Hope is good to .. "chase swallows with the salt"

Solution 1:

I am sorry that I can support my answer only with an 1834 German reference:

It says (roughly translated):

With their humorous salt they want to feed or catch time; and remind of children, which say to catch a swallow you only need to put salt on their tail.

I would say it means the erroneous believe, that salt is enough to catch a swallow, the hope that life is that easy a boy has, before he gets grave but experienced as a grown man. (But that is interpretation and wrong here.)

Edit: Thanks to the OPs comment I can now add another english reference stating the same act.

Solution 2:

What I suppose is the underlying message is that 'hope' is a characteristic of youth (the boy), and faith a virtue of an older person (the man). And that faith, which invests less in expectation is ultimately more satisfying.

The boy energetically goes into headlong pursuit of a near-impossible task, namely to catch swallows by 'chasing them with salt'. In doing so the thing that sustains him is 'hope'. But it is unlikely to deliver much.

Dictated by their experience, an older person is more realistic in their ambitions, and hence 'gravity'is more the man's outlook. But ultimately he is smiling, perhaps since a realistic approach is less likely to promise rewards that are unattainable. He will achieve more of what he expects from life.