MStechca,
The paralleled thermistors connect pin 2 (not pin 7 as it looked like) of the 555 to the positive supply. They are wrongly marked "100k" in the upper left of the schematic.
Pin 2 of a 555 is the base of a PNP darlington transistor so the thermistors won't have any current and the 555 will do nothing when the resistance changes.
The thermistor has such a low resistance that a pot in series to make a voltage divider would smoke and the massive current through the thermistor would heat it much more than the ambient ever will.