metapost spiral

Hi,

in https://tug.ctan.org/info/drawing-with-metapost/Drawing-with-Metapost.pdf
I found this code to draw a spiral:

path S;
S = origin
for t=1 upto 90:
   .. 1/12 t * dir 16t
endfor;
draw S;

I understand it starts at origin and adds a number of points – but where 
are the points (pairs)? There’s a number (distance?) and a direction 
(angle?). Could you explain this in layman’s terms?

If I try a different starting point than origin, the spiral still is 
drawn around origin. I'd like to add it to another path and also 
randomize the points a bit to make it look hand-drawn.

Hraban

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