🔍 Key Concepts

• Boiler gauge glass construction – know the order of parts from the boiler body out to the observer (boiler body → sealing surfaces → glass → protective pieces → cover plates). • Function of mica vs. gaskets vs. cover plates – mica protects the glass, gaskets seal under pressure, cover plates provide mechanical support, and glass inserts form the viewing window. • Location of items D and M in the cross‑section – look carefully at whether they are drawn as thin layers, thick blocks, or outer plates.

💭 Think About

• Compare the shape and thickness of D and M to the glass block shown – are D and M the main transparent blocks, or thin layers adjacent to them? • Ask what role D and M must play at that specific location: are they likely to seal, to protect, or to carry the load of the bolts and nuts? • Think about which component would reasonably appear at both ends of the gauge glass in exactly the same way, and which components would more likely be continuous along the whole length.

✅ Before You Answer

• Verify whether D and M are in direct contact with the boiler fluid space or with the metal seating surface – this hints at sealing vs. protection. • Check if D and M are drawn as thin linings or shims (suggesting gaskets or mica) or as thicker blocks (suggesting glass plate inserts). • Confirm which part in the illustration is clearly labeled as the outer cover plate; then eliminate any option that would duplicate that same function for D and M.