After the verb car 'to walk' preceded by sam , and when connected with a noun in the Instrumental - 3rd Case, the ātmanepada is used.,
As अश्वेन संचरते 'he rides on the horse-back.'
The rule does not apply when it is not in composition with a word in the instrumental case. As उभौ लोकौ संचरति इमं चामुं च देवल ! O Devala! thou wanderest through both regions, this and that.' Here though the sense of the Instrumental case is implied, yet as it is not expressly stated, we use the parasmaipada terminations.,