1060 Mr. E. J. Hartung on the Construction and 



superiority o£ his method, in spite of the greater difficulties 

 in construction and adjustment. However, the theoretical 

 and practical simplicity of the knife-edge as a point of sup- 

 port is a strong recommendation in its favour, and, moreover, 

 the results of the present work show that Pettersson's 

 criticisms are largely unfounded for balances of minimum 

 indication not less than Ijju. No trouble has been experienced 

 from ' dust and, far from deteriorating, the edges seem 

 actually to improve with careful use ; this is no doubt due to 

 the gradual removal of the infinitesimal raw edge left after 

 grinding. 



It is necessary to arrange the arrestments of the balance 

 so that the beam is merely arrested in its movement when 

 not in use and the knife-edges are not lifted away from the 

 plate. If the beam is lifted when the instrument is not in 

 use, the resting-point will not remain steady, particularly in 

 the more sensitive types. Steele and Grant mention this in 

 their paper, but do not explain the reason ; probably it is due 

 to irregular distortion of the knife-edge. Calculation shows 

 that in the sensitive balance described above, the centre of 

 gravity of the system is not more than *00l mm. below the 

 edge, while, in spite of the lightness of the beam and its 

 accessories, the pressure on the edge is intense owing lo its 

 very small bearing surface. A slight distortion may there- 

 fore have serious influence on the equilibrium position of the 

 beam, and it is probable that the wandering of zero after 

 lifting the beam is due to slow alteration in this distortion, 

 which only becomes permanent and constant when the knife- 

 edge rests continuously on the plate. 



The Fibre Suspension. 



Steele and Grant found that a fine silica fibre suspension 

 from one end of the beam was more satisfactory and much 

 more easily managed than the knife-edge and plane suspension 

 used on ordinary analytical balances. They drew the fibre 

 directly from the beam by hand. An improved process is 

 due to Mr. G. A. Ampt, of Melbourne University. He 

 fuses a weighted hook to the beam by a small, very thin rod, 

 and then passes a minute oxy-gas flame repeatedly and very 

 rapidly across this rod. The result is a fine fibre which 

 gradually extends and can be drawn to any desired thinness 

 at any point. Gray and Eamsay (Proc. Roy. Soc. A. lxxxiv. 

 p. 531"), 1911) fused a long fibre direct to the beam, and 

 obtained in this way a very free suspension for their niton 

 balance. The desideratum of a good fibre is that it bends 



