
Air guns were made use of at that period by a corps in the service
of his Imperial Majesty, constructed on a new and curious principle of
mechanism; the ramrods acling upon them as pumps, to load the bar-
rels. The inventor, if he still exists, is kept, as it is said, in close con-
finement at Vienna, lest the secret should by any means transpire.

There's an interesting variation of the description in what appears to be an earlier edition. ( http://books.google.com/books?id=A_FbAAAAQAAJ )

p. 34
"Amongst the various troops employed during the siege, there was a corps armed with air guns, on a particular construction; the ram-rods acting as pumps, to load them with a sufficient quantity of air, to discharge several balls. The inventor, is said to have been confined for life by the Emperor's orders, least so valuable a secret should transpire."

So, this gives credit to Captain Hewgill as the first writer, in English, to describe the Austrian Military Air Rifle the Girandoni, even though he gets some of the operational details wrong. That Hewgill does get what's going on with the air rifles wrong, I think is a strong indication that he actually observed the Austrians with their airguns. He just didn't understand exactly what they were doing when he saw them pumping up the air tanks.

That Hewgill would be interested in what the Austrians were doing with airguns can be explained by a previous book that he published, a translation of Tierce's "Field Engineer" in 1789 http://catalog.hathitrust.org/Record/002022016 Full title is, The field engineer; or, Instructions upon every branch of field fortification: demonstrated by examples which occurred in the Seven years war between the Prussians, the Austrians, and the Russians; with plans and explanatory notes. Translated from the 4th ed. of the German original of J.G. Tielke ... by Edwin Hewgill

Hewgill appears to be at the very beginning of the great English airgun era of 1795-1830.

Hewgill also did a series of prints that are well worth checking out.