The Newton scale was devised by Isaac Newton. He defined the "zeroth degree of heat" as melting snow and "33 degrees of heat" as boiling water. His scale is thus a precursor of the Celsius scale, being defined by the same temperature references. Thus the unit of this scale, the Newton degree, equals 100⁄33 Kelvin conversion or degrees Celsius and has the same zero as the Celsius scale.
Although initially defined by the freezing point of water (and later the melting point of ice), the Celsius scale is now officially a derived scale, defined in relation to the Kelvin temperature scale.
Zero on the Celsius scale (0 °C) is now defined as the equivalent to 273.15 K, with a temperature difference of 1 deg C equivalent to a difference of 1 K, meaning the unit size in each scale is the same. This means that 100 °C, previously defined as the boiling point of water, is now defined as the equivalent to 373.15 K.
The Celsius scale is an interval system but not a ratio system, meaning it follows a relative scale but not an absolute scale. This can be seen because the temperature interval between 20 °C and 30 °C is the same as between 30 °C and 40 °C, but 40 °C does not have twice the air heat energy of 20 °C.
A temperature difference of 1 deg C is the equivalent of a temperature difference 1.8°F.