Practice Test 21: Microplastics and the Problem of Longitude

A Plastic that is thrown away does not disappear; it breaks into smaller and smaller fragments. Pieces smaller than five millimetres are known as microplastics, and they have been found almost everywhere scientists have looked, from deep ocean trenches to mountain snow. Some are made small on purpose, such as the tiny beads once added to face scrubs, but most come from the gradual wearing down of larger objects.

B One major source is clothing. Synthetic fabrics such as polyester shed thousands of fibres in every wash, and these pass through sewage treatment plants into rivers and seas. Another is the wear of car tyres on roads, which sends fine rubber particles into drains. Researchers estimate that these two sources together account for a large share of the microplastics that reach the oceans, although the figures vary widely between studies.

C Once in the water, microplastics are swallowed by plankton, shellfish and fish, and so they enter the human diet. They have been detected in sea salt, in seafood and in samples of drinking water. What they do to human health is not yet clear. Laboratory studies suggest that some particles may cause inflammation in cells, but scientists stress that these experiments often use concentrations much higher than people are likely to encounter.

D Because the health evidence is uncertain, some argue that policy should wait for more data. Others reply that plastic persists for centuries, so waiting makes the problem harder to reverse. Practical steps already exist: filters can be fitted to washing machines, and several countries have banned microbeads from cosmetics. The challenge is to apply such measures widely without waiting for a perfect understanding of the risk.

A Sailors have long been able to find their latitude, their distance north or south of the equator, by measuring the height of the Sun or a star above the horizon. Longitude, their position east or west, was a different matter. Because the Earth turns steadily, every hour of difference in local time between two places corresponds to fifteen degrees of longitude. A navigator who knew the time at home port and the local time on board could therefore work out how far east or west the ship had travelled.

B The difficulty was keeping accurate time at sea. Pendulum clocks, the best timekeepers on land, were useless on a rolling ship, and changes in temperature and humidity made other clocks run fast or slow. The cost of the problem was measured in lives and money: ships wandered off course, ran short of food and were wrecked on rocks they did not know were there. In 1714 the British government passed an Act offering a large prize for a method of finding longitude at sea to a stated accuracy.

C Many astronomers believed that the answer lay in the sky and proposed methods based on the movement of the Moon among the stars. These worked in principle but required difficult observations and long calculations that few sailors could perform on deck. The prize was eventually pursued most successfully by a carpenter and clockmaker, John Harrison, who spent decades building a series of increasingly refined timekeepers designed to keep steady time despite the motion of a ship.

D Harrison's final design, a large watch known as H4, was tested on a voyage to Jamaica in the early 1760s and lost only a few seconds during weeks at sea. Even so, the prize committee was slow to accept his achievement, partly because some members favoured the astronomical approach. In the end, it was the chronometer, a precise portable clock, that became the standard instrument for navigation, and it remained so until the arrival of satellite systems.