The tides as the breathing of the Earth
The sea's flux and reflux likened to the lungs of man, with a vast computation of the Earth's 'lung', plus mechanics notes.
In the long left-hand column Leonardo develops his microcosm analogy: the sea's flux and reflux is the breathing of the earth, whose 'lung' he tries to size by comparison with the human lung (half a square braccio, breathing 270 times an hour). A tall right-hand column carries the resulting computation, running to millions of millions and divided by the times of human respiration. Shorter notes ask him to write to 'Bartolomeo the Turk' about the tides of the Black and Caspian seas, state that the sun is four thousand miles distant, and ask where a flame throws its smoke. A note added in another hand asks Leonardo to mark all the muscles that move the fingers; small sketches of pulleys and an inclined plane appear among the sums.
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Tides as the respiration of the Earth
Leonardo treats the sea's flux and reflux as the earth drawing in and expelling water, just as the human lung draws in and breathes out air. From this he tries to gauge the size of the earth's 'lung', taking the human lung as half a square braccio breathing 270 times an hour and asking how large a lung would breathe just once in twelve hours. He concludes that the volumes of water raised in the great oceans could never be drawn into so small a lung.
Reckoning the volume of the world's lung
The right-hand column carries the calculation to astronomical scale — 'eight million of million of million of million of million of million and eight hundred and twenty' — then divides by the number of a man's breaths, some 2,205,000,000,000,000,000,000 times. Columns of zeros and Roman thousands-marks fill the space beside the figures.
Weight generated by less oblique motion
A note states that the weight generated in a body will show itself the greater in proportion as its motion is less oblique. It sits beside small mechanical sketches of a pulley with two weights and an inclined plane.
A query on the tides of the Black and Caspian seas
A memorandum instructs Leonardo to write to 'Bartolomeo the Turk' about the flux and reflux of the Sea of Pontus (the Black Sea), and to have him find out whether such tides also occur in the Hyrcanian, or Caspian, Sea.
The earth reasoned as a living body
The whole argument rests on treating the earth as a living organism with a 'lung of the earth' comparable to the lung of man. Leonardo weighs whether the body of the earth could receive and expel the enormous volumes of tidal water in a single twelve-hour cycle, and concludes it could not, since that which draws is one and no more.