Recto: a sheet of instructions in Leonardo's handwriting describing how to paint a Deluge, illustrated by seven slight sketches. Verso: a description of a Deluge in Leonardo's handwriting, illustrated by three slight sketches. Melzi's number 178 (verso).

A cataclysmic storm overwhelming the earth was one of Leonardo’s favourite subjects during the last years of his life, in both his drawings and his writings. Several long passages recount the futile struggles of man and animal against the overwhelming forces of nature – tempests, floods, a mountain collapsing on a city, and finally the storm sweeping away all matter. But Leonardo’s descriptions of the deluge, far from being chaotic, are objective and detached, emphasising the attitudes of the figures, the appearance of the landscape, the optical qualities of cloud, rain, water, debris, dust and smoke, and thus of a piece with his notes throughout his life towards a treatise on painting, with every effect of interest to the painter now amplified and thrown together.

One of these long passages can be seen in RCIN 912665, running to almost 1,000 words on each side of the sheet, and laid out like a scientific treatise with marginal illustrations – a coastal view with rainfall, a plan of a dam with water flooding through a breach, a cloud formation with a downpour, a whirlpool sucking boats to its centre, a wave striking a block and curling back on itself, a wave breaking, and water flowing down a sluice. Not all the text can be quoted here, but a couple of passages give the flavour:

   Description of the deluge:

   Let there first be shown the summit of a rugged
   mountain surrounded by valleys. From its sides
   the soil slides together with the roots of bushes,
   denuding great areas of rock. And descending from
   these precipices, ruinous in its boisterous course, it
   lays bare the twisted and gnarled roots of large trees,
   throwing their roots upwards; and the mountains,
   scoured bare, reveal deep fissures made by ancient
   earthquakes. The bases of the mountains are covered
   with ruins of trees hurled down from their lofty
   peaks, mixed with mud, roots, branches and leaves
   thrust into the mud and earth and stones.

   And into the depths of a valley the fragments of
   a mountain have fallen, forming a shore to the
   swollen waters of its river, which has burst its banks
   and rushes on in monstrous waves, striking and
   destroying the walls of the towns and farmhouses
   in the valley. The ruin of these buildings throws up
   a great dust, rising like smoke or wreathed clouds
   against the falling rain. The swollen waters sweep
   round them, striking these obstacles in eddying
   whirlpools, and leaping into the air as muddy
   foam. And the whirling waves fly from the place of
   concussion, and their impetus moves them across
   other eddies in a contrary direction [...]

   The rain as it falls from the clouds is of the same
   colour as those clouds, in its shaded side, unless the
   sun’s rays break through them, in which case the
   rain will appear less dark than the clouds. And if the
   heavy masses of ruined mountains or buildings fall
   into the vast pools of water, a great quantity will
   be flung into the air, and its movement will be in a
   contrary direction to that of the object which struck
   the water; that is to say, the angle of reflection will
   be equal to the angle of incidence.

Text adapted from Leonardo da Vinci: A life in drawing, London, 2018