instance_of block 
The circumference ( from Latin " circumferentia ", meaning " carrying around ") of a closed ___MASK98___ or circular object is the linear distance around its edge . The circumference of a ___MASK50___ is of special importance in geometry and trigonometry . Informally " circumference " may also refer to the edge itself rather than to the length of the edge . Circumference is a special ___MASK7___ of perimeter : the perimeter is the length around any closed figure , but conventionally " perimeter " is typically used in reference to a polygon while " circumference " typically refers to a continuously differentiable ___MASK98___ . An airfoil ( in American English ) or aerofoil ( in British English ) is the ___MASK56___ of a ___MASK13___ , blade ( of a propeller , rotor , or turbine ), or sail ( as seen in cross - section ). Sailing comprises ___MASK54___ propulsion of a ___MASK49___ by means of sails or other airfoils and steering it over water , ice or land , depending on the type of ___MASK49___ . A sailor manages the force of the ___MASK54___ on the sails by adjusting their angle with respect to the moving sailing ___MASK49___ and sometimes by adjusting the sail area . The force transmitted from the sails is resisted by forces from the hull , keel , and rudder of a sailing ___MASK49___ , by forces from skate runners for an iceboat , and by forces from wheels for a land sailing ___MASK49___ to allow steering a course on a point of sail with respect to the true ___MASK54___ . In sailing , a block is a ___MASK16___ or multiple pulley . One or a number of sheaves are enclosed in an assembly between cheeks or chocks . In use , a block is fixed to the end of a line , to a spar , or to a surface . A line ( rope ) is reeved through the sheaves , and maybe through one or more matching blocks at some far end , to make up a tackle . The purchase of a tackle refers to its mechanical advantage . In general the more sheaves in the blocks that make up a tackle , the higher its mechanical advantage . The matter is slightly complicated by the fact that every tackle has a working end where the final run of rope leaves the last sheave . More mechanical advantage can be obtained if this end is attached to the moving load rather than the fixed end of the tackle . There are various types of blocks that are used in sailing . Some blocks are used to increase mechanical advantage and others are used simply to change the direction of a line . A ratchet block turns freely when a line is pulled in one direction but does not turn the other direction , although the line may slip past the sheave . This kind of block makes a loaded line easier to hold by hand , and is sometimes used on smaller boats for lines like main and jib sheets that are frequently adjusted . A cable is ___MASK75___ or more wires ___MASK58___ side by side and bonded , twisted , or braided together to form a ___MASK16___ assembly . The term originally referred to a nautical line of specific length where multiple ropes , each laid clockwise , are then laid together anti - clockwise and shackled to produce a strong thick line , resistant to water absorption , that was used to anchor large ships . A pulley is a wheel on an axle or shaft that is designed to support movement and ___MASK94___ of direction of a taut cable , rope or ___MASK85___ along its circumference . Pulleys are used in a variety of ways to lift loads , apply forces , and to transmit power . In nautical contexts , the assembly of wheel , axle , and supporting shell is referred to as a " block ." ___MASK65___ rope is rope made from ___MASK65___ . It consists of several strands of metal ___MASK65___ laid ( twisted ) into a helix . The term " cable " is often used interchangeably with " ___MASK65___ rope ", but narrower senses exist in which " ___MASK65___ rope " refers to diameter larger than 3 / 8 inch ( 9 . 52 mm ), whereas sizes smaller than this are designated cable or cords . Initially wrought iron wires were used , but today ___MASK24___ is the main material used for ___MASK65___ ropes . ___MASK54___ is the flow of gases on a large ___MASK40___ . On the ___MASK84___ of the Earth , wind consists of the bulk movement of air . In outer space , solar wind is the movement of gases or charged particles from the Sun through space , while planetary wind is the outgassing of light chemical elements from a planet ' s atmosphere into space . Winds are commonly classified by their spatial ___MASK40___ , their speed , the types of forces that cause them , the regions in which they occur , and their effect . The strongest observed winds on a planet in the Solar System occur on Neptune and Saturn . Winds have various aspects , an important one being its velocity ; another the density of the gas involved ; another its energy content or wind energy . A sheave is a pulley with a grooved wheel for holding a ___MASK85___ , wire rope , or rope . The grooved wheel spins inside the frame of the sheave . This allows the wire or rope to move freely minimizing wear and abrasion on cable . Sheaves can be used to redirect a cable or rope , lift loads , and transmit power . The words " sheave " and " pulley " are sometimes used interchangeably . ___MASK25___ of sail is a term used to describe a sailing ___MASK41___ ' s orientation in ___MASK4___ to the current wind direction . A wheel is a circular ___MASK63___ that is intended to rotate on an axle bearing . The wheel is one of the main components of the wheel and axle which is one of the ___MASK60___ simple machines . Wheels , in conjunction with axles , allow heavy objects to be moved easily facilitating movement or transportation while supporting a load , or performing labor in machines . Wheels are also used for other purposes , such as a ship ' s wheel , steering wheel , potter ' s wheel and flywheel . A rudder is a primary control ___MASK84___ used to steer a ship , boat , submarine , hovercraft , aircraft , or other conveyance that moves through a fluid medium ( generally air or ___MASK14___ ). On an aircraft the rudder is used primarily to counter adverse yaw and p - factor and is not the primary control used to turn the airplane . A rudder operates by redirecting the fluid past the hull ( watercraft ) or fuselage , thus imparting a turning or yawing ___MASK8___ to the craft . In basic form , a rudder is a flat plane or sheet of material attached with hinges to the craft ' s stern , tail , or after end . Often rudders are shaped so as to minimize hydrodynamic or aerodynamic drag . On simple watercraft , a tiller  essentially , a stick or pole acting as a lever arm  may be attached to the top of the rudder to allow it to be turned by a helmsman . In larger vessels , cables , pushrods , or hydraulics may be used to link rudders to steering wheels . In typical aircraft , the rudder is operated by pedals via mechanical linkages or hydraulics . On boats and ships , keel can refer to either of ___MASK75___ parts : a structural element that sometimes resembles a fin and protrudes below a ___MASK41___ along the central line , or a hydrodynamic element . These parts overlap . As the laying down of the keel is the initial step in the ___MASK38___ of a ship , in British and American shipbuilding traditions the ___MASK38___ is dated from this event . Only the ship ' s launching is considered more significant in its creation . A rope is a ___MASK15___ of yarns , plies , or strands that are twisted or braided together into a larger and stronger form . Ropes have tensile strength and so can be used for dragging and lifting , but are too flexible to provide compressive strength . As a result , they cannot be used for pushing or similar compressive applications . Rope is thicker and stronger than similarly constructed cord , line , ___MASK70___ , and twine . Fiber rope is made from fiber , whereas wire rope is made from wire .