478 



Mr. L. Southerns on the 



a tension T. In fig. 1 6 represents the inclination of either 

 fibre to the vertical and li the horizontal distance between P 

 and Q. The distance P Q is supposed to be small compared 



Ficr. 1. 



with the length of the fibres, but great with respect to h m 

 Small motions of P or Q will not appreciably affect either 6 

 or 1. In fig. 2, which represents a plan of the arrangemen t, 



Fio-. 2. 



the point Q has moved perpendicularly to the plane of fig. 1 

 from its normal position Q by reason of a smali deflexion, 

 say \Jr, of the balance-pointer and beam. The deflexion of the 

 mirror is represented by <f>. H represents the horizontal 

 component of the tension in the fibre Qb; this and the 



