5"2 Efcflpemetits for Pendulums, 



This cfcapement not being adapted to fuch vibrations as are performed through arcs of 

 a few degrees only, another conftruftion has been made which has been in conftanc ufc 

 for about a century in clocks, with a ^ong pendulum beating feconds. Fig. 2. A B repre- 

 fents a vertical wheel called the fwing wheel, having thirty teeth. CD reprefents a pair 

 of pallets connefted together, and moveable in conjun£lion with the pendulum on the centre 

 or axis F. One tooth of the wheel in the prefent pofition refts on the inclined furface of 

 the inner part of the pallet C, upon which its difpofition to Aide tends to throw the point of 

 the pallet farther from the centre of the wheel, and confequently affifts the vibration in 

 that dire£lion. While the pallet C moves outwards and the wheel advances, the point of 

 the pallet D of courfe approaches towards the centre in the opening between the two 

 neareft teeth ; and when the afting tooth of the wheel flips olF, or efcapes from the pallet 

 C, another tooth on the oppofite fide immediately falls on the exterior inclined face of D, 

 and by a fmiilar operation tends to pufh that pallet from the centre. The returning vibra- 

 tion is thus aflifted by the wheel, while the pallet C moves towards the centre, and re- 

 ceives the fucceeding tooth of the wheel after the efcape from the point of D. In this 

 manner the alternation may be conceived to go on without limit. 



'The fame obfervation wliich was made upon the efcapement fig. i. is applicable alfo 

 to this -, namely, that the vibrating part is conftantly under the influence of the maintain- 

 ing power, except during the interval of the drop, or aftual efcape of the wheel from one 

 pallet to the other. The indireft aftion of an inclined plane, with the influence of oil 

 which it neceflarily requires, may alfo be mentioned to fl-.ew that this efcapement is not 

 equal to what might have been produced by a flcilful variation of the other. One principal 

 recommendation of this efcapement feems to have been the facility with which it aflxirds 

 an index for feconds in the face of the clock. Though the pendulum, according to this 

 conftruftion, is conftantly conne£led with the maintaining power in a clock, yet the va- 

 riations of that power have not the fame mifchievous efl"e£l: as in a watch, becaufe the 

 momentum of the pendulum, compared with the impulfe of the maintaining power, is pro- 

 digioufly greater in the former of thefc inftruments. A very confiderable change in the 

 maintaining' power of a clock with a long pendulum, will not caufe a greater variatioa 

 than a few feconds in the daily rate. 



The celebrated George Graham improved this efcapement very much by taking off part 

 of the Hope fartheft from the points of the pallets ; infiead of which part he formed a cir- 

 cular or cylindrical face, having its axis in the centre of motion. Pallets of this kind are 

 feen on the oppofite fide of the wheel at E and G, having H for their centre or axis. A 

 tooth of the wheel is feen lefting upon the circular inner furface of the pallet G, which 

 is not therefore affefled by the wheel, excepting fo far as its motion arifing from any 

 other caufe may be. aflx£led by the fri£lion of the tooth. If the vibration of the pendulum 

 be fuppofed to carry G outwards, the flope furface will be brought to the point of the 

 tooth, which will Aide along it and urge the pallet outwards during this Aiding aftion. 

 When the tooth has fallen from the point of this pallet, an oppofite tooth will be. received 

 on the circular furface of E, and will not afi"e6l the vibration, excepting when the flope fur- 

 face of E is carried out fo as to fuffer the tooth to Aide along it. In the two former efcape- 

 Hients there is always a certain portion of vibration takes place after the drop which drives 

 the pallets back, and caufes the index alfo to recede through a fmall arc. This has been 



diftingulflicd 



