An insect performs no miracle in walking up a wall or uopn the surface of a pond; the small force of gravity pulling it down or under is easily overcome by the surface forces which act to keep it in position. Throw an insect off the roof and it floats gently down as the forces of friction from the air acting upon its surface overcome the weak influence of gravity.

Which word does 'acting upon' modify' here?

I think it's 'the forces (of friction)', but my book says it's 'the air'...
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Comments  (Page 2) 
So, your idea is that 'acting upon' refers to 'the air'. Right, goldmund?
Dear Taka,

Yes, that is my opinion.

The body moves through air. The effect of movement through air is friction. The friction slows the movement of the body. The friction therefore «overcomes gravity».

The writer means: friction is the action of air on the moving body.

It is perhaps a badly written sentence. Emotion: smile

Kind regards,

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I see.

By the way, where are you from, may I ask?
Hi Taka,
To my eyes, there is no punctuation exsisting in the original sentence...

Exactly. That's why it means 'the air'.
Sherlock Holmes said something to the effect that the significance of the dog's barking was that it didn't do it.

Emotion: tongue tied
I don't really understand the connotation of your joke, Clive...
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Talking about friction, I've just heard that a cat that falls from say the 17th floor never falls faster than about 59 mph, because of the friction of air against its hairs.
AND that it is safer for it to fall from the 17th floor than say the 2nd or the 10th, all because of that friction, which creates a kind of parachute effect.
It pins you to the wall, doesn't it?
(I wonder how many experiments they conducted to reach that conclusion, and what happened to the lower-floor cats)
Hi Taka,
It's hard to explain humour, but here it lies in the suggestion that the significance of something is in its absence.
The significance of the dog's barking was that it wasn't barking.

Perhaps humour doen't travel well between countries?
I understand the joke itself, Clive. But I have some trouble understanding this expression---because punctuation actually doesn't exist :
the existing way it's punctuated

Or, by 'the exsisting way', did you mean 'any possible way' or something?
Hi again,
By 'the existing way it's punctuated', I simply meant that the sentence is punctuated with no commas and only a period at the end.
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