Again about the upcoming War: how is the nitrate situation? How important the nitrate supply from South America is going to be, and what chances for the RN to interdict this stuff getting to enemy ports?
I suppose we are still quite far from Haber's synthesis.
I've been puzzled by this for years, nitrates as a strategic resource. I suppose that while nitrates can be harvested on a small scale from any barnyard, the sheer scale of gunpowder expenditure in a serious war dwarfs what can be obtained by a reasonable level of effort from such sources.
And while small nitrate deposits exist in many places in the world, even in Europe, the only really big one known in the 1880s was the one in the coastal desert contested by Chile and Bolivia. And even if the French side has some sort of ace in the hole in terms of local influence in the contending powers there, still it's the British who are going to control the sea routes to haul it back to Europe. The same holds even if we consider other deposits, such as on islands near the South American coast or even Nauru.
So--it's the Franco-Russian-Austrian alliance that has to worry about running low. Though I suppose if the French naval forces are really good, they can interdict shipments to North Germany somewhat.
By the way Jonathan I hope I haven't been remiss and too late in naysaying about submarines in this war. It occurred to me that you might have the French playing that card. Maybe they can. But I doubt it. As with aircraft, I fear the crucial issue is power plants. Holland was OTL the advanced visionary who came up with the solution that worked best for making submarines practical warcraft, and even he was rather stymied and behind in coming up with the classical, definitive answer. Because oddly enough, Americans had a hard time coming up with decent, reliable diesel engines, and so the American submarine force in particular was still using gasoline engines when the OTL war broke out.
There are certainly at least in theory alternatives to the classic combination of diesel (or in a pinch, gasoline, but these stank and were hazardous too) engines to generate electricity (and also direct torque for propulsion) when surfaced, to charge storage batteries for electric propulsion when submerged. OTL, one attempted method was to use steam engines even when submerged, by taking advantage of latent heat in hot water to develop useful pressure even after the fire was quenched. In general, I gather, though steam was a tempting and very well developed tech for generating primary power, it worked badly in submarines. The subs were horribly hot and humid; it took a very long time to work up the steam head; submerging was slowed down considerably by the requirements of banking the fire.
Other methods, like using pneumatic air pressure for energy storage and power while submerged, were terribly limited; the range of pneumatic powered subs was well under 10 miles. Yet other methods, such as the Walter process of using hydrogen peroxide as oxidant in what was basically a Diesel engine (I think, I confess I don't really get exactly how the Walter process was supposed to work, I think by simply using the peroxide as an oxygen storage medium and outgassing it to run a diesel) proved too visionary and risky even in the 1940s and post OTL WWII, even in the 1950s when both British and Soviet experiments tried it and gave it up in favor of nuclear power. I suppose if nuclear power weren't an option peroxide might have been made to work but it was marginal even sixty years after your Great War period, so I daresay it's clean out for even an 1890s advanced by hothouse priority R&D.
The most promising alternative, path not tried OTL, I can think of is using either peroxide or liquified oxygen, if that could be possible in this time frame, and conventional fuels to burn to drive a kind of Stirling engine. I believe that Stirling engines, despite some attractive features (including relatively quiet running, very important for war subs in particular) have been bypassed OTL for some good reasons. But on the other hand, the basic concept and practical tech had been developed well before IC engines OTL, and they are after all quiet (though I suppose that in the 1890s, the potential for using sonic location would be low enough to allow systems that would be unacceptably noisy even in the OTL 1910s, so this advantage ironically might not count). I'm most encouraged because I know of modern OTL air-independent submarines built by the Swedes that use such a method--stored, presumably liquid, oxygen burns with a fuel to drive a modern Stirling, as an alternative to nuclear power. Compared to nukes it is much inferior of course but compared to diesel-electrics it's pretty good. But that of course is with modern familiarity with liquefying oxygen, and modern very expensive advanced Stirling engines. An 1890s version might have to use peroxide to store the oxygen, and suffer much in lower efficiency and a heavier engine for a given power output, but it's competing with alternatives that are also terribly poor compared to modern standards.
I wish I knew how much of the risk and danger of using peroxide had to do with the Walter process engine system, and how much is just inherent in trying to store hydrogen peroxide at all; I fear the problems are mostly from the latter. I know that Walter's WWII U-boat designs wanted to store the peroxide in "plastic," whatever that means in the context of 1940s Germany, bags outside the pressure hull, and obviously the options in the 1890s for such storage are more limited.
Anyway, OTL no subs tried the Stirling solution in the 19th century and there may be good reasons for that.
The method that in hindsight worked best, diesel-electric, is aside from limits of the state of the art of suitable IC engines, limited by the state of the art of electric tech, batteries and motors.
I fear that even with a feverish effort put behind it leading up to the war, the possible submarines of the 1890s can't amount to serious war machines in the way they did just a couple decades later. The batteries just won't be good enough, the alternatives are too poor; a Stirling system just might emerge at the top of the pool and function acceptably well, but the lack of even attempts at that OTL argues against it.
Whatever clever tricks the French might have against the RN, they might try subs but I don't think they can achieve many good results with them this early.
Oh well, if you've already written posts detailing the deadly effect of French (or conceivably Russian or even Austrian designed) subs I might still credit it, especially if you've got technical advice more practically informed than mine. But I hope any clever tricks you give the French side don't rely on good subs, because it's just too early for them.