Quantity of Matter

Materials quite obviously take up space; we say they have volume. In Britain we buy petrol by the gallon, on the continent we buy it by the litre. The gallon, the litre, are all units of volume – measures of a quantity of material bought or sold. But it is also quite common to buy and sell things by weighing them in ounces, pounds, kilograms or tons.

The simplest form of weighing machine consists of a balanced level with equal arms. When two identical lumps of material are hung from the ends of the arms, they exactly balance each other. If one of the lumps, say the coin is replaced by something quite different but which still balances it, then we say that the two things have the same quantity of matter. To quantity of matter measured in this way we give the name mass.

In this balancing method we are really balancing two forces, the weights of the objects. It is important to distinguish between the mass which we measure this way and the weight which helps us to do so. The heaviness of objects is due to the attraction which our planet, the Earth, has for them. This heaviness is different at other places. Thus it has been calculated that objects on the Moon have only one-sixth of their earth-weight; a 10-stone boy would weigh only 23 pounds on the Moon, would find it possible to jump 30 feet. And throw a cricket ball a quarter of a mile. The space traveller of the future will find that 50 pounds of luggage become less heavy the further he goes from the Earth. But the quantity of it – its mass – will not change; it will still balance 50 pounds on a level balance.

The standards of mass on the metric system and on the British system are the kilogram and the pound respectively The abbreviations for these are kg and lb. The abbreviations for the forces which the Earth has on them are Kg (for the kilogram-weight) and Lb (for pound-weight).

Time

A time scale must be based on some happening which takes place regularly. The regular rotation of the Earth, which governs the rising and setting of the Sun or the passage of a star across the true north-south line (meridian), gives us our time-unit, the day, which is subdivided into hours, minutes and seconds. The time between successive transits of a star across the meridian is known as a sidereal day (sider, sideris, a star), and standard clocks are checked against this time.

Although sidereal time is of great importance to the astronomer, it is the mean solar day (the average time between successive transits of the Sun across the meridian) which is the unit on which the hours, minutes and seconds of daily life are based.

The division of the day into its parts is brought about by means of clocks. The Egyptians made use of the rate at which water or sand flowed through a hole in a vessel, while some people used the regular burning of a candle or oil-lamp. Most modern clocks are based on some type of oscillating system. A pendulum swings to and fro in a time which is almost independent of the extent of the swings.

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