The circular door would be kept open and had a grill to prevent the canary escaping. Once the canary showed signs of carbon monoxide poisoning the door would be closed and a valve opened, allowing oxygen from the tank on top to be released and revive the canary. The miners would then be expected to evacuate the danger area.
While you can't visit our canaries just yet, why not have a go at making a canary themed optical illusion at home? When the two images are twisted quickly they become one and it will look like a canary in a cage!
What you need:
- A pencil or stick
- Two circles of card
- Crayons for drawing