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(Completely rewritten after downvotes & closing down)

Plutarch's Decline of the Oracles (Moralia vol. V) famously opens with the report by Cleombrotus, the wannabe theologian, that the perpetual lamp in the temple of Zeus-Ammon burns less and less oil each passing year, which the priests explain by a shortening of the year; this sparks a debate with Demetrius the grammarian and Ammonius the philosopher.

In modern times, the whole passage is widely believed to distort the opinions of Plutarch's informants, who may or may not be from Siwa, to mock the building of grandiose theories on the petty side effects of menial operations: de minimis non curat praetor.

Disclaimer The whole passage and its interpretation is emphratically not what my post is about. I am trying to evaluate whether Plutarch 1. faithfully reports the findings of his informants (as opposed to their opinions) and 2. misunderstands on what grounds they believe their metrology is sound (as opposed to disagreeing with them).

@OddThinking Please humor me a with a wee bit of trolling: yes, this is the grandiose aim of my question; no, I am not aware of any exegete expressing disbelief specifically about the following claims. If you know better, now is the time to show your hand. End of trolling.

End of disclaimier.

Since many claims of Plutarch's are widely believed to be abusive, let us focus on claims expressed by Cleombrotus:

  1. Plutarch's informants have discovered their perpetual lamp burns less oil per year after several centuries of operation.

  2. They know this by periodical gaging of the fuel: I myself actually saw the measure; for they had many of them to show, and that of this past year failed to come up to the very oldest. (As translated in the Loeb edition, see e. g. here)

This strikes me as both necessary for the initial discovery of the anomaly, and overly fastidious as a routine check. The normal way to manage a buffer stock of oil would be to count amphoras in the celler: why bother with precision measurements before there is any reason to suspect they might bring intringuing restults?

However, there is at least one reasonable scenario of which this oddity is just a natural consequence: this specific lamp had a capacity of less than 1 day and, by necessity, doubled as a timepiece. Refilling would be effected at constant intervals, say 8 or 12hrs, in day time as measured by the sundial and, in night time, by the only available means: gaging the oil chamber.

This explains the periodical gaging and ensures any sudden change of the burning rate from one day to the next (e. g. on the first use of a new amphora) will be detected and analyzed.

This makes it possible that the oil chamber was never less that half-full in the course of several centuries and accumulated soot, tars and oxydized compounds; so that the fuel actually sucked up by the flame differed in composition from the oil regularly added to it and burnt significantly slower.

This in turn makes it possible, in spite of Plutarch's sarcasms, the priests did understand why it mattered to supply oil of constant quality and could both believe they did and fail to convince Plutarch, while failing to understand this was not enough to ensure a constant burning rate.

Hence the questions.

Q1: suppose a bronze lamp is filled to the rim with fuel of constant composition at regular intervals, when the oil chamber is at least, say, 60% full, and burns it through an asbestos wick which is never trimmed nor replaced. The chamber is never purged: over the course of several centuries, will its contents slowly differ in composition from the input fuel and burn at a slower rate?

Q2: is it possible, with early 1st Mill. BC technology and an adequate quality insurance system, to compensate for small changes in composition between oils from various orchards and to produce batches of lamp fuel that show no appreaciable change in burning rate over dozens of years?

Q3: will such a system maintain the constant burning rate of the fuel indefinitely, or must it allow it to drift over longer periods than dozens of years, say over some centuries?

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    Perhaps there is a muddling. When I read this question I envision an answer that discusses typical uses of these ancient things. That's a good question on history.stackexchange.com. If you're comment above is in earnest, that you are trying to see what these specific people actually did, whether the story is even true, etc., that is a question we can handle here, but the story is so old that it is still necessarily a question requiring a particularly heavy use of historical analysis. When you have history.stackexchange.com available to you, maybe that's better either way.
    – fredsbend
    Nov 7 at 15:50
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    For the larger implications of this supposed scientific discussion, hsm.stackexchange.com might produce interesting answers.
    – fredsbend
    Nov 7 at 15:51
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    After a quick look at the linked Decline of the Oracles translations, it looks more to me like Plutarch has written the thing in the form of a discussion of real events by real people, but is actually presenting a philosophical argument by way of fictional events told by fictional characters. The story about the oil is a sort of stand in for the real argument - it makes you think "silly priests arguing from such a stupid base" to then draw a parallel to arguments about the decline of oracles.
    – JRE
    Nov 8 at 15:35
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    In other words, there's no point in discussing how the priests measured the oil consumption because they never did it, and never argued that days were getting shorter on that basis.
    – JRE
    Nov 8 at 15:36
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    @FrançoisJurain: You have missed my point. You argued that, because Plutarch was widely admired philosopher hundreds of years ago, that his claim is widely believed today. I gave an example of an even more widely admired philosopher with an even more famous claim, and argued that because that statement is clearly not widely believed, the form of your argument must be unsound.
    – Oddthinking
    Nov 16 at 11:43

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