36 Captain Kater*s account of the 



I first tried cast iron ; but though the beam was made as 

 light as was consistent with the requisite degree of strength, 

 the inertia of such a mass appeared to be so considerable, 

 that much time must have been lost before the balance would 

 have answered to the small differences I wished to ascertain. 

 Lightness was a property essentially necessary, and bulk 

 was very desirable in order to preclude such errors as might 

 arise from the beam being partially affected by sudden alte- 

 rations of temperature. I therefore determined to employ 

 wood, a material in which the requisites I sought were com- 

 bined. The beam was made of a plank of mahogany about 

 70 inches long, 22 inches wide, and 2^ thick, tapering from 

 the middle to the extremities. An opening was cut in the 

 centre, and strong blocks screwed to each side of the plank 

 to form a bearing for the back of a knife edge which passed 

 through the centre. Blocks were also screwed to each side 

 at the extremities of the beam on which rested the backs of 

 the knife edges for supporting the pans. The opening in 

 the centre was made sufficiently large to admit the support 

 hereafter to be described, upon which the knife edge rested. 



In all beams which I have seen, with the exception of those 

 made by Mr. Robinson, the whole weight is sustained by 

 short portions at the extremities of the knife edge, and the 

 weight being thus thrown upon a few points, the knife edge 

 becomes more liable to change its figure and to suffer injury. 



To remedy this defect, the central knife edge of the beam 

 I am describing was made six inches, and the two others five 

 inches long. They were triangular prisms with equal sides, 

 of three quarters of an inch, very carefully finished, and the 

 edges ultimately formed to an angle of 120°. 



