Averaging over columns

[michael watson (IAH-C)]

Hi

I've been reading the help for by and aggregate but can't get my head
round how to do this.  

I have a data frame - the first three columns are replicate
measurements, then the next 3 are replicates etc up to 36 (so 12
variables with 3 replicate measurements each).  I want to compute the
mean for each of the 12 variables, so that, for each row, I have 12
means.

A grouping variable across columns can easily be created by
rep(1:12,each=3), but I can't figure out which function to use to get R
to calculate the means I want.

Thanks in advance
Mick