.376 
CAPTAIN KATER ON THE ERROR IN 
near the extremities of the bow ; these served as bridges, and one was of a 
sufficient length to support the whole when laid upon a horizontal surface. 
The nature of the surface upon which it is placed may thus be examined at 
leisure by observing its proximity to the wire. Should the surface be convex, 
a wire must be placed under each extremity of the bow-string of sufficient 
diameter just to clear it from all but the most convex part of the surface. As 
this simple contrivance may be applied to a great variety of useful purposes 
where a straight edge is required, I trust I may be pardoned for giving the 
accompanying sketch of it. 
By means of this apparatus I examined a variety of surfaces, any one of 
which I formerly should have considered as well calculated to serve as a sup- 
port for scales, which were to be compared together. The following were the 
results. 
Length 
in 
inches. 
Curvature. 
Versed 
Sine 
inches. 
A mahogany dining-table 
inches. 
42 
61 
62 
62 
48 
Concave. 
Ditto 
Ditto 
Ditto 
Ditto 
inches. 
0.04 
0.12 
0.04 
0.10 
0.03 
A marble chimney-piece 
Another 
Another 
Top of a piano-forte of rosewood finely finished. . 
The front board of a piano-forte of rosewood forty-seven inches long. This 
was very highly finished, and the general surface was found to be nearly plane, 
but irregular. Here it is worthy of observation, that I could detect the nature 
and extent in some degree of the irregularities of the surface, by tapping with 
my fingers upon the wire whilst it was pressed by the weight of the bow upon 
the board. Where it yielded no sound, the wire was of course in contact with 
the surface, which in that case was either convex or plane. When the wire 
yielded a sound by tapping upon it with the finger, the surface was concave; 
and some idea might be formed of the extent, by the acuteness or gravity of the 
sound produced ; the edges of the concavity serving as bridges, which limited 
