354 On the Construction and 



allow a flat plate of hardened steel, which is screwed upon 

 the top of the stirrup to rest upon the points. To prevent the 

 points wandering, and causing the stirrup to touch the beam 

 and impede its motion, two shallow holes are worked in the 

 surface of the steel plate of the stirrup, which are just deep 

 enough to keep the points of suspension in the centre, and 

 the holes are very finely polished until a strong lens shews 

 no scratches. 



The scale-pan is hooked to an eye at the point of the stir- 

 rup, but the strings of the scale-pan are not immediately at- 

 tached to the hook, but to the edge of a little circular 

 dice | an inch in diameter, into the middle of which the 

 hook is screwed. By this the swinging of the scale-pans is 

 in a great measure prevented, and the rotatory motion which 

 is common when the strings are attached to the hook itself, 

 is altogether obviated. 



Levelling screws are quite unnecessary, as the indications 

 of the beam are estimated from the point to which it was 

 adjusted. 



This instrument when loaded with 3000 grains and ad- 

 justed to 24 second vibrations, will give a decided result of a 

 full degree of inclination on the addition of — th of a grain, 

 as of course for so large a radius | a degree is distinctly visible 

 without mistake ; it is sensible with rapid vibrations for prac- 

 tical purposes to the 600-thousandth part of the load ; when 

 carefully adjusted to 40 second vibrations, it is sensible to 

 nearly three times this quantity. This heavy beam even with 

 the 200-grain counterpoise, will shew the 50 or 60-thousandth 

 part. 



For adjusting small weights, &c. I use a second beam ex- 

 actly similar to the large one, except that it is only £ inch 

 wide, and weighs without the bob only about 500 grains, 

 which with a load of 400 grains, gives a change of angle 



of inclination of 8° by the addition of J-th grain. 



J ioo ° 



Knife-edges are generally used in the construction of fine 



