The literary device used in this case is a simile. A simile is the device that used for comparing two different objects such that the qualities of the objects are compared. Similes shows the similarities that takes place between two things through the use of words such as "as"sing, "like", "than", "so" etc.
In the question given, when the evening is spread out against the sky like a patient etherised upon a table indicates that evening is being compared to death.
