You won't know but is through my discovery of and my participation in our esteemed hosts first incarnation of this site that I came to accept theories are self fulfilling fantasies.
These days I just have speculation based in personal experience coupled with whatever information comes to my attention by digital, or real world texts, photos and to a much lesser degree video and film.
So in the case of the telltale compasses the available evidence for their use points to them being developed for the purpose I stated. Whether that is true or not I will never find out but it does make sense and given the lack of evidence to the contray a likely truth. To me thats good enough.
I have a little to add to the prominent E its to do with the winds. Sailing vessels masters pay more attention to the prevailing wind than anything else. Its also the direction the sun rises which also has a bearing on the E being more prominent but buggered if I can recall with more precision. Westerlies and easterlies being the direction of travel that moves the sailing ships across the ocean so having a prominent E provides a clear indication of direction in terms of destination even in a gloomy cabin.
There are myriads of structures and infrastructure we seem incapable of comprehending today. However what we contemporary humans seem even more incapable of, which is a tad scary quite frankly, is imagining the actual process of how things were done. We always go from what we do today as a start point instead of asking ourselves how would I do it, what would I use.
Korben and I have had many a ding dong in regards horse and oxen power and what it is capable of which has led to some fascinating research which revealled all manner of cranes neither of us knew existed for one example.
Another is the fact both scaffolding and cranes used the building that was going up as the supporting structure for themselves. It's still done the same way today but only for buildings which go beyond the reach of mobile cranes or tower cranes fixed to the ground but in the past they went up with the building from the get go. They were easy to dismantle and assemble so at the end they were dismantled and removed leaving next to no trace of their existence. Pulley blocks and rope for example have enormous lifting power yet can be used by two men and leave no trace of their use as can windlasses. Used with cranes, levers, and rollers and there isn't much man power alone cannot build.
We discovered a simple metal device called a Lewis which is used to lift and hold the weight whilst the block is positioned. Its just a simple pair of levers but there is nothing simpler or more effective yet invented.
I know this has gone way off topic in a way but they serve to show how I approach figuring things out. Always to my good enough measure.
I've looked and looked into the etherric energy idea and found nothing but one device which korben actually discovered if memory serves, which if proven to work and no-one here has ever gone to find out, blows the idea of antennas on buildings masquerading as flagpoles whilst capturing ethertic energy out of the window because it is a portable device and it sits inside a building not connected to the outside in any way.
So using my good enough measure and the complete lack of evidence for a second machine or any electrical equipment or cabling under flagpoles that suggests etheric electricity collection and usage is doubtful.
Most devices like this big bird vessel tend to be inexplicable simply because they are. As with this one details are ropey at best. It always strikes me as very dodgy what comes through as documentary record and what doesn't like the missing key for the OP picture or the book it came from.
I find it odd that this is a stand alone page from the book when its clearly got a page number on it. Given the sheer number of books on shelves in stately homes succumbing to dust mites and the passage of time on this little island alone there is likely to be a book with its pages intact where we could read the key but the chances of finding it are nil.
As you may have noticed I do not contribute to many threads. I did when I first began this forum posting lark but I was like a kid in a sweetshop. After reading a copy of Voltaire's History of Charles XII of Sweden for a thread on the original forum, it dates to 1742 and is hard copy not a digital, to find it reads like a novel or magazine yet it is treat by historians as a work of true historical record, well it was enough to approach every historical named character as probably a fiction and suffice to say most like these two Jesuits do collapse into fiction.
One other thing in regards machines and the events in which they were said to have been used like big bird vessel I have learnt, again through interacting with Korben to look at the logistical aspect of things as a bona fide marker for a veracity claim.
In this case if it is heated air doing the lifting there has to be some chamber or vessel where the hot air is produced and none is visible. There has to be fuel onboard if it is meant to travel any distance and there isn't any.
There has to be wings and or rudders to change direction made of something that can take the stresses. Wood and metal are the two obvious choices so what metals might be up to it. Wood seems too heavy.
How did the machine land, did it land and take off from water if so was it sea or lake, what was the water proofing, how could it be repaired etc etc.
Even simple things like what did the pilot do when he needed a shit or a piss?
How much water could it carry, where is it stored?
All these and more run through my mind when pondering what the hell am I looking at or reading about.
Just to throw a bit of context into the discussion which has a direct bearing on the OP have a look at the machines in this autochrome photo. Most people below a certain age alive right now would likely assume these machines could not fly but they did. Back then they were cutting edge today they are obsolete.
It doesn't take long for knowhow to all but vanish.
Image title: "Here is another old autochome I've restored for you, taken 111 years ago at the first Paris Air Show in September 1909. (It isn't colourised)."