Sentence:  There is no breeze, no slightest shift of air-particles; yet down the gorge comes this cloud,--a cloud <WSD>unsensible</WSD> except to nostrils,--eddying as if swirling around the edges of leaves, riding on the air as gently as the low, distant crooning of great, sleepy jungle doves. 
Meaning of the word marked with <WSD> tags:  That cannot be sensed; imperceptible.