But there was a contradictory side to him too. He was a great artist who painted only a few pictures, and finished even fewer. He was a sculptor who never completed a single statue. He was an inventor whose war machines were seldom built. He was an aeronautics designer whose aircraft would never have flown. He was a hydraulics engineer who failed to understand that water was carried to the tops of mountains by clouds.

Yet his influence in all those fields was significant. How did he come to be such a radical thinker? Unlike the early Greek philosophers, who believed the human mind capable of analyzing and deducing all scientific principles without experiment, Leonardo based his scientific theories, like his artistic innovations, on careful observation and precise documentation. He understood, better than anyone of his century or the next, the importance of precise scientific observation. And he kept copious notes and sketches.

His writings and records.
Unfortunately, just as he often failed to complete his artistic projects, he never completed his planned expositions on a variety of scientific subjects. Instead, his theories are contained in numerous notebooks, which were written in mirror script. Because they were not easily decipherable, his findings were not dispersed in his own lifetime. If they had been, it would not be unfair to say they would have revolutionized 16th century science, and every century thereafter.

He died in France in 1519, arguably the greatest mind in an age of extraordinary enlightenment. Yet his ideas were nearly lost completely. It was only in the 1690s (175 years later) that his notebooks were discovered in Rome by Giuseppi Ghezzi. Two of them were later sold to the Lord of Leicester (in 1717). They remained in England for the next 250 years, where they became known as the Leicester Codex. They are a remarkably eclectic jumble of scientific writings and sketches. Da Vinci was left handed (unusual for his era). It has been suggested that he wrote in mirror style to prevent his discoveries being misinterpreted by the Church. Heresy was a capital offence, and others had paid dearly for choosing to speak out against the doctrine of the Holy Roman See.

In 1883, two volumes were compiled by Jean Paul Richter. They remain one of the few sources of information today. Apart from that effort, his notebooks were effectively lost to scientists for almost 400 years. In 1980, the Leicester Codex were bought by financier Armand Hammer (of Occidental Oil), who renamed them the Hammer Codex. They was sold again at Christies in 1994 to Bill Gates (Microsoft), who restored their original name. Even today, there is still no complete, sequential translation of this remarkable man's work.

Leonardo the geologist.
Da Vinci was drawn to geology not through any specific interest in rocks and minerals themselves, but because of one of the great puzzles of the age. What were fossils? Prevailing opinions were divided into two camps. Were fossils ancient life entombed in stone, or were they minerals that merely RESEMBLED ancient life?

The question intrigued Leonardo, and he set about solving this dilemma by first embarking on a period of close observation of Nature, notably in the dolomite formations of northern Italy. In a notebook, he recorded data that seem obvious enough to us today:

• 1. Rock layers represented sequential time scales (and a river cutting a valley often had the same strata on both banks).
• 2. Angular rocks were found at stream sources, rounded pebbles midway down rivers, and silts and clays at river estuaries. He correctly concluded they were, in fact, all the same material.
• 3. Fossils were found in a number of strata, not in a single layer.
• 4. Some strata had marine tracks that had themselves been fossilized into stone.
• 5. Many fossils were intact, not fragmented.
• 6. Numerous fossils had growth rings, which da Vinci compared to the growth rings on bulls' horns.
• 7. Fossils were only found in layered (sedimentary) strata.