float c = 25.0;

void setup() {
  size(100, 100);
  smooth();
  noLoop();
}

void draw() {
  arch(c);
}

void arch(float curvature) {
  float y = 90.0;
  float sw = (65.0 - curvature) / 4.0;
  strokeWeight(sw);
  noFill();
  beginShape();
  vertex(15.0, y);
  bezierVertex(15.0, y-curvature, 30.0, 55.0, 50.0, 55.0);
  bezierVertex(70.0, 55.0, 85.0, y-curvature, 85.0, y);
  endShape();
}