Possible Clues:

• Tool with a bubble
• Basic tool for carpenters and surveyors
• Device showing if someone's been at the gin?
• Means of checking horizontality
• What surveyor may use in flat after ghost is seen
• Horizontality checker with a bubble
• What a builder might use to maintain equanimity?
• Tool to get even with drink
• Device for checking whether a surface is horizontal
• What told builder if someone had been at his whisky?