Lucretius, De Rerum Natura
William Ellery Leonard, Ed.

     The bolts of thunder, then, must be conceived
As all begotten in those crasser clouds
Up-piled aloft; for, from the sky serene
And from the clouds of lighter density,
None are sent forth forever. That 'tis so
Beyond a doubt, fact plain to sense declares:
To wit, at such a time the densed clouds
So mass themselves through all the upper air
That we might think that round about all murk
Had parted forth from Acheron and filled
The mighty vaults of sky- so grievously,
As gathers thus the storm-clouds' gruesome might,
Do faces of black horror hang on high-
When tempest begins its thunderbolts to forge.
Besides, full often also out at sea
A blackest thunderhead, like cataract
Of pitch hurled down from heaven, and far away
Bulging with murkiness, down on the waves
Falls with vast uproar, and draws on amain
The darkling tempests big with thunderbolts
And hurricanes, itself the while so crammed
Tremendously with fires and winds, that even
Back on the lands the people shudder round
And seek for cover. Therefore, as I said,
The storm must be conceived as o'er our head
Towering most high; for never would the clouds
O'erwhelm the lands with such a massy dark,
Unless up-builded heap on lofty heap,
To shut the round sun off. Nor could the clouds,
As on they come, engulf with rain so vast
As thus to make the rivers overflow
And fields to float, if ether were not thus
Furnished with lofty-piled clouds. Lo, then,
Here be all things fulfilled with winds and fires-
Hence the long lightnings and the thunders loud.
For, verily, I've taught thee even now
How cavernous clouds hold seeds innumerable
Of fiery exhalations, and they must
From off the sunbeams and the heat of these
Take many still. And so, when that same wind
(Which, haply, into one region of the sky
Collects those clouds) hath pressed from out the same
The many fiery seeds, and with that fire
Hath at the same time inter-mixed itself,
O then and there that wind, a whirlwind now,
Deep in the belly of the cloud spins round
In narrow confines, and sharpens there inside
In glowing furnaces the thunderbolt.
For in a two-fold manner is that wind
Enkindled all: it trembles into heat
Both by its own velocity and by
Repeated touch of fire. Thereafter, when
The energy of wind is heated through
And the fierce impulse of the fire hath sped
Deeply within, O then the thunderbolt,
Now ripened, so to say, doth suddenly
Splinter the cloud, and the aroused flash
Leaps onward, lumining with forky light
All places round. And followeth anon
A clap so heavy that the skiey vaults,
As if asunder burst, seem from on high
To engulf the earth. Then fearfully a quake
Pervades the lands, and 'long the lofty skies
Run the far rumblings. For at such a time
Nigh the whole tempest quakes, shook through and through,
And roused are the roarings,- from which shock
Comes such resounding and abounding rain,
That all the murky ether seems to turn
Now into rain, and, as it tumbles down,
To summon the fields back to primeval floods:
So big the rains that be sent down on men
By burst of cloud and by the hurricane,
What time the thunder-clap, from burning bolt
That cracks the cloud, flies forth along. At times
The force of wind, excited from without,
Smiteth into a cloud already hot
With a ripe thunderbolt.