ON BALANCES. 53 



been sent for exhibition, No. 332 in the catalogue. There 

 is also a very fine specimen of a steelyard in the room 

 below, exhibited by the Spanish Government, which could 

 not conveniently be brought up here for you to see. 



10. I may here shortly refer to some other balances of 

 peculiar construction, which are exhibited in the collection. 

 Mohr's balance (No. 4467 in the catalogue) is stated to be 

 a balance with one arm. But you will see it is merely a 

 balance with two unequal arms. There are also in the collec- 

 tion some hydrostatic balances intended for weighings in 

 water. Hydrostatic weighings may, however, generally be 

 made in an ordinary equal-armed balance of precision, by 

 means of a short pan specially constructed for the purpose 

 with a hook underneath, from which the body to be hydro- 

 statically weighed is suspended by a very fine wire, and is 

 immersed in a vessel of distilled water placed underneath the 

 short pan. 



11. I shall now go on to describe more particularly the 

 principles of construction of equal-armed balances of precision, 

 as balances for scientific weighings are usually called. 



The chief requirements of such a balance are as follows : 



(I). The points of suspension of the pans from the beam 

 ought to be exactly in the same line as the centre of motion, 

 or the fulcrum on which the beam turns when set in action. 

 The line joining these three points is the axis of the 

 beam. 



(II.) The two points of suspension of the pans should be 

 exactly equidistant from the centre of motion. This is a 

 sine qua non for a just equal-armed balance. 



(III.) There should be a minimum of friction at the centre 

 of motion and the two points of suspension of the pans. 



(IY.) The centre of gravity of the beam should be placed 

 a little below the centre of motion. 



12. As to the effects of the relative positions of the centre 

 of motion or fulcrum with the points of suspension and the 

 centre of gravity of an equal-armed balance, loaded with 

 equal weights, it is to be remarked : 



(a) When the centre of gravity coincides with the centre 

 of motion of the beam, and the three knife-edges are in the 

 same line, the beam of the balance will have no tendency to 

 one position more than another, but will rest in any position 

 in which it may be placed, whether the pans be suspended 



