[ 98 ] 



than the tremors of the inftrument itfelf, and are more to be 

 prevented. 



2d'y The greater the weight is, the more will both be fup- 

 prcfl'cd j fo that the redification of the inftrument is within 

 one's power ; it fhould not be much lefs than two pounds : it 

 feemed to me that the greater the tremors were, the lefs 

 were the vibrations and vice verfa ; but the former were, as 

 might be cxpeded, diminifhed, and the latter increafed, by 

 Ihortening the firing /, or by lengthening the ftem, and con- 

 trarywife.* In calm weather this level will, when the arm K 



fupporting 



• I have tried different lengths of the ftring and quantities of the weight, without 

 being able to make out a theory of their effefts, which yet is the more important, 

 as it is part of the theory of the pendulum; for the level with it? weight is a 

 compound pendulum, confiding of two mafles of matter connefted by a ftring or 

 ■wire : when I fufpefted that the elafticity of the ftring might fuffer it to extend, 

 and fo to permit the tremulous agitations of the mirror and ftem without propa- 

 gating motion to the weight, and exciting its whole reGftance, I fubftituted a ftrong 

 wire for the ftring (each two feet long) but found that the tremors of the mirror 

 were more lafting, and the vibrations of the weight fooner excited by the wire 

 than the ftring ; fo that the latter better anfwered the defign of the inftrument : its 

 length feemed of the lefs confequence, fince any length tending to fupprefs the tremors 

 encreafed the vibrations and vice verfa : I therefore contrafted the ftring to the 

 length of an inch. 



The tremors of the ftem were much quicker than its ofcillations from its own 

 gravity would be, and far more fo than thofe of the weight could be fuppofed ; 

 thefe tremors therefore were like the vibrations of a mulical chord, faving that 

 this is ftretched beyond its natural length, except when in the middle point of each 

 vibration ; whereas it is more likely that during the tremors of the mirror, the 



weight 



