_r_ = _N_(_t_ + 267) log _v_. (2)
This is the motive power produced by the expansion of the air which,
under the temperature _t_, has passed from the volume 1 to the volume
_v_. If instead of working at the temperature _t_ we work in precisely
the same manner at the temperature _t_ + _dt_, the power developed will
be
_r_ + δ_r_ = _N_(_t_ + _dt_ + 267) log _v_.