The computer is a wonderful machine. It is the most important invention since the type of engine used in cars. Today it has the greatest effect on science, industry and business, but it is being used more in education and medicine. By the end of this century it will touch the lives of everyone, even people in distant villages. It is a revolutionary invention. The oldest kind of computer is the abacus, used in China since the sixth century. In the seventeenth century an adding machine was invented, but the first large, modern computer was built in 1973. A few years later a computer could do 5,000 additions per second. Now the computations are so fast that they are measured in nanoseconds. Today most computers are stored-program computers, that is, they have a memory. They are getting smaller and smaller, and computing faster and faster. Even in a large computer, the part that does the actual computing is about the size of the end of a finger. Computers can do all kinds of work. When someone buys something in a department store, in formation about the sale goes into a computer. A scientist can talk to the computer about the rocks, and the computer answers the questions. A doctor can talk to the computer and explains what is wrong with a patient. If the doctor asks why, the computer goes through its stored information and ex plains exactly why. When early humans began fanning, it was a revolutionary change in human life. It was hundreds of thousands of years later that people developed a writing system. In less than fifty years people have developed computers that can do most of the things humans can do. This could be a frightening development. The oldest kind of computer, the abacus, has been used since ______.