From Distillers Wiki
The vinometer is operated by holding it upside down and filling the mouth (C) with the liquid to be measured. The liquid is slowly gravity fed down the neck of the vinometer (B) until 5 or 6 drips exit at (A). The vinometer is then inverted which causes the solution to slowly drain down the neck from (A) toward (C). The liquid will pause at the appropriate proof, allowing an accurate reading.