References in classic literature ?
The thermometers and barometers had resisted, all but one minimum thermometer, the glass of which was broken.
If you do not known what the upper and lower temperatures are in your garden, then little of what I have said above will be of any use and so, all gardeners should have some method of measuring the normal air temperature around the garden, whether through one of the modern electronic weather stations that are so popular these days, or through a good old-fashioned maximum - minimum thermometer, strategically positioned.
Insulation merely slows down the heat loss or gain, so a maximum and minimum thermometer (preferably an accurate digital one) on top of the box will allow you to monitor the conditions: Above 11AC, the tortoise may become active; below 2AC is potentially dangerous.

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