DECLARE EXTERNAL PICTURE YA
PRINT
PRINT "赤玉が作る重力場（３次元）の断面のイメージ"
PRINT "      （ 場の大きさは距離の２乗に反比例します。）"
PRINT
SET WINDOW -5,5,-5,5
LET Z=0
DO WHILE Z<9
   SET LINE WIDTH .2
   IF INT(Z/2)=Z/2 THEN
      SET LINE COLOR 0
      PLOT LINE -5,0;5,0
      PLOT LINE -.001,5;-.001,-5
      PLOT LINE -5,-5;5,5
      PLOT LINE -5,5;5,-5
      FOR i=1 TO 7
         DRAW CIRCLE WITH SCALE(i)*SHIFT(0,0)
         DRAW CIRCLE WITH SCALE(i*SQR(2))*SHIFT(0,0)
      NEXT i
   ELSEIF INT((Z+3)/4)=(Z+3)/4 THEN
      SET LINE COLOR 15 
      PLOT LINE -5,0;5,0
      PLOT LINE -.001,5;-.001,-5
      FOR i=1 TO 7
         DRAW CIRCLE WITH SCALE(i)*SHIFT(0,0)
      NEXT i
   ELSEIF INT((Z+1)/4)=(Z+1)/4 THEN
      SET LINE COLOR 15
      PLOT LINE -5,-5;5,5
      PLOT LINE -5,5;5,-5
      FOR i=1 TO 7
         DRAW CIRCLE WITH SCALE(i*SQR(2))*SHIFT(0,0)
      NEXT i
   END IF
   SET LINE COLOR 1
   SET AREA COLOR 4
   DRAW DISK WITH SCALE(.2)*SHIFT(0,0)
    
   FOR x=-5 TO 5
      FOR y=-5 TO 5
         IF x=0 AND y>0 THEN
            DRAW YA(1/(x^2+Y^2)) WITH ROTATE(-PI/2)*SHIFT(x,y)
         ELSEIF x=0 AND y<0 THEN
            DRAW YA(1/(x^2+Y^2)) WITH ROTATE(-3*PI/2)*SHIFT(x,y)
         ELSEIF x<0 AND y=0 THEN
            DRAW YA(1/(x^2+Y^2)) WITH SHIFT(x,y)
         ELSEIF x>0 AND y=0 THEN
            DRAW YA(1/(x^2+Y^2)) WITH ROTATE(-1*PI)*SHIFT(x,y)
         ELSEIF x<0 THEN 
            DRAW YA(1/(x^2+Y^2)) WITH ROTATE(ATN(Y/x))*SHIFT(x,y)
         ELSEIF x>0 THEN 
            DRAW YA(1/(x^2+Y^2)) WITH ROTATE(PI+ATN(Y/x))*SHIFT(x,y)
         END IF
       NEXT  Y
   NEXT x
      LET Z=Z+1
       
      WAIT DELAY 6
   LOOP 
   PRINT
   PRINT "このプログラムは終了しました。"  
   PRINT 
END

EXTERNAL PICTURE YA(D)   !基本の矢印を描く
   SET LINE WIDTH 3*D
   PLOT LINES: 0,0;0.8*D,O
   PLOT LINES: 0.8*D,O;.5*0.8*D,-.3*0.8*D
   PLOT LINES: 0.8*D,O;.5*0.8*D,.3*0.8*D
END PICTURE