LASERS

An important recent development is that of the laser, an acronym for light amplification by stimulated emission of radiation. In lasers , which may have gases, liquids, or solids as the working substance, a large number of atoms are raised to a high energy level and caused to release this energy simultaneously, producing coherent light where all waves are in phase. Similar techniques are used for producing microwave emissions by the use of masers. The coherence of the light allows for very high intensity, sharp wavelength light beams that remain narrow over tremendous distances; they are far more intense than light from any other source. Continuous lasers can deliver hundreds of watts of power, and pulsed lasers can produce millions of watts of power for very short periods. Developed during the 1950s and 1960s, largely by the American engineer and inventor Gordon Gould and the American physicists Charles Hard Townes, T. H. Maiman, Arthur Leonard Schawlow, and Ali Javan, the laser today has become an extremely powerful tool in research and technology, with applications in communications, medicine, navigation, metallurgy, fusion, and material cutting

Historians surmise that the earliest ships appeared around 16,000 bc in Europe, and perhaps earlier in Asia and Africa. Little archaeological evidence for these prehistoric vessels survives because they were made from perishable materials. Prehistoric drawings illustrate that reindeer hunters of central and western Europe made hulls of animal skins sewn together around a birch wood frame, and archaeologists have discovered vessels made from skin and reindeer antlers dating from 9000 bc. Seal hunters plied the frigid waters of the northern Atlantic in boats constructed from sealskin stretched over frames of wood or whalebone. Ancient peoples used coracles, round, skin-covered vessels with wicker frames, to fish the lakes and rivers of what are now Ireland and Wales. Larger but similarly constructed currachs could sail the open waters of northern Europe. Hunters and fishers all over the world constructed similar hulls from birch bark, balsa wood, papyrus, ox hide, and other local materials.

The boat is quite similar to those used in ancient Egypt, which were made out of papyrus stalks bunched together. Many people believe Egyptians used this kind of boat to sail to America. Although there is no definite proof that this is true, explorer Thor Heyerdahl accomplished the journey in a papyrus boat modeled on ancient lines, proving that it is possible

How Aeroplanes Fly

Airplane,, engine-driven vehicle that can fly through the air supported by the action of air against its wings. Airplanes are heavier than air, in contrast to vehicles such as balloons and airships, which are lighter than air. Airplanes also differ from other heavier-than-air craft, such as helicopters, because they have rigid wings; control surfaces, such as movable parts of the wings and tail, which make it possible to guide their flight; and power plants, or special engines that permit level or climbing flight.

Airplanes range in size from the single-seat, single-engine private plane to massive jumbo jets capable of carrying hundreds of passengers. A versatile and relatively fast means of transportation, many models of the airplane have been designed for a diverse set of purposes.

Pontoons allow this bush plane to land on Lake Abitibbi in Ontario, Canada. Seaplanes have a variety of uses, but they are particularly valuable to bush pilots who must frequently make landing in sea