Whenever the qualified electors of any incorporated city whose population has decreased since its incorporation to less than 500 inhabitants as shown by the last federal census and the qualified electors of such city show a desire to vacate the incorporation of the same, the city council upon petitions being presented to it at any regular or special meeting, signed by at least 1/4 of the legal voters of such city as shown by the registration list of the last preceding registration held in such city, praying that the incorporation of such city be vacated, shall immediately thereupon order a special meeting of the electors of such city to be held for the purpose of voting upon the question of vacating the incorporation of the same, and shall give 30 days’ notice of the time and place of holding such meeting by posting up written or printed notices which shall state the object of such meeting by reciting the substance of such petition, in 6 of the most public places within the limits of such city.