Analysis of 406 [unfinished]

I do not know the meaning of the sign,
But bend before its power, as a reed bends
When the black tornado fills the valley to the lips.
Three times in twenty years its shape has come
On lines of fire on the black veil of mystery;
At first, tho' strange, it seemed familiar,
And lingered on the mind as if at rest;
The second time if flashed a thrill came, too,
For supernature spoke, or tried to speak;
The third time, like a blow upon the eyes,
It stood before me, as a page might say:
'Read, read,—and do not call for other warning'.