If you do not have thermometers in a given location, look at the nearest surrounding thermometers you DO have. Do some interpolation weighted with weather patterns and historical trends. You can come up with a pretty good estimate.The POINT is, if you cannot place thermometers in Siberia, the Antarctic, Congo or Australian Outback, there's no possible WAY to "average global temperatures" in 1850 so trying to compare TODAY'S temperature with 1850's is an exercise in futility.