False vacuum and true vacuum form a metastable system. Metastable state is a state which does not change with time but is susceptible to falling into lower-energy states with only slight interaction. It is analogous to being at the bottom of a small valley when there is a deeper valley close by. True vacuum is the name applied to a …

It is an expanding bullet designed to grow on impact, increasing in diameter to limit penetration and/or produce a larger diameter wound. There are many expanding bullet designs though the most commonly encountered are hollow-point bullet and the soft-point bullet. Such bullets are sometimes known as ‘dum dum’ bullets. After an early British example produced in the Dum Dum arsenal, …

A dowser is another name for a water-diviner someone who can tell where there is water lying underground. The dowser uses a whole variety of tools to find the water, but one of the most common is the Y-shaped twig from a hazel tree. The dowser holds the twig by the two prongs of the Y, with the long stem …

There are about 500 active volcanoes around the world, not including those hidden by the sea. A few, such as Stromboli, a volcanic island off the north-eastern coast of Sicily, are active nearly all the time. But most undergo long periods when they are dormant, or sleeping. Some volcanoes stay dormant for so long that they are thought to be …

A digital pet (also known as a virtual pet or artificial pet) is a type of artificial human companion. They are usually kept for companionship or enjoyment. People may keep a digital pet in lieu of a real pet. Digital pets are distinct in that they have no concrete physical form other than the hardware they run on. Interaction with …

Look in any jeweller’s window, and you are bound to see diamonds, the white, glittering gems which are so valuable and so beautiful to look at. It’s hard to think that the diamonds which sparkle in the shop windows came out of the ground as lumps of dull rock, isn’t it? But this is how they are found. Diamonds are …

The word ‘dhow’ is used to describe any kind of Arab sailing vessel. They are the regular Arab trading craft, and make along voyages from the Red sea and Persian Gulf down to the African coast. The commonest are the small, square-sterned dhows known as ‘sambuks’. Then there are the smart ‘booms’, and the short, wide ‘bedeni’ from Muscat in …

A detergent is a substance, which has the power to cleanse. This description applies to soap, as well as to soap less shampoos and washing powders. A detergent must have surface activating properties, which means it must be able to tc break down the surface tension of water. In the process of cleaning the detergent acts as a bridge between …

Cold air flows outwards from the poles, while warm subtropical air moves polewards. The cold and warm air meet along a line called the polar front. Because of the great difference in temperature, the cold and warm air do not mix. Instead, warm air flows into bends in the polar front and cold air flows in behind it. This sets …

A datum (plural datums or data) is a reference point from which measurements are made. In surveying and geodesy, a datum is a set of reference points on the Earth’s surface against which position measurements are made. Horizontal datums are used for describing a point on the Earth’s surface in latitude and longitude or another coordinate system. Vertical datums measure …