RAT

The name of a large, destructive, and very prolific species of the genus Mun ( Mus decuman us, Linn.), introduced into the British Islands from Asia; not, as is commonly believed, from Norway. It has spread over all the country, and multiplied at the expense of the old British species called the "black rat" {Musratius, Linn.). See Mvs.., RATCHKT, in Clock and Watch \\ ork, is the name given to an arm or piece of mechanism, one extremity of which abuts against the teeth of a ratchet wheel, and the other extremity is either freely jointed to a reciprocating ilriver for the purpose of communicating a continuous motion to the wheel, or is attached to a fixed centre to ensure the wheel against reverse motion. In the former case it is also called a click or paul, in the latter a detent. [s. 1037]