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Lichen

A lichen is not one organism. It is a fungus and an alga living as one body. The fungus provides the structure and holds water. The alga photosynthesises and makes sugar. Separated, the fungus starves and the alga dries out. Because it needs no soil, a lichen can grow on bare rock, on gravestones and on the roofs of cars that have not moved for a decade. Lichens grow extremely slowly — some crustose species less than a millimetre a year. Scientists use that rate to date rock surfaces, a method called lichenometry. Lichens also absorb whatever is in the air. Where the air is dirty, they disappear, which makes them a living measure of pollution.

The Longitude Problem

A ship's latitude — how far north or south it is — can be found from the height of the sun at noon. Sailors could do that for centuries. Longitude, how far east or west, was the problem. It has no natural marker. The solution was known in principle: compare the time on board with the time at a fixed place. The Earth turns fifteen degrees an hour, so every hour of difference is fifteen degrees of longitude. That required a clock that stayed accurate on a rolling, salty, temperature-swinging ship. No such clock existed. John Harrison, a carpenter, spent forty-three years building four. The fourth, finished in 1759, was the size of a large pocket watch and lost five seconds in eighty-one days at sea.