The target a hippo offers to a Mauser rifle bullet is not much more than half the size of a man's hand, including only the ear and eye and the narrow space between them.

By daylight at a hundred yards that is nothing for a cool shot to complain about, but in half-moonlight, at that angle, it is none too much.

I swore silently, wishing again and again that Fred would pass the rifle to Will, or to Brown--or to me! Yet if he had passed it to me I should have trembled worse than any one.
Visions began to haunt me of what would happen if Fred should miss! What would the effect be on wild folk tortured by hunger and keyed to the pitch of frenzy by suspense?
Then, even while we watched, another problem added itself.

Over on the water there began to come a wind, driving ripples and little waves in front of it.

The moment those came near the hippo he would vanish from view, for they only care for moonlight when they can see it mirrored on a perfectly still surface.
I cursed Fred between set teeth, almost loud enough for him to hear me; for the hippo did move.