Farmar's Wine & Spirit Merchant's Rule

Liverpool: F C Farmar & Son (Sole Maker and Patentee), ca1900. As issued.. Very good/fine.. 23 inch, single slide boxwood rule with bronze end caps; ruled both sides. Based on patents held by Farmar beginning 1899

It is not an excise rule but has scales for gauging, stocktaking, alcohol percentage, profit, proof and other similar calculations. Farmar's spirit slide rule was a tool used, among other things, to calculate how much water is needed to reduce the strength of a liquor. The calculation is not complex. It is, however, tricky because of the fact that when alcohol and water are mixed there is a net contraction in the volume, thereby introducing an error into the "proof" result which is based on volumes. It is from this that the proper excise tax was calculated. Hence the importance of an accurate measure of "proof". Farmar's is the first rule to account for this. See Journal of the Oughtred Society, vol. 30, #1, 2021, pp. 5 - 17 for the very detailed Greg Miller and Tom Martin article.