never resting never still

The riddle 'Never resting, never still. Moving silently from hill to hill. It does not walk, run or trot, all is cool where it is not.' suggests an object or concept that is constantly in motion, yet seems to have no fixed location. The allusion to a heatwave adds to the imagery of something that is elusive and ever-changing. Considering the context of the riddle, the most likely answer is Sunlight. Sunlight is an excellent analogy as it provides a continuous source of light and heat without ever ceasing, and indeed, the concept of the sun moving across the sky is metaphorically used in the riddle to suggest an endlessly repetitive motion.