Sentence:  The above work is edited by Sir Charles's sister-in-law, and is composed of many pleasantly-written letters, collected from friends and relations, most of them written by Sir Charles under the <WSD>stimulancy</WSD> of fresh scenery, or visits to famous geological sections, or to celebrated geologists. 
Meaning of the word marked with <WSD> tags:  The act of stimulating; stimulation.