The Sun's Rays and the Element of Fire
Solar rays keep their nature through the cold air; the sun draws fire toward itself, not to us
Leonardo continues arguing about the sun's heat and its rays. He observes that solar rays pass unchanged through the cold region of the air and through glasses of cold water, so the claim that the cold would cancel their heat, or that mountain peaks like the Caucasus should be hotter than the valleys, is vain. He rejects the idea that the rays push the element of fire down to us, since such local motion would take time and would cool the opposite horizon; instead, since a lesser fire bends toward a greater, the sun must draw fire toward itself. A small marginal figure shows the sun with its rays crossing the sphere of fire.
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Rays pass unchanged through cold air and cold water
The solar rays pass through the cold region of the air and through glasses full of cold water without changing their nature, as though passing through an equal thickness of air. Leonardo argues that if they took on cold after taking on the fire's heat, the cold would annul the heat and the rays would reach us without warmth, which experience denies. The claim that the cold merely tempers their heat fails too, else the peaks of the Caucasus in Scythia, rising above the middle air, would be hotter than the valleys.
The rays do not carry the element of fire down to us
Leonardo denies that solar rays push the element of fire toward us by local motion, since such motion across so great a height cannot happen without length of time, all the more when the sun is 3500 miles more remote at the horizon than at mid-sky. If it did so, it would cool our opposite horizon by carrying away the element of fire on that side.
The sun draws fire toward itself rather than driving it to us
Since a lesser fire attracts and bends toward a greater fire, as experience shows, the sun must draw the element of fire to itself rather than drive it away and push it to us. The heat of fire does not descend unless igneous matter follows it; and were it to do so, it would be material and therefore visible.