Recreation and sport in which the participants use roller skates (shoes with sets of wheels attached) to move about on special rinks or paved surfaces. The invention of roller skates is traditionally credited to the Belgian Joseph Merlin in the 1760s, but the first practical four-wheel skate was designed in 1863 by James Plimpton of Medford, Mass. Roller-skating speed events became popular in the early 20th century. Later, team competitions in “roller derbies” on banked tracks became a spectator sport. Other roller-skate contests, such as acrobatics and hockey, followed. In the late 20th century, roller skates gave way to in-line (Rollerblade) skates, in which a single row of wheels is used in place of the standard rectangular configuration.
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One of the two types of rolling, or antifriction, bearings, the other being the ball bearing. Like a ball bearing, a roller bearing has two grooved tracks, but the balls are replaced by rollers. The rollers may be cylinders or shortened cones. If the rollers are cylindrical, only radial loads (perpendicular to the axis of rotation) can be carried, but with conical rollers both radial and thrust, or axial, loads (parallel to the axis of rotation) can be carried. In a given space, a roller bearing can carry a greater radial load than a ball bearing can.
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The rollers are an Old World family of near passerine birds, related to the kingfishers and bee-eaters. The group gets its name from the aerial acrobatics some of these birds perform during courtship or territorial flights.
Rollers resemble crows in size and build, ranging from 25 to 27 centimetres in length. They share the colourful appearance of those groups, blues and browns predominating. The two inner front toes are connected, but not the outer one.
They are insect eaters, with Eurystomus species taking their prey on the wing, and those of the genus Coracias catching it on the ground. They often perch prominently whilst hunting, like giant shrikes.
These are birds of warm climates. They nest in an unlined tree-hole, and lay 2-4 eggs. The eggs hatch after 17-20 days, and the young remain in the nest for approximately another 30 days.
The eleven species are: