Since pre-historic times mankind has been using tools to improve the quality of life and work efficiency. We are one of the few creatures on the planet that uses tools to assist in daily tasks. Our tools have improved significantly over the past few generations, particularly the last 150 years. One improvement that has significantly changed human machinery is hydraulics. Hydraulic systems are machines and tools that rely on high-pressure liquid power in order to function. The pure state of this liquid is essential to its function and so hydraulic liquid must be run through an oil purification system to filter out any contaminants. Various motors and cylinders transfer this hydraulic liquid through hoses, tubes, and control valves to the motors and cylinders. Many important machines that are in operation every day use hydraulic machinery in their function. Bulldozers are machines that are classified as crawlers, which means that they move using continuous tracks or rigid plates joined to each other set on a system of wheels. The purpose of continuous track movement is to distribute the weight of the machine over a larger surface than wheels are able to distribute. A bulldozer is equipped with a blade known as a "dozer blade." Although the term "bulldozer" is often used to describe many different kinds of machinery, its correct use describes only a tractor with a dozer blade. The blade is used to move sand, soil, or debris, particularly during construction work. In early bulldozers, the blade was controlled by a winch/brake system, but now they are fitted with hydraulic cylinders, which allow more precision in the blade's operation. A backhoe is another piece of machinery that uses hydraulics in order to function. A backhoe is a type of excavating equipment often referred to as a "digger." Essentially it is a tractor fitted with a two-part articulated arm with a digging bucket on the end for excavation purposes. The arm nearest the vehicle is called a "boom" and the second part of the arm is called the "dipper." The arm and bucket are powered by advanced hydraulic systems that allow precision of movement. Fork lifts are also powered by hydraulic systems. These machines are powered industrial trucks used in the lifting and transporting of materials, particularly lifting pallets in warehouses and distribution centers. The mast of a fork lift is what does the work of raising and lowering a load. The mast is usually operated by one or more hydraulic cylinders.