Drawing a Spiral in BASIC

A while ago I came across this short snippet of BASIC code, originally written for QuickBASIC 4.5:

CLS
RANDOMIZE (TIMER)
x = 320
y = 240
r = 100
c = 1
SCREEN 12
PSET (x, y - r), 0
FOR a = 0 TO 72 STEP .01
    LINE -(x + r * SIN(a), y - r * COS(a)), c
    r = r - .01
    c = RND(1) * 15
NEXT

This uses simple geometry to draw a circle, but slowly reduces the radius so …