  We describe a new aerogel threshold Cherenkov detector installed in the HMS
spectrometer in Hall C at Jefferson Lab. The Hall C experimental program in
2003 required an improved particle identification system for better
identification of pi/K/P, which was achieved by installing an additional
threshold Cherenkov counter. Two types of aerogel with n=1.03 and n=1.015 allow
one to reach 10^{-3} proton and 10^{-2} kaon rejection in the 1-5 GeV/c
momentum range with pion detection efficiency better than 99% (97%). The
detector response shows no significant position dependence due to a diffuse
light collection technique. The diffusion box was equipped with 16 Photonis
XP4572 PMT's. The mean number of photoelectrons in saturation was ~16 and ~8,
respectively. Moderate particle identification is feasible near threshold.
