The board of county commissioners, from time to time, if a levy is made for a firebreak fund, shall divide the county into as many fire districts as may be necessary. Each district must be known and designated as “Fire District No.         “. The board may map out each fire district in any form, consistent with the amount of funds available, so that when firebreaks are made a prairie fire will be confined to the smallest possible area. When forming fire districts, the board may take advantage of any creek, river, broken or plowed field, or other natural or artificial barrier to prairie fires.