B A C is the average angle formed by a string passing over the bridge of a Violin, and the tension acts equally in the direction A B, A C.
Take A C=A B.
From the point B draw B D parallel to A C. And from the point C draw C D parallel to A B, cutting B D at D.
Join A D.
Then, if a force acting on the point A, in the direction of A B, be represented in magnitude by the line A B, an equal force acting in the direction A C will be represented by the line A C, and the diagonal A D will represent the direction and magnitude of the force acting on the point A, to keep it at rest.
N.B.—The bridge of a Violin does not divide the angle B A C quite equally, but so nearly that A D may be taken as the position of the bridge.
Also, the plane passing through the string of a Violin, on both sides of the bridge, is not quite perpendicular to the belly. To introduce this variation into the calculation would render that less simple, and it will be sufficient to state that about the 150th part must be deducted from the downward pressures given in the above table from the first and fourth strings, and about the 300th part for the second and third strings. The total to be deducted for the four strings will not exceed three ounces.
On the line A B or A C set off a scale of equal parts, beginning at A, and on A D a similar scale beginning at A.