Equating Mean and Average is a gross over simplification Mean .
Using the 18th century data (assuming we could get accurate data) would lower both the mean and the median. But, if you were to go all the way back to when Greenland was green (and the climate was much warmer), that might raise them.
The same would be true of the Twentieth Century. Accurate data for global temperatures do not exist before the 1980s and the advent of infrared satellites.
Those simple and easy to understand facts are why the whole global warming hoax is laughable. There simply is not enough data to support the hypothesis.
In the absence of a adjective like "geometric" or "harmonic", most of us equate "mean" with "arithmetic mean".
Even your citation says that:
The same would be true of the Twentieth Century. Accurate data for global temperatures do not exist before the 1980s and the advent of infrared satellites.
Yes, I posted that earlier as well. Before that, there has been so much potential for measurement error, as well as measurement bias.
I remember an article a few years back: someone literally took pictures of every measurement station they could find. There were many siting problems: stations were next to heat sources like parking lots, roofs, exhaust vents: things that would introduce an upward bias.
Coincidentally, the temperature record has leveled off about the time we started getting satellite measurements.