using tapply to extract 2 arguments

["Dakin, Jonathan" <j.dakin@imperial.ac.uk>]

Could anyone help me with syntax to extract two arguments from a dataframe
using tapply ?  So that each is a column of the dataframe, split according
to a third factor column.

eg I would like to apply a linear model to 2 columns of a dataframe, by
section.

Many thanks.

Jonathan Dakin
Imperial College