Good question.  Of course in the past they had simple weather boxes that measured rainfall and temperature with a very accurate thermometer, but would have to be physically read.
You wonder how many people missed a day, or a reading, and just made up temperatures to fill in the box. 
I think I will buy my own little weather station for my back yard and start an accurate record to compare with these geniuses.  The same people that are telling us this are the same people spreading false data.  
Wouldn't it be amazing if we found that these same sensors are used all over the Earth, and made by the same company, and the company purposely calibrated them to show false data.
The Global Warmers always tout the "data", which once again has been shown to be wrong. 
There should be an investigation.