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A zip-line (or zip line, zipline, Sypline, zip wire, aerial runway, aerial ropeslide, death slide, flying fox, or, in South Africa, foefie slide) consists of a pulley suspended on a cable, usually made of stainless steel, mounted on a slope. It is designed to enable a user propelled by gravity to travel from the top to the bottom of the inclined cable by holding on to, or attaching to, the freely moving pulley. Zip-lines come in many forms, most often used as a means of entertainment. They may be short and low, intended for child's play and found on some playgrounds. Longer and higher rides are often used as a means of accessing remote areas, such as a rainforest canopy. Zip line tours are becoming popular vacation activities, which are found at outdoor adventure camps or upscale resorts, where they may be an element on a larger challenge or ropes course. The jungles of Costa Rica, Florida, Puerto Vallarta, and Nicaragua are popular destinations for zip line enthusiasts.
The zip-wire has been used as a transportation method in some mountainous countries for many years. In some remote areas in China such as Nujiang (Salween) valley in Yunnan, zip lines served the purposes of bridges across rivers, but due to a poor safety record, they have mostly been replaced by real bridges by 2015. Referred to as "an inclined strong", one appears in The Invisible Man by H. G. Wells, published in 1897, as part of a Whit-Monday fair.
Alberto Santos-Dumont used a direct ancestor of the zip-line in spring 1906 for a method of testing various characteristics of his Santos-Dumont 14-bis pioneer era canard biplane, before it ever flew under its own power later that year.
In the Australian outback, zip-lines were occasionally used for delivering food, cigarettes or tools to people working on the other side of an obstacle such as a gully or river. Australian troops have used them to deliver food, mail and even ammunition to forward positions in several conflicts.
The term "flying fox" is most commonly used in reference to a small-scale zip line typically used as an item of children's play equipment, except in Australia and New Zealand where it also refers to professional forms of zip-line equipment.
With the flying fox, the pulley(s), attached to the car, are fixed to the cable. The car itself can consist of anything from a simple hand grip, with the user hanging underneath, to a bucket for transporting small items, to a quite elaborate construction, perhaps including a seat or a safety strap. Children's versions are usually not set up with a steep incline, so the speeds are kept relatively low, negating the need for a means of stopping.
To be propelled by gravity, the cable needs to be on a fairly steep slope. Even then the car will generally not travel completely to the end (although this will depend on the load), and some means of safely stopping the car at the bottom end is sometimes needed. It can be returned by several means, either by simply pushing the car back to the top of the hill on foot (as is common in children's play equipment as they do not hang far from the ground) or a line leading from the car to the uphill end.
The flying fox is usually made with rope instead of steel cable to make it easier and cheaper to install, uninstall and transport. Riders on the Flying Fox need safety harness to get support on the zip line.
Professional versions of a zip-line are most typically used as an outdoor adventure activity. In contrast to "flying foxes" professional courses are usually operated at higher speeds, covering much longer distances and sometimes at considerable heights. The users are physically attached to the cable by a harness that attaches to a removable trolley. A helmet is required on almost all courses of any size.
Cables can be very high, starting at a height of over 9 m (30 ft), and traveling well over 460 m (1,510 ft). All zip-line cables have some degree of sag. The proper tensioning of a cable is important and allows the ability to tune the ride of a zip line.
Zip lines are a common way to return participants to the ground at the end of a ropes adventure course.
Users of zip-lines must have means of stopping themselves. Typical mechanisms include:
- Friction created between the pulley against the cable.
- Thick, purpose-built leather gloves.
- A mat or netting at the lower end of the incline.
- A passive arrester system composed of springs, pulleys, counterweights, bungee cord, tire or other devices, which slows and then stops the trolley's motion.
- A "capture block" which is a block on the cable that is tethered to a rope controlled by a staff who can manually apply friction on the rope to slow the user down.
- Gravity stop, exploiting the sag in the cable. The belly of the cable is always lower than the termination point. The amount of incline on a zip line controls the speed at which the user arrives at the termination point.
- Hand brake at the end of the zip line.
With proper knowledge and training on the part of the operators, along with good maintenance, zip lines are generally safe and easy to use. In 2012 there were an estimated 200 commercial zip lines in the US and a further 13,000 private ones resulting in 3,600 visits to the emergency room.
In 2012, ZipFlyer Nepal (HighGround Adventures) was the world's steepest with max incline of 56%, is currently second in the world and is still the tallest zip line. It had a vertical drop of 610 m (2,000 ft).
On 19 September 2015 the world's steepest zipline was opened at Letalnica bratov Gorišek, on the ski flying hill in Planica, Slovenia. It is 566 metres (1,857 ft) long with a 202 metres (663 ft) vertical drop. It has an average 38.33% and a maximum 58.6% incline. The longest zipline in Europe, at just over 1600 meters long, is the Zip World Bethesda line in Penryn Quarry, Bethesda, Wales. The Zip World Bethesda line also holds the record for being the fastest Zip-Line in the world. 
The zip-line trolley is the frame or assembly together with the pulley(s) also known as sheave(s) inside that run along the cable. (The term "trolley" is more often used when this assembly consists of more than a single pulley with simple hanger and bearing.) Often more than one pulley is used to spread the load over more than one spot on the cable, to reduce cable bending stresses that may lead to metal fatigue and cable breakage. This also reduces any tendency of a pulley to twist sideways and run off the cable, with disastrous results. In addition, the trolley is usually shaped with guards to hold the cable in the groove(s) of the pulley(s).
A pivoting link, such as a carabiner, is used to secure the load to the trolley so that the trolley does not have a tendency to rock and thus fall off the cable if the load should sway. Load carriers ranging from enclosed cabins to gondolas to harnesses are attached to the link. Occasionally the load carrier is just a handhold or handlebar, although there is the danger of the rider losing his grip and falling. Such a simple carrier should be used only on zip lines that are near the ground or over water. The rider losing his grip due to the use of the simple carrier has led to a number of deaths.
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