Science: Technoquest

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Q IF THE tide is said to be six metres high, what's that six metres measured from?

The six metres is the height of the tide above "Chart Datum", which is different for every port around the coast because of variations in the shape of the land, the depth of the sea, and so on. It is defined as being approximately the level of the Lowest Astronomical Tide, which is the lowest tide that can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions.

This means that on rare occasions, when a particularly high tide is predicted to occur, the following low tide may be low enough to have a negative value.

Q DOES GEOGRAPHICAL evidence support Milancovitch's theory on the ice ages?

Milancovitch suggested that there was and still is a cycle of ice ages and warmer climates. As a result different layers of sediment would be laid down over history. There is some possible evidence in sediments for this theory of oscillating climates to hold true. Undoubtedly there are cycles of limestone, shale, sandstone and coal measures in Carboniferous and Jurassic rocks. These changes in rock type represent changes in climate on a cyclic basis. There are even subtle changes in layers of chalk - which repeat themselves and could reflect climatic cycles.

But inferring rates of climatic change from rock types is very difficult and so far there is no conclusive geological proof of the Milancovitch theory.

Q PORTERS IN Nepal carry heavy weights suspended from their heads, whereas Westerners tend to carry such weights suspended from shoulders, in rucksacks. Which is the better method?

Most rucksacks, or at least the larger ones, have waist straps to take most of the weight. In a well-fitting rucksack, most weight will be taken on the hips, with the shoulder straps just used for balance.

It is thought that this is better for you than using straps around the head, which put an enormous strain on your neck and back vertebrae - not the strongest parts of your skeleton. Your hips, on the other hand, as long as the load is spread evenly, can take quite a weight before being damaged.

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