The invention relates to therapeutic uses of a 2–pyrimidone derivative represented by formula :Wherein R1, R2, R3, R4 and n are as defined herein. Cooling fluids having different temperatures are supplied to each of the cooling paths of the lower electrode so that the multiple cooling paths are at different temperatures from one another. In relation to the longitudinal axis of the road milling machine, the intake opening points to the side of the road milling machine, so that air is drawn in from the side.