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°. 
