In the plot of temperature of the global warming, the measurements start from 1860. How was the global temperature measured at that time? How accurate was the measurement?

The standard mercury-filled thermometers that we still use today were used in 1860, and the accuracy of each measurement was very close to what you get today, except that thermometers were not as accurately calibrated as they are now. The most recent calibration scales were implemented in 1990; before then, the margin of error was slightly higher but not to the point where data collected before then is invalid.

We can presume that mercury thermometers are very accurate given that modern equipment, which is much easier to use and can be monitored remotely, is still checked against one on a weekly basis.

Sadly, there were not nearly as many data points available in 1860 as there are today, which reduces the accuracy of the data when one must average the data for the entire planet to identify a warming or cooling trend.

Global temperatures in the 1860s were primarily measured using thermometers placed in various locations around the world. These measurements were less accurate compared to modern methods due to limited coverage and technology.