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since James Jeans was that famous British physicist, shouldn't there be:

Jeans's instability

instead of: 

Jeans instability

(wiki link)

I have read couple (maybe not all) of saxon genitive related topics here but I haven't found the answer.

Kuba
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    "Jeans mass" is the mass of a cloud. "Jeans' mass" is the mass of Jeans himself. "Car antenna" is a type of an antenna. "Car's antenna" is the antenna belonging to a particular car. Really quite straightforward. So, do you mean to say that Jeans was instable or do you not? There's your answer right there. – RegDwigнt Aug 21 '13 at 12:55
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    @RegDwighт, and yet it is called both Down’s syndrome and than Down syndrome. These things are not quite as regular as we’d like them to be. – Janus Bahs Jacquet Aug 21 '13 at 15:50

2 Answers2

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Normally, when a noun ends in s and it is singular, you add 's to form a Saxon genitive.This is also normally true for Christian names (for example, St. James's park in London).

When it comes to surnames, the matter is less clearly settled. When reading about the novels that Dickens wrote, you can find both Dickens's and Dickens'. Note however that the apostrophe should always be added.

In the case you mention, I suppose that the reason for the lack of an apostrophe is that the name "Jeans" is used as an attribute of the term instability, meaning that it defines of which instability we are talking about, not something which belongs or characterizes Doctor Jeans. In other words, Jeans is used as an adjective and for this reason it does not change.

Paola
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You're begging the question by coupling the title

Formal Saxon genitive usage

with the question

since James Jeans was that famous British physicist, shouldn't there be:

Jeans's instability.

The implication is that the 'formal [S]axon gen[i]tive usage' is always mandatory.

The move towards the dropping of apostrophes in associative rather than true possessive constructions (Jeans neither designed nor bought the instability) is discussed at Is it correct to say "I write children books" (not possessive case)? . However, here, it is not clear whether we have a dropped apostrophe (as in Blyth Working Mens Club) or the use of Jeans as a noun-modifier. The fact that your article uses Jeans length and Jeans' length right next to each other, and Jeans instability as well as the Jeans instability shows that some authorities at least can't seem to decide on a single format.

Community
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Edwin Ashworth
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  • @Paola:' Begging the question (Latin petitio principii, "assuming the initial point") is a type of informal fallacy in which an implicit premise would directly entail the conclusion; in other words, basing a conclusion on an assumption that is as much in need of proof or demonstration as the conclusion itself.'(Wikipedia) So would you please correct your incorrect edit. An apology would also be appreciated. – Edwin Ashworth Aug 21 '13 at 13:33
  • I surely apologize for wrongly assuming a spelling mistake. It's the first time I've ever heard such an expression, even in its Latin form, and I can assure you that Latin expressions are frequently used in Italian, as they are normally easy to grasp. And in the end, there's no need to sound haughty in one's remarks. – Paola Aug 21 '13 at 13:37
  • Perhaps one should check a person's site history before 'correcting' (especialy when one is at risk of making a mistake in that 'correcting'). I would have thought that the rest of the answer and linked material would suggest that a simple spelling mistake would be unlikely here. Changing a correct answer to a wrong one is simply bad practice, and contrary to the aims of this site. And perhaps a simple check - looking up "beg/ging the question" - could have avoided this misleading edit. And does a teacher sound haughty just because they are (you'd better check concord) correcting an error? – Edwin Ashworth Aug 21 '13 at 18:00
  • do you mean to say that I should check the usage of a plural form when the sentence starts with a singular noun? I know about it, thank you. No, what sounds haughty is your last sentence in the first comment. I surely could have read the whole answer with greater care, still making a mistake is something which may occur even to the best. For example, one may write "especially" with one "l" only, but that is clearly a typo. – Paola Aug 21 '13 at 18:27
  • Making a mistake is certainly not a cardinal sin, and I make them as well. However, I take great care to have my facts straight before I edit - not to do so would be cavalier, arrogant and potentially misleading. – Edwin Ashworth Aug 21 '13 at 21:16
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    @Paola: Having twice this week had a post subjected to an incorrect edit, I would reinforce Edwin's sentiment: Don't correct when you think something is wrong, or a typo, but only when you are damn sure it is wrong or a typo. When at all uncertain, post a comment to the author of the post instead. – Pieter Geerkens Aug 22 '13 at 02:08
  • Thank you, Pieter. (But I admit I'd like to correct the comma splice in Paola's last comment. We'll say the lower-case 'd' is a typo.) – Edwin Ashworth Aug 22 '13 at 07:15