Sentence:  Its [the barometer's] normal register in the Paumotus [the Tuamotus] was 29.90, and it was quite customary to see it <WSD>vacillate</WSD> between 29.85 and 30.00, or even 30.05; [...] 
Meaning of the word marked with <WSD> tags:  To sway unsteadily from one side to the other; oscillate.