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Childbed fever comes from decaying matter, from corpses or from the living, like a cancer or a festering wound, carried to the mothers on hands and instruments, and now and then through a ward’s air. Wash hands in chlorinated lime between every examination, and the fever goes.

In October, the doctors examined a woman with a foul-smelling cancer of the womb, washed only with soap, and went on to twelve women in labor. Eleven of them died, though no corpse was involved.

Childbed fever kills mothers four times as often in the doctors’ clinic as in the midwives’ clinic next door, though both breathe the same air. Since the doctors began washing their hands in chlorine after dissecting corpses, deaths have fallen from 18 in a hundred births to 2. The cause comes on the doctors’ hands, not from the air both clinics share.

Childbed fever comes from particles of corpses, which doctors and students carry on their hands from the dissecting room to the mothers they examine. Wash them off in chlorinated lime, and the fever goes.

The Analytical Engine can’t originate anything. It can do whatever we know how to order it to perform, and follow our reasoning, but it can’t anticipate any truths we haven’t worked out ourselves.

The Analytical Engine needn’t stop at numbers. It works on symbols by rules, and symbols can stand for anything whose relations can be put into rules, like the notes of music. So it could compose music, or work in any field whose rules we can write down.

A machine can carry out any calculation at all, however long. Punched cards tell it which operations to perform on which numbers. It keeps the numbers in a store, works on them in a mill, and chooses which cards to follow next by its own results.

Diseases like cholera and fevers come from miasma: bad air given off by rotting filth, sewage, and swamps. Clean away the filth and its smell, and the disease goes with it.

The parallax is there, only tiny. Over the year, 61 Cygni moves back and forth by 0.31 seconds of arc either way against two fainter stars beside it, just as it should if the Earth circles the Sun. The stars are simply that far away: 61 Cygni is about 660,000 times farther from us than the Sun.

Mathematical tables can be worked out and printed by machinery. Almost any table can be built up by adding differences, and gears can add, so a machine of number wheels can do it, without the mistakes that human computers and typesetters make.

I put a small round disk in a narrow beam of light, and the bright spot is there, at the center of its shadow, just as Fresnel’s theory says.

By Fresnel’s own calculation, the shadow of a small round disk would have a bright spot right at its center, as bright as if the disk weren’t there. That’s absurd.

Light does bend into shadows, only very slightly, because its waves are so short. That makes the fine fringes along the edge of a shadow, and my calculation of the waves predicts exactly where they fall.

Light added to light can make darkness. Hold a thin card in a narrow beam of light, and bright and dark stripes appear in its shadow; block the light passing one side of it, and they vanish. Waves can cancel each other out. Streams of particles can’t.

Gamma Draconis, and the other stars I’ve measured, shift back and forth over the year by about 20 seconds of arc, always toward the direction the Earth is moving in at the time. Light takes time to arrive, and the Earth’s motion slants the direction it seems to come from. A motionless Earth can’t produce this.

A wave would bend around an obstacle and spread into its shadow, as sound bends around a corner. But light casts sharp shadows.

Light is a stream of tiny particles thrown off by shining things. They fly in straight lines, which is why shadows are sharp. Glass and water pull them in as they enter, which bends their path and speeds them up.

Light is a wave: a disturbance spreading through an invisible medium, the ether, as sound spreads through air. It travels more slowly in glass and water than in air, which is why its path bends as it enters them.

Every object pulls on every other object. The pull grows with their masses and weakens with the square of the distance between them. This one law explains the orbits of the planets and why things fall on Earth.

Write every idea in a universal language of symbols, and fixed rules for calculating with them, which a machine could follow, will settle any question that doesn’t turn on facts about the world. Then such disputes can be settled without arguing: the two sides can sit down and calculate.

The Earth stands still at the center. The Moon, the Sun, Jupiter, and Saturn roughly circle it, and Mercury, Venus, and Mars go around the Sun on Kepler’s ellipses, so they fit Tycho’s measurements.

Everything on the Earth shares its motion, just as everything on a smoothly sailing ship shares the ship’s. A stone dropped from the top of the mast lands at its foot, and a cannonball keeps the Earth’s motion too, so it flies as far to the east as to the west. Equal distances show nothing about whether the Earth spins.

Jupiter carries four moons around with it as it moves. So a moving Earth can keep its Moon, too.

Venus shows phases like the Moon’s, from nearly full to a thin crescent, and looks largest when it’s a crescent. In Ptolemy’s system, Venus always stays between the Earth and the Sun, so it could only ever show us a crescent.

The Earth is a planet. It spins once a day and goes around the Sun once a year, as the other planets do. Mars moves on an ellipse, a stretched circle, with the Sun at one of the ellipse’s two foci, so that the line from the Sun to the planet sweeps out equal areas in equal times, and the other planets, I expect, move the same way.