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By Root 3100 0

of a principle which proved immensely fruitful in dynamics, the principle that, when several forces act simultaneously, the effect is as if each acted in turn. This is part of a more general principle called the parallelogram law. Suppose, for example, that you are on the deck of a moving ship, and you walk across the deck. While you are walking the ship has moved on, so that, in relation to the water, you have moved both forward and across the direction of the ship's motion. If you want to know where you will have got to in relation to the water, you may suppose that first you stood still while the ship moved, and then, for an equal time, the ship stood still while you walked across it. The same principle applies to forces. This makes it possible to work out the total effect of a number of forces, and makes it feasible to analyse physical phenomena, discovering the separate laws of the several forces to which moving bodies are subject. It was Galileo who introduced this immensely fruitful method.

In what I have been saying, I have tried to speak, as nearly as possible, in the language of the seventeenth century. Modern language is different in important respects, but to explain what the seventeenth century achieved it is desirable to adopt its modes of expression for the time being.

The law of inertia explained a puzzle which, before Galileo, the Copernican system had been unable to explain. As observed above, if you drop a stone from the top of a tower, it will fall at the foot of the tower, not somewhat to the west of it; yet, if the earth is rotating, it ought to have slipped away a certain distance during the fall of the stone. The reason this does not happen is that the stone retains the velocity of rotation which, before being dropped, it shared with everything else on the earth's surface. In fact, if the tower were high enough, there would be the opposite effect to that expected by the opponents of Copernicus. The top of the tower, being further from the centre of the earth than the bottom, is moving faster, and therefore the stone should fall slightly to the east of the foot of the tower. This effect, however, would be too slight to be measurable.

Galileo ardently adopted the heliocentric system; he corresponded with Kepler, and accepted his discoveries. Having heard that a Dutchman had lately invented a telescope, Galileo made one himself, and very quickly discovered a number of important things. He found that the Milky Way consists of a multitude of separate stars. He observed the phases of Venus, which Copernicus knew to be implied by his theory, but which the naked eye was unable to perceive. He discovered the satellites of Jupiter, which, in honour of his employer, he called 'sidera medicea'. It was found that these satellites obey Kepler's laws. There was, however, a difficulty. There had always been seven heavenly bodies, the five planets and the sun and moon; now seven is a sacred number. Is not the Sabbath the seventh day? Were there not the seven-branched candlesticks and the seven churches of Asia? What, then, could be more appropriate than that there should be seven heavenly bodies? But if we have to add Jupiter's four moons, that makes eleven—a number which has no mystic properties. On this ground the traditionalists denounced the telescope, refused to look through it, and maintained that it revealed only delusions. Galileo wrote to Kepler wishing they could have a good laugh together at the stupidity of 'the mob'; the rest of his letter makes it plain that 'the mob' consisted of the professors of philosophy, who tried to conjure away Jupiter's moons, using 'logic-chopping arguments as though they were magical incantations'.

Galileo, as everyone knows, was condemned by the Inquisition, first privately in 1616, and then publicly in 1633, on which latter occasion he recanted, and promised never again to maintain that the earth rotates or revolves. The Inquisition was successful in putting an end to science in Italy, which did not revive there for centuries. But it failed to prevent men of science