- made in musical keyed Instruments. 8331 
position A, acting on the extremity of the spiral ; and as 
the bellows sink; the spiral turning in the opposite diree- 
tion, will gradually unwind the string so that the counter 
poise will act on a radius continually decreasing, as the 
force of the bellows is increasing: the accuracy with which 
the regulator will equalize the force of the blast, depends” 
on three circumstances; First, the form of the spiral, the 
size of the pulley J, 1, 2, 3, and the weight of the counter- 
poise 7; the spiral curve is to be formed by the following 
rule: Describe a circle of any convenient diameter, and 
supposing the whole circumference to represent the size of 
the greatest angle which the ribs of the bellows make with the 
bottom- or top-boards. Assume any pomt, and from thence 
divide the circumference into segments (always measured 
from the same point).respectively proportional to the sines 
of the angles up to that greatest angle. It will be sufficiently 
accurate to take the sines of the five first degrees, thence 
the sines of each half degree up to 15°, and thence to the 
greatest angle (the entire revolution) each quarter degree, 
draw radii to all these points. Then from the centre of 
the circle, measure off each radius proportionally to the se- 
cant of its respective angle; and trom this point draw a per- 
pendicular to the radius; these perpendiculars by their 
mutual intersections, will form an irregular polygon, ap- 
proaching to the curve required. The scale of equal parts 
by which the radii are measured proportionally off, to the 
secant of the angles, will be greater or smaller, according 
to the size of the bellows to be regulated. For a chamber 
organ of four or five stops, the secant of the greatest angle 
may be about ten inches : for large bellows it may be con- 
siderably larger; otherwise the weight or counterpoise 
might be inconyeniently great: the size of the pulley is to 
be such that its circumference shall be exaetly equal to the 
rise of the bellows, when the ribs make with the bottom- 
or top-boards, the greatest angle for which the spiral is 
made: the size of the pulley will therefore, ceteris paribus, 
depend on the breadth of the ribsp——Thus, if the breadth of 
the rib be five inches, the circumference of the pulley 
should be equal to twice the size of the greatest angle, as 
put down in the ordinary table of sines, tangents, &c. 
galling the first figure in the table, mches, and the rest, 
decimals of an inch. — If the ribs be less or more than five 
inches, the circumference of the pulley will be found from 
the tabular sine by a statement in the Rule of Three. First, 
as five is to the tabular of sine, so is the rib to a fourth pro- 
portional ; which being doubled, is the circumference is 
, quired, 
