> That 1930's heliograph is quite intriguing.
> I had no idea something like that was in production so late....

Since this seems a bit OT, send me a PM if you want to know more. However - most heliographs sold on Ebay are British heliographs made in WWII.

I read an account today of a military foot patrol in Pakistan in May 1951. For communication, the patrol carried a walkie-talkie, carrier pigeons, and a heliograph (the book included a full-page photo of the latter in action.)

In my last trip to the mountains we had three cell phones (from two different carriers), two amateur radios, plus aimable signal mirrors ranging up to 12"x12". But we didn't think to bring carrier pigeons!

> I wonder if the glass ones would even hold up where I live?
> Glass and severe cold usually don't get along that great.
> I've lost a couple of compass mirrors due to the cold.

I think it likely would, though minimizing thermal shock would be prudent.

The US military mirror spec Doug Ritter posted here: Thermal Test Requirement for MIL-M-18371E Signal Mirror includes taking the mirror to -65 +/- 2 deg Fahrenheit ( -54 +/-1 deg Centigrade) and holding it there for 24 hours. (Also, to +160 deg F). This test cycle is to be repeated twice. This mirror is made by the same manufacturer (S.I. Howard Glass) as the MIL-M-18371E mirror, and seems to be made by the same methods.

The US Military "Cold Climate Survival Kit" from the early 1960s included a glass signal mirror. Antarctic survival kits for several nations include signal mirrors, but I don't know what type. I have read two accounts of improvised glass signal mirrors in use in Antarctica: Improvised Antarctic Signal Mirror and 1909: "the western party used a heliograph, consisting of a shaving mirror"