The board of supervisors of any county may, for the purpose of ascertaining the probable expense of any proposed subway, expend out of the county general fund a sum not exceeding thirty-five hundred dollars. When any such proposed subway reaches partly into one county and partly into another, such counties shall equally divide the expenditure necessary to ascertain the probable expense of any such proposed subway, but such expenditure shall not exceed in the aggregate the sum of thirty-five hundred dollars.