RECAPITULATION, 453 



3. Bevelled-wheels are much used for changing 

 the direction of motion in wheel-work. 



4. Hook's universal joint is sometimes used for 

 the same purpose. 



5. The ends of the teeth of wheels ought never 

 to be circular, but formed of parts of an epicycloid. 



Pendulums, 



1. All the vibrations of the same pendulum, 

 whether great or small, are performed in equal 

 times. 



2. The longer a pendulum, the slower are its 

 vibrations. 



3. Heat expands, and consequently lengthens, 

 pendulums ; and cold contracts, and shortens them. 



4. A pendulum, to vibrate seconds, must be 

 shorter at the equator than at the poles. 



5. Methods have been used for correcting the 

 irregularity arising from expansion and contraction: 

 one of these is the gridiron-pendulum. 



6. Deal is the best substance for pendulum-rods, 

 as it is very little affected by heat and cold. 



Chronometers. 



1. Chronometers are instruments for mea- 

 suring time. 



2. Clocks are moved either by a weight or a 

 spring. Watches differ from the latter only in 

 being portable. 



3. The pendulum in a clock, and the balance 

 in a watch, are the parts which properly measure 

 time by their vibrations. 



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