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## A FEW WORDS ABOUT LENGTH, MASS, TIME

The study of physics..enables us to give the answers to some of the many fascinating questions which concern the behaviour of machines, of electricity and of magnets, and of vibrations of light, heat and sound. /Many simple observations in physics may be made by naked eye, by touch or by ear. A blacksmith judges the temperature of hot iron by the colour of the glowing metal, knowing that there is a relation between brightness of glow and degree of hotness; a railway mechanic tests for flaws in the metal of carriage wheels by the sound of his hammer blows; the photographer often judges lighting conditions by eye. Our senses alone, however, are often not sufficiently trustworthy for our purposes. So we use measuring instruments in order to make our observations more precise and less affected by errors of the sense.

People may differ in their estimate of what is warm and what is tepid; a thermometer gives a more reliable value of the temperature. A great deal of attention has been given to the designing of scales of measurement, e. g. scales of length and of volume, and scales of weight, of temperature and of time.

There is generally a scale of centimetres, subdivided into millimetres: This scale is obtained from another standard of length, the international standard metre, which is also defined as the distance between two marks on a standard metal bar at a definite temperature. The prefix centi means a hundredth, so a centimetre is а hundredth of a metre; the prefix milli means a thousandth, so that a millimetre is a thousandth of a metre and is equal to a tenth of a centimetre. In similar fashion kilo means a thousand, so that a kilometre is а thousand metres.

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