COMMON THINGS CLOCKS AND WATCHES. 



can be commanded for its ascent and descent, and neither it nor 

 the pendulum is applicable except to timepieces which rest in a 

 fixed and stable position. 



In the case of timepieces whose position is fixed, but where 

 the vertical space for the play of the weight cannot be conveniently 

 obtained, the mainspring is applied as a moving power, combined 

 with the pendulum as a regulator. Chimney and table clocks 

 present examples of this arrangement. The height being limited, 

 it is necessary also in these cases to apply short pendulums. The 

 length of a pendulum which vibrates seconds being about 39 

 inches, such pendulums can only be used where considerable height 

 can be commanded. 



It has been shown, that the lengths of pendulums are in the 

 proportions of the squares of the times of their vibration. It 

 follows, therefore, that the length of a pendulum which would 

 vibrate in half a second, will be one-fourth the length of one 

 which vibrates. in a second, and since the latter is 39 inches, the 

 former must be 9| inches. Such a pendulum can therefore be 

 conveniently enough applied to a chimney or a table clock high 

 enough to leave about ten. inches for its play. 



The pendulum is so good a regulator, and the anchor-escape- 

 ment renders it so independent of the variation of the moving 

 power, that in timepieces where it is combined with a main- 

 spring a fusee is found to be unnecessary. In such cases, there- 

 fore, the axis of the first wheel is placed in the centre of the 

 mainspring, as represented in fig. 17. 



47. The cylindrical escapement, shown in fig. 22, is nearly as 

 independent of the variation of the moving power as the pendulum, 

 and therefore in common watches, where this escapement is used, 

 the fusee is dispensed with. 



In the class of timepieces called chronometers, used for the 

 purposes of navigation, and in general for all purposes where the 

 greatest attainable perfection is required in a portable timepiece, 

 all the expedients to insure regularity are united, and accordingly 

 the detached escapement is combined with fusee and mainspring. 



Besides the expedients above mentioned, for insuring unifor- 

 mity of rate, provisions are made in the most perfect chronometers 

 to prevent the variations of rate which would arise from the 

 expansion and contraction of the metal composing the balance- 

 wheel by the variation of temperature. These expedients are very 

 various ; but in general they consist of contrivances by which the 

 expansion, of the rim of the wheel causes a part of it so to bend, 

 as to throw a heavier part nearer to the centre, to compensate for 

 the increased distance of another part produced by its general 

 enlargement. 

 -44 



