So, to wrap up this accidental series. To check whether professional astronomical journals displayed the same patterns in discussing 'Mars' and 'canals' as the more popular/amateur ones I again looked at the peak decade 1891-1900, this time selecting only the more serious, respected journals. However, because of the French problem I had to exclude L'Astronomie and Ciel et Terre (the former was apparently more popular anyway). So for my top three I ended up with Astronomische Nachrichten, Publications of the Astronomical Society of the Pacific (PASP) and Monthly Notices of the Royal Astronomical Society (MNRAS). Astronomische Nachrichten ('astronomical notes') was the leading astronomical journal of the 19th century, founded 1821. It published articles in a number of languages including English. Fulltext Service seems to be multilingual, as it picks up the German (at least) equivalents of Mars/Martian and canal/canals. That doesn't help with the French problem, but that will only affect a small minority of Astronomische Nachrichten's articles. The Astronomical Society of the Pacific was founded in California as a joint amateur-professional organisation. Its PASP is now a very highly regarded journal, although I must admit I don't know if this was always the case. MNRAS is the journal of the Royal Astronomical Society in Britain. It also happens to be where my solitary peer-reviewed astronomy article was published (and when I say 'my', I think approximately 1 sentence relates to research I actually undertook), but even so it really is a highly-respected journal.
Astronomische Nachrichten published by far the most articles mentioning 'Mars' and 'canals'. Interestingly, this was in the 1900s, not the 1890s. The numbers decline rapidly thereafter: there were still a fair few in the 1910s, but hardly any in the 1920s and none at all after then. PASP's peak was in the 1890s, when it published exactly as many articles as Astronomische Nachrichten. There was a big fall in the 1900s, but unusually its level of interest in Martian canals, at least as measured by the number of articles mentioning 'Mars' and 'canals', stayed reasonably constant throughout the 20th century, at a bit over one a year. It must be said, however, that one article a year is not exactly indicative of an obsession. We are getting into some small numbers here. MNRAS's numbers are even smaller, though like Astronomische Nachrichten they peaked in the 1890s and then exceeded that level in the 1900s.
Just for completeness, here's the ratio of articles mentioning of 'Mars' and 'canals' to those mentioning 'Mars' alone. Because of the small numbers not much weight can be placed on this, but it does suggest that PASP's interest in the canals in the 20th century tracked its interest in Mars fairly closely. Also noteworthy is that Astronomische Nachrichten has peaks in both graphs in the 1900s, which is also when it published the most mentioning 'Mars' (859 in total). That is, even as the number of articles on Martian canals (yada yada) increased in absolute terms, they also increased as a proportion of the number on Mars altogether, to nearly 10%. So this is consistent with the picture of the 1900s as the peak decade for the canals controversy, not the 1890s as I had originally assumed.
The upshot of this is not a lot. I think these plots do match the patterns I found in the previous posts, but only weakly. What's really worrying is the small numbers. If these are the three professional journals which published the most articles on the Martian canals (and there may be other gaps in the ADS database), then apart from Astronomische Nachrichten in the 1890s and 1900s (and maybe in the 1910s) and PASP in the 1890s (and maybe the 1920s and 1930s) they didn't really publish very much; and, of necessity, the other professional journals published even less. So can it really be said that professional astronomers -- as opposed to amateur astronomers and for that matter the public -- as a whole were particularly interested in (let alone persuaded by) the Martian canals hypothesis? I'm not sure. Indeed, I'm not sure whether this exercise has shown anything at all, other than the need to look very carefully at your data, even when the story it is telling seems to make sense. The only way to find the answers, really, is to dig much deeper into the data, and start reading the articles themselves and put them in context: quantitative history alone isn't good enough. Which is quite reassuring, actually.