Final answer:
The metaphor comparing the Titanic to a 'big floating layer cake' highlights the ship's multi-tiered structure and emphasizes its grandiosity and internal complexity.
Step-by-step explanation:
The author's description of the Titanic as "like a big floating layer cake" is a metaphor that visualizes the ship's multiple decks stacked on top of each other, just as a layer cake has several tiers of cake piled high. This comparison also captures the grandiosity and distinct, separate levels within the ship, each with its own amenities and types of passengers, similar to how each layer of a cake might have different flavors or fillings. The metaphor serves to emphasize both the size and the structured complexity of the ship, conveying more than just its physical appearance but also suggesting its societal stratification.