Section 121. If, as of the last day of any March, June, September or December, the reserve fund shall exceed by thirty per cent or more the normal reserve fund, and during the three preceding months the income shall have exceeded the cost of the service, the company shall, within thirty days thereafter, put into effect the next lower grade of fare adopted as aforesaid; and if, as of the last day of any March, June, September or December, the reserve fund shall be less than seventy per cent of the normal reserve fund, and during the three preceding months the income has been less than the cost of service, the company shall, within thirty days thereafter, put into effect the next higher grade of fare, and the fare shall continue to be decreased or to be increased, if the amount of the reserve fund is above or below said limits, as of such quarterly dates. The company, with the consent of the department, may put into effect the next higher or lower grade of fare at any time when the reserve fund is below or above the normal amount.