FTL Design
History of Technology

John Dicas Alcohol Proof Rule
circa 1795

John Dicas was a scientific instrument maker, active in Liverpool, England, between 1774 and 1797. He was succeeded by his daughters Mary and Ann, who continued to operate the business until some time after 1837 [1].

In 1780 John Dicas was awarded UK patent 1,259 for “constructing hydrometers with sliding-rules to ascertain spirit strength”. According to Peter Hopp, the rule was of very simple form: “actually a line of numbers, 0-360, on a rod opposite which is placed the strength of spirits. This shows the strength of spirits at any degree of heat” [1].

In 1790 Dicas's hydrometer was listed as the US standard for measuring the proof of alcohol in an Act of Congress passed on 19 July of that year and effective 1 January 1791 [2]. Dicas's hydrometer continued to be the standard for the next 60 years [3].

I can find no date for the introduction by Dicas of the US standard hydrometer, subsequent to his patented design of 1780, but it is likely that at about the same time Dicas also developed the ivory proof rule shown here to be used with the US standard hydrometer and with other instruments.

The rule is 208 mm long (about 8.25") by 32 mm wide, and the slide has a travel of about 60 mm, corresponding to the length of the scale marked from 30 to 100 at the top left of the slide on the Alcohol side of the rule.

“Alcohol” side of the rule

Ivory proof rule marked on the slide “Dicas Patentee Liverpool”
The stock's top scale is marked “Proof” and the bottom scale “Alcohol”
Detail of scale

“Water” side of the rule

Ivory proof rule marked on the slide “Dicas Patentee Liverpool”
The stock's top scale is marked “Water” and the bottom scale “Proof”

The Museum of the History of Science at Oxford, England, has a similar rule in its collection.

The earliest documentation for this style of rule by Dicas is found in the 1795 edition of General View of the Agriculture of the County of Lancaster, published in London “from the Communications of Mr. John Holt of Walton, near Liverpool” [4]. This describes a lactometer to be used for testing the quality of milk, and the illustration shows a rule of identical construction to the proof rule above, but with scales specific to the measurement of milk.

Illustration of the Dicas lactometer rule from
Agriculture of the County of Lancaster

This extract from The Vintner's Guide, published in 1825 [5], describes the operation of the Dicas hydrometer:

Title page

Page 226

Page 227

Page 228

Directions for proving the Strength of
Spirituous Liquors, by the

Invented by the late J. DICAS, and made by T.
SAUNDERS, (late of College-Green,) Eden-Quay, Dublin..

The Hydrometer is fitted up in a mahogany box, with 36 weights accurately adjusted thereto, numbered, 0, 10, 20, 30, &c. to 350, and upon the cap of the Hydrometer is marked 360 to extend the scale still farther.—Those weights are to be applied only one at a time, upon the top according to the destiny of the spirit, and the difference between each of them, is pointed out by the ten divisions upon the stem, by this means the hydrometer weight of any spirit may be readily obtained.

The box also contains a thermometer for taking the temperature of the spirit, and likewise an ivory rule for determining the strength; upon the middle part of this sliding rule is laid down the hydrometer weight of the spirit, going from 0 to 160, on one side, and from 100 to 370, on the other. The hydrometer being constructed so as to represent the weight of water as nothing, and that of spirit to increase as it becomes stronger; opposite to the hydrometer weight of the spirits the different strengths are placed, proceeding from water to proof on one side, and from proof to alchohal on the other, shewing how many gallons they are either above or below proof, and as heat and cold produce different effects upon spirits of different densities.

The rule is graduated so as to accommodate to each strength that particular variation it is subject to, on being brought from a cold to a warm temperature, or the reverse, for which purpose, the degrees of heat from 30 to 80, corresponding with Fahrenheit's scale, are laid down on each side of the sliding rule, towards the left hand, with a flower de luce opposite, as an index to fix the slide to the temperature of the spirit.


First, find the temperature of the spirit, by immersing the thermometer therein, and fix the sliding rule so that the flower de luce shall be opposite the same degree which the mercury rises or falls to, then put in the hydrometer arid try which of the weights will sink it to some of the divisions upon the stem. Add the number on the weight and that of the division together, for the hydrometer weight of the spirit, and having found the hydrometer weight upon the middle or sliding part of the rule, directly opposite thereto, till be shown the exact strength.


Suppose in 55 degrees of heat, with the weight of 150 upon the top, the hydrometer sinks to 6 upon the stem, the weight of the spirit becomes 156; the rule being fixed so that the 55 degrees of heat shall be opposite to the flower de luce; then facing 156 will be found 7, which is the number of gallons in the 100, the spirit is above proof, of which 290 upon the top, the hydrometer sinks to 1 upon the stem; the rule being fixed to the temperature as before against 291, will be found 53 gallons in the 100 over proof; if in 74 degrees of heat, the hydrometer with the weight 70 upon the top sinks to 5 upon the stem, 74 degrees being placed opposite the flower de luce against 75, the hydrometer weight will be found 42, the number of gallons in the 100, the spirit is below proof, &c. &c.

N.B. If foreign spirits are sold lower than 15¾ per cent. under proof, which the star on the scale denotes, they are seizable according to act of parliament. The letters S.W. denote very good spirits of wine; on the outer edge of the sliding rule, are placed the different strengths, as they are made by Clark's hydrometer.


To know the value of any quantity comprising 2 figures and upwards, at 2s., double the last figure for shillings and the remaining figures, or first figures become pounds; or for example, 36 at 2s. make £3. 12s. and so for any larger quantity or complement; as for instance, 378 at 2s., double the last figure 8 for shillings makes it 16, and the 37 for pounds makes the amount £37. 16s. Any aliquot part of 2s.. for instance, 6d. the one-fourth, 8d. the one-third; 4d. the one-sixth, 2d. the one-twelfth, by dividing the amount at 2s. by any of those aliquot parts, the amount is immediately discovered.

A Dictionary of Chemistry, published in 1828 [6], while noting that use of the Sikes hydrometer was required by law in Britain, also describes the operation of Dicas's hydrometer:

The strength of spirits is determined, according to the existing laws, by Sikes's hydrometer; but as many dealers use Dicas's, I shall describe it here.

It consists of a light copper ball, terminating below with a ballast bottom, and above with a thin stem, divided into ten parts. The upper extremity of the stem is pointed, to receive the little brass poises, or discs, having each a hole in its centre. These poises are numbered 0, 10, 20, 30, &c. up to 350, which is the lightest of the series. The intermediate units are given by the subdivisions on the stern. A graduated ivory scale, with a sliding rule and thermometer, accompanies the hydrometer, to make the correction for temperature.

The first thing in using this instrument is to plunge the thermometer into a glass cylinder containing the spirits to be tried. The sliding rule has then the degree of temperature indicated, moved opposite to zero. The hydrometer is now placed in the liquid, and such a poise is put on as to submerge a portion of the stern. The weight, added to the number on the stem, gives a sum, opposite to which on the scale we find a quantity by which the particular spirit may exceed or fall short of proof. Thus, if it mark 20 under proof, it signifies that every 100 gallons of that spirit would require to have 20 gallons of water abstracted from it to bring it up to proof. If it mark 10 over proof, we learn that every 100 gallons contain too little water by 10 gallons. When the thermometric degree of 60° is put opposite to zero, then the weights and value of the spirits have the following relations on the scale.

102.5 denotes 20 under proof
122.0   10 "
143.5   Proof  
167   10 over proof
193   20 "
221   30 "
251   40 "
284.5   50 "
322.5   60 "
350.5   Alcohol  

There is, besides, an upper line on the scale, which exhibits the relation of spirit to water reckoned unity. Thus, above 10 per cent. over proof in the second line, we find in the upper line 8. From which we learn, that 8 of that spirit by bulk, will take 1 of water to bring it down to proof. At 60° Fahr. I find that 10 over proof on Dicas corresponds to specific gravity 0.9085:

10 over proof 0.9085
over proof 0.9169
Proof   0.9218

Act of Congress dated 19 July 1790 authorizing a duty on spirits distilled in the United States from molasses “at the rate of three cents for every gallon of such spirits, not more than ten per cent. under proof, according to Dicas's hydrometer”.

Images courtesy of the Library of Congress


1. Hopp, Peter M.: Slide Rules. Their History, Models, and Makers. Astragal Press, Mendham, New Jersey, 1999.

2. An act making further provision for the payment of the debts of the United States. Printed by John Fenno, Philadelphia, 1790.

3. Bud, Robert & Warner, Deborah Jean: Instruments of Science. Science Museum London; Smithsonian Institution, Washington DC; Garland Publishing, 1998.

4. Holt, John: General View of the Agriculture of the County of Lancaster. G. Nicoll, London, 1795.

5. Phipps, William: The Vintner's Guide. John Bromell, Boyle, Ireland, 1825.

6. Ure, Andrew: A Dictionary of Chemistry, Third Edition. London, 1828.

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Last revised: 12 December, 2019