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Q Why don't concrete ships sink? (Asked by John Battle, the Minister for Science and Technology - the 50,000th question that Science Line has answered since it was set up three years ago.)

Because of their shape. Any object placed in water displaces its weight in water - Archimedes' Principle. The water being displaced pushes up on the immersed object; gravity pulls the object down. If the object is shaped correctly, then the upward force equals the gravitational force before the object is immersed. Concrete is denser than water, but boats are designed with high enough sides that the displaced water can't get over the top to sink them. So a concrete slab sinks, but a concrete boat doesn't.

Q Shouldn't a science minister know Archimedes' Principle already? (The 50,001st question, asked by Charles Arthur, science editor of `The Independent')

Maybe he thought the answer would be interesting to the general public. Or maybe he knew, but wanted to refresh his memory.

Q Who discovered sunspots?

According to most Western science books, sunspots were discovered by Galileo in 1610. His theories were seen as blasphemous, denying the perfection of the heavens.

As with a number of Galileo's "discoveries", they had been discovered before. Italian observers noticed sunspots in 1457; even further back, the Arabian astronomers Ibn Rushd (in 1196) and Abu al-Fadl Ja'far ibn al-Muqtafi (in 840) had described them. The earliest mention of sunspots in the West was in Einhard's Life of Charlemagne in AD807.

However, to find the earliest systematic reports of sunspots we must travel to China: in the fourth century BC, Kan Te described sunspots for the first time. The date of this discovery may seem unbelievable from our "modern" Western scientific viewpoint, but China was far in advance of the West.

Q How far above the Earth did Mir and the Shuttle dock?

When the two craft docked at the end of June 1995, they were 295km (184.3 miles) above the Earth's surface.

Q Why don't railway lines buckle in the heat?

They do sometimes, but are laid to reduce the amount of buckling. There are two types of track: jointed rail and continuous welded rail. Jointed rail comes in 60ft (18m) lengths and is bolted at the ends by a fishplate. Each track end has two holes in it and the fishplate has four. The plate is bolted to the track and the bolts are tightened. However, a small gap is left to allow for expansion and contraction between the ends of each rail. In addition, a technique called "sun locking" is used, whereby the track is laid, and then on a hot day the bolts are finally tightened fully.

Continuous welded rail comes in a variety of lengths which have about a metre of overlap at each end. The ends are shaped like wedges, so that the track can expand and contract length-wise without there being any break in the entire length.

Jointed rail track makes the noise we all associate with train travel - clickety-clack, clickety-clack. The continuous welded rail should be quieter.

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