[R] TS: Calculating the mean by month

[Victor]

I have the following (monthly) ts time-series:
> t
      Jan  Feb  Mar  Apr  May  Jun  Jul  Aug  Sep  Oct  Nov  Dec
2002  5.3  9.4 12.1 14.4 17.9 23.5 24.3 23.6 19.3 16.3 13.9  9.0
2003  7.5  5.4 10.4 13.3 19.9 25.6 26.2 27.6 20.6 15.5 12.5  8.0
2004  6.3  7.8  9.6 13.6 16.5 22.0 24.3 24.6 21.3 18.1 11.7  9.0
2005  5.7  5.4  9.7 13.0 18.9 22.9 25.0 23.4 21.4 16.5 11.3  6.7

I would like to calculate the mean of data by month.

How can I do it with a ts time series?

Many thanks
Vittorio