  We examine the conditions of the plasma along a sample of 'classical'
Herbig-Haro jets located in the Orion and Vela star forming regions, through
combined optical-infrared spectral diagnostics. Our sample includes HH 111, HH
34, HH 83, HH 73, HH 24 C/E, HH 24 J, observed at moderate spatial/spectral
resolution. The obtained spectra cover a wide wavelength range from 0.6-2.5 um,
including many transitions from regions of different excitation conditions.
This allows us to probe the density and temperature stratification which
characterises the cooling zones behind the shock fronts along the jet. The
derived physical parameters (such as the extinction, the electron density and
temperature, the ionisation fraction, and the total density) are used to
estimate the depletion onto dust grains of Calcium and Iron with respect to
solar abundances. This turns out to be between 70% and 0% for Ca and ~90% for
Fe, suggesting that the weak shocks present in the beams are not capable of
completely destroying the dust grains. We then derive the mass flux rates
(Mdot_jet is on average 5 10^-8 M_solar yr^-1) and the associated linear
momentum fluxes. The latter are higher than, or of the same order as, those
measured in the coaxial molecular flows, suggesting that the flows are jet
driven. Finally, we discuss differences between jets in our sample.
