ABSTRACT
A motor vehicle is a complex machine. It can be useful, but it can be dangerous. Danger from external causes may arise from the speed at which it is driven, the road surface and the density of external hazards. The driver may be aware by eyes and ears of the condition of the road surface and of other hazards. By visual and bodily sensations he may know when he is driving at high or low speed. But to know at what speed he is travelling, especially when speed is regulated in the interest of public safety, he relies on a speedometer. Dangers and impediments from the machine itself may arise from excessive engine temperature, shortage of fuel, faults in the electrical system, faults in the braking system, and so on. Engine, gas tank and other parts of the vehicle are all physically distant from the driving seat. The driver depends, for warning and guidance on the state of the vehicle, entirely on instruments-gauges, dials, tell-tale lights. He is warned by the maker of the vehicle that parts of the machine need service after so many hours or kilometres of running; for distance travelled he relies on a distance meter, an odometer. These are the minimum instruments of a modern road vehicle. The instrumentation of modern ships, airplanes and space vehicles is far more complex, for the machines are more intricate, the hazards more numerous, and the consequences of failure more serious. Accurate, sensitive instruments and sets of instruments are integral parts of their design and use.
