To expand a little; the surface temperature sensors are early warning devices.
In the dark even the most skilled of men could overlook some warning device.
"It could be a warning device to keep those beast boys away from the food," Scott said.
The warning device must be placed where it can be seen or heard easily.
Something had triggered the warning devices at one of the listening posts.
Well, if the data is close to real time, then I would think it could serve as an early warning device.
My empathic sense will certainly be useful as a warning device.
This was one of the best warning devices they car- ried.
What good is a warning device if no one monitors it?
On a planet with no life, it was a simple but effective warning device.
