It finally got cold enough at the house to turn on the heat, for the first
time since last winter - and - and - nothing happened.

The heat didn't go on all night, so this is what I saw in the morning:
http://farm8.staticflickr.com/7419/1...2b188768_o.gif

I really do not understand how home heating systems work.

I figured I'd start by taking apart the thermostat to see if I can test
if it's giving the right signal to it's Payne 394JAW propane heater:
http://farm3.staticflickr.com/2890/1...c9c93247_o.gif

The little mercury bulb appears to work, at least at the extremes:
http://farm3.staticflickr.com/2837/1...d6aa35f3_o.gif

And, there's this calibration thingy that seems to be already set:
http://farm4.staticflickr.com/3796/1...1339eb39_o.gif

But, I couldn't see what I'm supposed to *test* without removing screws:
http://farm8.staticflickr.com/7452/1...ff18c35f_o.gif

Yet, once I removed the three flathead screws, I was met with this!
http://farm4.staticflickr.com/3797/1...d18b3e08_o.gif

May I ask:
Q: How do I test this thermostat to see if it's actually working?