What are the images of the two similes used in lines 21 and 22 to create the picture of the fountain bursting forth from the chasm?

Huge fragments vaulted like rebounding hail,or chaffy grain beneath the thresher's flail : the water flowing out of the chasm is strong and powerful. the purpose of these similes is to show how the water sprays and bounces around similar to the way hail falls and bounces once it hits a surface. it is also compared to grain being turned and tossed in a thresher, rolling and flying everywhere.