That county engineers of the several counties of the state shall be allowed by the board of county commissioners of their respective counties not less than the following salaries, to be paid out of the general fund the same as other county bills are paid or out of the consolidated road and bridge fund: In counties having a population of more than 10,000 and not more than 15,000, $1,300 per annum; in counties having a population of more than 15,000 and not more than 20,000, $1,400 per annum; in counties having a population of more than 20,000 and not more than 25,000, $1,600 per annum; in counties having a population of more than 25,000 and not more than 30,000, $1,800 per annum; in counties having a population of more than 30,000, $2,000 per annum: Provided, That in counties having a population of 10,000 or less the county engineer’s salary shall not be less than $1,200 per annum: And provided further, That where two or more counties unite to form a county engineer district the salary shall be determined by the population of the district according to the same schedule and in the same manner as for single counties. The population for the purposes of this act shall be determined from the last returns of the assessors to the county clerks. The board of county commissioners shall in addition pay the county engineer’s actual and necessary traveling expenses.