(This is only a segment of my Chapter Four essay, but I am still finishing that up, and I think it can stand alone.)
As, forced from wind-guns, lead itself can fly,
And pond’rous slugs cut swiftly thro’ the sky…
–Alexander Pope, The Dunciad, Book I
The more I learn about the technological world of the Shadows of the Apt books, the more I realise that its most important inventions are not imaginary, but real-world dead-ends and paths-not-taken. Totho’s ‘air battery’, introduced in this chapter, is no exception: compressed air, as a way of powering weapons, is (surprisingly to me) not at all anachronistic to the 17th century Eurasian world I see as the main historical inspiration of the series.
Stenwold, coming across the battery in the workshop, is unfamiliar with its mechanism; he describes it as ‘a tube about as big as his fist’, with ‘some manner of pump within it.’ (61) It’s a new development since his artificing days. Totho clarifies:
‘You see, sir, there’s a chamber here with air in … see the one-way valve I’ve put in here … now it’s full and … you cock it like a repeating crossbow, with this lever here – just with your thumb, though, three or four times … and then you’ve put the air under pressure, lots of pressure … and then, with this lever here, you can release it all at once … and you produce almost as much force as a firepowder charge.’ (62, ellipses in original)
Already, Totho intends to use his invention for weaponry. (63) In the real world, the air gun, also known in English in the seventeenth and eighteenth centuries as the wind-gun, has a rather uncertain early history.  It was apparently invented as early as the fifteenth century in the German lands, with improvements made in its design over the next few centuries, for example by the Nuremberg mechanic Hans Lobsinger in 1566. (I have seen multiple claims online that the earliest surviving air gun is from 1580, with reference to various museum collections,  but the oldest example I could find actually attested on a museum website is this German-made example in the Livrustkammaren in Stockholm, dated simply to the late 1500s.)
In the seventeenth century, the air gun developed a somewhat odd association with the experimental method, as a number of men of science developed new prototypes whilst trying to understand more about vacuums. Otto von Guericke – a political as well as academic figure, who one senses would fit in rather well in Collegium – demonstrated the possibility of evacuating rather than compressing air in a series of dramatic experiments in the 1650s. Denis Papin’s air gun, presented to the Royal Society in London in 1686, was probably based on a vacuum pump of the kind invented by von Guericke. 
By this time, air guns were reasonably popular for hunting in northern Europe (though they seem not to have caught on in Britain), and there are a number of well-preserved examples from the period in museums.
In the following century, repeating air rifles were in use in a number of European armies, among them the Girandoni air rifle (later of Lewis and Clark fame) – but that is to follow the history of the technology beyond its development in this chapter, and will be a story for another time.
Although there are multiple methods of powering air guns, Totho’s air battery in this chapter seems to work on a multi-stroke pneumatic model: the lever is pumped multiple times to store compressed air, and is able to release one shot before re-cocking is required.  To me this diagram from a seventeenth-century book looks like the kind of thing:Compare this image, which I think is a spring-piston mechanism:
Air guns had a number of advantages over early modern firearms: they were quieter and more discreet (because they did not issue smoke), and were more reliable in wet weather, when the fuse of a gunpowder rifle would be hard to light. More advanced versions were also capable of frequent shots, and could fire multiple shots without recharging. Although the importance of Totho’s battery is not especially emphasised in this chapter, with some knowledge of the history of pneumatic weapons, it is clear that it could represent a not-insignificant advance on the technology we have seen Apt characters use so far.
As firearms became more powerful and reliable, however, air-powered guns and artillery tended to fall out of military use. I’m not sure they have ever had the military importance in our world that they will develop in the world of the books…
Page references within the text refer to the 2012 Tor edition of Empire in Black and Gold.
 I found the most useful sources on the early history of the air gun to be Peter O.K. Krehl, History of Shock Waves, Explosions and Impact: A Chronological and Biographical Reference (Berlin, 2009), pp. 196-7, 880, and Johann Heinrich Zedler, Großes vollständiges Universal-Lexikon aller Wissenschaften und Künste (Halle and Leipzig, 1732−1754), Vol. 57, pp. 660-668, accessed at zedler-lexikon.de.
 Krehl, History of Shock Waves, p. 223. There is a considerable literature on von Guericke and on Robert Boyle, who further publicised his work with his own experiments in the 1660s. A useful brief summary is Anne C. van Helden, ‘air pump and vacuum pump’, in J.L. Heilbron (ed.), The Oxford Companion to the History of Modern Science (Oxford, 2003).
 My understanding of different kinds of air gun mechanisms is based on this Wikipedia article, as well as conversations with my ever-patient partner.
 Johann Ulrich Müller, Neu-eröffnete Schatz-Kam[m]er, verschiedener Natur- und Kunst-Wunder… (Nuremberg, 1694), p. 286.
If you’re knowledgeable about historical air guns, I would love to pick your brains about the trade-offs required between cost, power, range, accuracy and weight, and the limits of pneumatic technology! Please do leave a comment below.