Fuel?
Yes; and a wind.
The humming in the air grew, the scent of fire came plainly.

The plover rose around their nests and circled, crying piteously.

The scattered hares became a great body of moving gray, like camouflage blots on the still undulating waves of green and silver, passing but not yet past--soon now to pass.
The girl, her hands arrested, her arms out, in her terror, stood trying to remember.

Yes, it was three short puffs and a long pillar.

She caught her shawl from her shoulder, stooped, spread it with both hands, drove in her stiffest bough for a partial support, cast in under the edge, timidly, green grass enough to make smoke, she hoped.
An instant and she sprang up, drawing the shawl swiftly aside, the next moment jealously cutting through the smoke with a side sweep of the covering.
It worked! The cut-off column rose, bent over in a little detached cloud.