<P> With the fire across the river and moving rapidly towards the heart of the city, panic set in . About this time, Mayor Roswell B. Mason sent messages to nearby towns asking for help . When the courthouse caught fire, he ordered the building to be evacuated and the prisoners jailed in the basement to be released . At 2: 30 a.m. on the 9th, the cupola of the courthouse collapsed, sending the great bell crashing down . Some witnesses reported hearing the sound from a mile (1.6 km) away . </P> <P> As more buildings succumbed to the flames, a major contributing factor to the fire's spread was a meteorological phenomenon known as a fire whirl . As overheated air rises, it comes into contact with cooler air and begins to spin creating a tornado - like effect . These fire whirls are likely what drove flaming debris so high and so far . Such debris was blown across the main branch of the Chicago River to a railroad car carrying kerosene . The fire had jumped the river a second time and was now raging across the city's north side . </P> <P> Despite the fire spreading and growing rapidly, the city's firefighters continued to battle the blaze . A short time after the fire jumped the river, a burning piece of timber lodged on the roof of the city's waterworks . Within minutes, the interior of the building was engulfed in flames and the building was destroyed . With it, the city's water mains went dry and the city was helpless . The fire burned unchecked from building to building, block to block . </P> <P> Finally, late into the evening of the 9th, it started to rain, but the fire had already started to burn itself out . The fire had spread to the sparsely populated areas of the north side, having consumed the densely populated areas thoroughly . </P>

What two sets of fire boundaries are set during a fire