http://www.bettyboop.com/forums/showthread.php?1961-Want-to-Ask-Any-Booper-a-Question/page27

In the above website, I encountered an odd sentence that's the underlined one below.

Old people lose height and muscle and fat, so they get smaller. Their spines begin to degenerate and compress, making them shorter. That's partially gravity taking hold. We all get shorter in the course of a day because of the compression of the water in the discs of the spine.

It seemed odd, I racked my brains, but still could not figure out how this sentence could make an integral sentence.

So I googled and found out so many sentences with the below structure.

This isthe sort of action that's taking hold all over America.

Now, this is my conclusion. The original underlined sentence seems wrong. The one who wrote it somehow confused it with "That's partially gravity that's taking hold." The replaced sentence seems much better to me semantically and syntactically.

Am I right?

If I am wrong, could you tell me why, explain the structure of the sentence in question, and, if possible, give me another example sentence?

(Sorry about too much asking, but the sentence is killing me^^;)
Both of these uses are correct, in my opinion.

We often say "This/that is partially A and partially B."

This is gravity.

This is partial gravity. (not full gravity) "Partial" is an adjective.

This is partially gravity. (The rest is something else altogther.) "Partially" is an adverb.

This isthe sort of action that's taking hold all over America.

"This" is a demonstrative pronoun. The choice between "this" and "that" would be determined by the antecedent.

Eg., is it the near one or the far one?

In your example sentence, "that" is a relative pronoun. ("This" could not be substituted.)
Thanks for quick answer.

So do You mean "taking hold" is modifying "gravity" as "reduced relative pronoun"?

(What I mean is THAT'S THE GRAVITY [THAT'S] TAKING HOLD.)
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That's partially gravity taking hold.

Ouch! What's a reduced relative pronoun? Sorry! Emotion: embarrassed

How can she act like that? (reply) That's partially Jane just being herself. (The rest of it may be that she's got a migraine.)

The rest of it may be gravity doing something else, or it could be an entirely different force or phenomenon doing its thing.

Yes, "taking hold" modifies "gravity."

(What I mean is THAT'S THE GRAVITY [THAT'S] TAKING HOLD.)

I'm not sure I exactly understand your question.

If I say, "That's gravity taking hold," I'm specifying both what it is and what it's doing.

If I say, "That's partially gravity taking hold," "That" is still the subject. "Is" is still the verb.

If I say, "That's partially a cloud covering the moon," I may mean to imply that something else is helping to obscure it, such as the shadow of the earth in a partial eclipse.

I'm having a hard time explaining how "partially" modifies the whole phrase "gravity taking hold" adverbially.

"Partial" may apply both to the thing and to what it's doing.

If I may struggle on, both of your versions are correct, but with slightly different meanings.

Question 1. What is taking hold? (reply) That's partially gravity that's taking hold.

(Something else is also taking hold.)

Question 2. What is happening? (reply) That's partially gravity taking hold.

(Something else is also happening.)
Stenka25Old people lose height and muscle and fat, so they get smaller. Their spines begin to degenerate and compress, making them shorter. That's partially gravity taking hold. We all get shorter in the course of a day because of the compression of the water in the discs of the spine.
I don't mean to rain on your parade, but water is not compressible.

Anyway, the mysterious subject of the sentence in your example is the gradual shortening of us old folk. (And o, tis true, tis true.)

So the question is not, "What is taking hold?" (answer - It's partially gravity that's taking hold.)

The question is, "What is happening?"

In my opinion, the "that" doesn not belong in this answer.

Your example is complicated by the fact that the author is separating the two effects: the temporary daily shortening which happens to everyone because of gravity; and the gradual long-term shortening of the old folk.
Thanks, Avangi, for your sincere answer.

Thanks a million.