135 



Pendulum, Blackburn's. A pendulum with double suspension, so 

 as to give simultaneously oscillations in two vertical planes at 

 right angles.the bob consisting of a funnel containing fine sand 

 or other material which, as it flows out, traces Lissajou's 

 curves. Used in investigations of simple harmonic motion. 



Pendulum, Borda. A form of simple pendulum (q.v.) used 

 for the determination of " g," consisting of knife-edge, 

 heavy ball, wire suspension and bracket support with 

 plate glass surface. The period of the knife-edge without 

 the suspension wire can be adjusted by means of a movable 

 weight. 



Pendulum, Compensation, Harrison's. A compound pendulum 

 consisting of a framework containing rods, alternately brass 

 and iron, so arranged that any rise or fall in temperature is 

 compensated, as the steel rods lengthen downwards while 

 the biass rods lengthen upwards, thus keeping constant the 

 distance between the point of suspension and the centre of 

 gravity of the pendulum. 



Pendulum, Compound. An instrument differing from the Simple 

 Pendulum essentially in the fact that in place of a light 

 string we have a rigid rod, the mass of which is not neglig- 

 ible compared with the mass of the bob. It may be of 

 cylindrical or rectangular bar form, and is usually fitted 

 with knife-edge for suspension. 



Pendulum, Cycloidal. A simple pendulum in which the string 

 near the point of suspension presses, when swinging, on 

 cheeks, so that the bob is made to swing in a cycloidal arc, 

 and the time of swing is independent of the amplitude. 



Pendulum Instrument. See Harmonograph. [19, 26.] 



Pendulum, Kater's Reversible Compound. A compound pendu- 

 lum, first used by Kater in 1818, for the determination of 

 gravity. It consists essentially of a metal rod, weighted at one 

 end, so that the centre of gravity is much nearer one end 

 than the other, and having two knife-edges, one at either 

 end. In addition, there is some method of adjusting the 

 periods to equality when the pendulum is suspended from 

 either knife-edge. 



Pendulum Level. A plumb level. [6, 24, 25, 27.] 



Pendulum, Mercury, Graham's. A compensated pendulum in 

 which the ball or " bob " is a cylindrical glass vessel con- 

 taining mercury. When the temperature rises the rod and 

 stirrup expand downwards, lowering the centre of gravity ; 

 but the mercury, expanding, rises in the cylinder and pro- 

 duces an inverse effect. 



Pendulum Myograph. An instrument for noting, by means of a 

 smoked glass plate forming the bob of a pendulum, the 

 amount and duration of the contraction when electricity is 

 sent through a muscle. See also Myograph. [4.] 



