IV. 

 \ 11:1 r.vi. \i:i.. n IBS. 



,.rocccd to establish a general theorem respect- 

 ing the equilibrium of a body or system of bodies, called the 



i 



When a system of particles is in equilibrium, and we 

 fttppoM each of t! cd in a po*i 



!i it jvnlly ooeO] 



nexi- parts of tlie system with each other, the straight 



: st iiositioii of a particle with the second 

 Is called the virtual velocity of that 



term velocity is used because we may conceive all the 

 icements to be made in the tame indefinitely small 

 lien the spaces described are proportional to the velocities. 



itimnu- iisplacemenU 



really made, but only supposed. We retain the 

 established phraseology, but it is < m these explana- 



tions words i light be conveniently 



rephuv.l by J t >//.t/ t > 'ir.tl ,//>y,/, f , -. //,, nt. 



By the words, without </.' the connexion of the part* 



of the system \cith ca, we mean, that any rigid pody 



MS iu the system is supposed to remain of invariable 



ami that any rods or strings which connect different 



<>f the system are to remain unbrokt: -. at least, 



:ninary statement to assist the reader, 



subject again ; see Art 257. 



, by reason of this limitation the virtual velocities of 

 4 of a system are frequently so connected 

 i those of a definite number or points are assumed, 

 those of all the rest necessarily follow. 



24 s virtual velocity of a particle estimated in a 



- the projection of the v y on 



; it is considered positive when the direction 



