-1 4 setxrange
-.5 6 setyrange

/xM 2 def
/yM 4 def
/M {xM yM} def

/L1 2 def
/l1 .7 def
/L2 3 def
/l2 1 def

/intermasses 1 def
/dt .3 def

%% les centres des masses
/I1 {M 0 intermasses l1 2 div add addv} def
/I2 {M 0 l2 -2 div addv} def

%% points d'attache ressort 1
/A1 {I2 L2 -2 div l2 -2 div addv dt 0 addv} def
/A2 {A1 projx} def

%% points d'attache ressort 2
/A4 {A3 projx} def
/A3 {I2 L2 2 div l2 -2 div addv dt neg 0 addv} def

%% points d'attache du piston
/B1 {A1 A3 milieu} def
/B2 {B1 projx} def

%%[A1 A2 A3 A4 B1 B2] points

%% syntaxe : A Lx Ly rectangle
/rectangle {
2 dict begin
   /Ly exch 2 div def
   /Lx exch 2 div def
   gsave
      stranslate
      newpath
   	 Lx Ly smoveto
   	 0 Ly -2 mul srlineto
   	 Lx -2 mul 0 srlineto
   	 0 Ly 2 mul srlineto
   	 closepath
      stroke
   grestore
end
} def

axeOyarrow
M point
traceOy
traceOx
yM ytick

medium setlinewidth

I2 L2 l2 rectangle
I1 L1 l1 rectangle
A1 A2 .25 18 (-) ressort
A3 A4 .25 18 (-) ressort
B2 B1 1 1 1.6 .6 .75 piston*

setTimesItalic
   (M) M utext
   (m) I1 text
setTimes
   (1) indice
setTimesItalic
   (m) I2 text
setTimes
   (2) indice
setCourrier
   (0) 0 yM ltext