Science: Technoquest; Quake pointers/ Balanced Moon/ Glowing time/ Night blossoms

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Q Who invented the seismograph?

This has been attributed to De la Hautefeuille (1703); the first modern forms were developed in 1848. Some books attribute the first modern pendulum seismograph to the German Emil Wiechert, in 1928. However, the forerunner can be traced to 132AD during China's Han Dynasty. It was important then (as now) for the government to be forewarned of earthquakes.

Chang Heng, the Astronomer Royal, designed a fine-cast bronze vessel, 6ft across, with a domed lid and eight dragons' heads with bronze balls in their mouths, and eight bronze toads below looking up with their mouths open. A central column moved when an earthquake occurred, and pushed a slider; this dislodged the ball in a dragon's mouth, to fall into a toad's mouth, giving a rough indication of the direction of the disturbance. There was a mechanism so only one ball could fall, as a protection against being set off by secondary tremors. Unfortunately none of these incredible objects has survived.

Q Where do the Moon's and Earth's

gravitational fields exactly balance?

About 62,000km (39,000 miles) from the Moon - which itself is 384,000 km (240,000 miles) from Earth on average - the pull from Earth and the pull from the Moon cancel out. The Moon's gravity is about one-sixth that of Earth's, and this distance is about one-sixth of the way between them. This point is called the L1 (Lagrange-1) point between Earth and Moon. Apollo 8 had a radio call from mission control when they passed this point in December 1968, on their way to the first human orbit of the Moon.

Q Are luminous watch faces dangerous?

Not now. But about 35 to 40 years ago it was common to use radioactive radium paint to make parts of wrist watch dials luminous. This posed a health hazard - particularly to the dial painters, many of whom developed cancers. The watch industry eventually acknowledged the hazards, and so turned to tritium (radioactive hydrogen) and phosphor paints for luminescence. This technology is still used by some watchmakers, though a better technology now uses tritium gas enclosed in glass vials, which excite or activate a phosphor coating to provide a self-illumination. This minimises any hazard, since the tritium gas cannot penetrate the glass vial; if a vial is broken, the gas simply dissipates into the atmosphere.

Q Why do some wild flowers flower at night?

Flowers control various pollinating mechanisms - including attracting insects and other animals to carry pollen from one flower to another. Those opening at dusk tend to have moths and/or bats pollinating them; it would be a waste of energy to open at other times when pollinators weren't available. The flowers are usually very pale, and show up in available light at dusk. Sweet scent is a common feature.

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