We can calculate with fair accuracy the number of miles an automobile will go in an hour. We can gauge pretty closely the amount of merchandise a given sum of money will buy. But a good deed or a kind impulse is not measurable. Their influence works in devious ways and lives on when perhaps we can see them no more.

I shout an arrow into the air,
It fell to earth, I knew not where;
For, so swiftly it flew, the sight
Could not follow it in its flight.

I breathed a song into the air,
It fell to earth, I knew not where;
For who has sight so keen and strong,
That it can follow the flight of song?

Long, long afterward, in an oak
I found the arrow, still unbroke;
And the song, from beginning to end,
I found again in the heart of a friend.

Henry Wadsworth Longfellow.