problem getting MIDI output from lilypond

[Colin Cotter]

Hi,
I'm a new convert to lilypond and have been very pleased with the output 
it produces as well as the ease of input. I have a question about 
generating MIDI output.
I have included the following in my .ly file
\midi {
\tempo 4=100
}
and have typed 
lilypond -m trousers.ly
(where I have chosen -m to suppress the .tex output)
but I get no messages talking about MIDI and no .midi files are created.
Have I typed something wrong, or do I need additional software to use MIDI 
with lilypond?
Thanks in advance,
Colin Cotter