It was as simple as 0 = freezing and 100 = boiling for a specific standard of water and standard of atmosphere and pressure. Practically, those points always varied according to other factors, but it was only recently redefined to match the Kelvin scale and be tied to a universal constant. It was attached to the transition points of water for centuries.
Apparent temperature is a whole other can of worms. 80° F could feel pleasant or unbearable depending on humidity or wind.
It was as simple as 0 = freezing and 100 = boiling for a specific standard of water and standard of atmosphere and pressure. Practically, those points always varied according to other factors, but it was only recently redefined to match the Kelvin scale and be tied to a universal constant. It was attached to the transition points of water for centuries.
Apparent temperature is a whole other can of worms. 80° F could feel pleasant or unbearable depending on humidity or wind.