How do they know the continents are drifting?

As long ago as 1620, the English philosopher Francis Bacon pointed out the close fit between the coastlines of South America and Africa. The shapes suggested that they had once been joined but had drifted apart.

In 1912 the German meteorologist Alfred Wegener put forward detailed arguments in support of the theory. But it was not proved to the satisfaction of most geologists until the 1960s.

The fit between the two continents is good, particularly the continental edge rather than the shorelines. Their shape has been altered by the eroding effects of tides. But the continental outlines where the ocean is 3000ft (900m) deep show the average ‘misfit’ when joined to be only about 50 miles (80km).

Other evidence that the continents were once linked includes common geological features, such as similar types of rock of a similar age. And many plants and creatures appear to share a common origin. For example, many freshwater fish in South America are closely related to African species, and it is difficult to accept that they could have swum the Atlantic Ocean.

The guinea pig is found in the wild in both Africa and South America, but nowhere else. Also, monkeys are indigenous only in those continents and it is unlikely they would have evolved independently in each place.


Picture Credit : Google

How the speed of light was measured?

A French physicist, Jean Foucault, measured the speed of light remarkably accurately in the mid-19th century, using two mirrors set 66ft (20m) apart.

One mirror was fixed and another rotated at 800 revolutions a second. Beams of light were directed at the rotating mirror. When a light beam hit the mirror while it was at just the right angle, it was reflected to the fixed mirror, bounced back to the rotating mirror and then reflected back to the source.

In the time it took to make the return trip between the mirrors, the rotating mirror had turned through a small angle so the beam that returned to the source deviated slightly from the original path.

By using the deviation of the beam to measure the angle through which the mirror had moved, and knowing the speed of rotation, Foucault could work out how long the light had taken for its trip, and its speed. Foucault’s final result, reported in 1862, worked out at 187,000 miles a second (300,939km a second).

Foucault’s method was refine in the 1920s by Albert Michelson, an American physicist, who sent light through a vacuum tube a mile (1.6km) long to remove the effect of air on its speed. Modern measurements have refined the figure to 186,282 miles (299,793km) a second.


Picture Credit : Google