  We show that a mesoscopic system such as Feynman's ratchet may operate as a
heat pump, and clarify a underlying physical picture. We consider a system of a
particle moving along an asymmetric periodic structure . When put into a
contact with two distinct heat baths of equal temperature, the system transfers
heat between two baths as the particle is dragged. We examine Onsager relation
for the heat flow and the particle flow, and show that the reciprocity
coefficient is a product of the characteristic heat and the diffusion constant
of the particle. The characteristic heat is the heat transfer between the baths
associated with a barrier-overcoming process. Because of the correlation
between the heat flow and the particle flow, the system can work as a heat pump
when the particle is dragged. This pump is particularly effective at molecular
scales where the energy barrier is of the order of the thermal energy.
