The said motor ambulance shall be constructed upon good, substantial and easy springs. It shall be covered and closed, and shall have windows on the sides or ends. It shall be of sufficient size to convey at least two injured persons with two attendants, at one time, and shall be provided with spring mattresses or other comfortable bedding, to be placed on roller frames, together with sufficient covering and protection for convenient movement of the injured. It shall also be provided with seats for the attendants. The stretchers shall be constructed of such material and in such manner as to afford the greatest ease and comfort in the carriage of the injured persons. The motor ambulance shall at all times be properly heated.