Final answer:
The device used to indicate the water level in a steam boiler is a water level gauge or glass. This device is essential for the safe and efficient operation of the boiler, similar to how other fluid level or velocity measuring devices are used in various engineering fields.
Step-by-step explanation:
A device used to indicate the water level of a steam boiler is critically important for the safe operation of the boiler. This device is known as a water level gauge or glass. It provides a visual representation of the water level inside the boiler, which is essential for preventing dry firing (where the boiler operates without enough water) and overfilling, which can cause inefficient operation and potential damage. Water level gauges are part of the boiler's safety apparatus and must be maintained and monitored regularly. Similar to how a civil engineer might use an instrument to monitor well water levels, this gauge helps operators in maintaining the appropriate water level for efficient steam generation.
In many applications, such as aircraft, similar devices are used for measuring fluid velocities, known as airspeed indicators. Though the applications are different, the fundamental principle of measurement remains the key aspect of these devices.