Tralles’s hydrometer differs from Gay-Lussac’s only in being graduated at 4° C. instead of 15° C., and taking alcohol of density .7939 at 15.5° C. for pure alcohol instead of .7947 as taken by Gay-Lussac (Keene’s Handbook of Hydrometry).

In Beck’s hydrometer the zero of the scale corresponds to density 1.000 and the division 30 to density .850, and equal divisions on the scale are continued as far as is required in both directions.

Fig. 8.—Sike’s Hydrometer.

In the centesimal hydrometer of Francœur the volume of the stem between successive divisions of the scale is always 1⁄100th of the whole volume immersed when the instrument floats in water at 4° C. In order to graduate the stem the instrument is first weighed, then immersed in distilled water at 4° C., and the line of flotation marked zero. The first degree is then found by placing on the top of the stem a weight equal to 1⁄100th of the weight of the instrument, which increases the volume immersed by 1⁄100th of the original volume. The addition to the top of the stem of successive weights, each 1⁄100th of the weight of the instrument itself, serves to determine the successive degrees. The length of 100 divisions of the scale, or the length of the uniform stem the volume of which would be equal to that of the hydrometer up to the zero graduation, Francœur called the “modulus” of the hydrometer. He constructed his instruments of glass, using different instruments for different portions of the scale (Francœur, Traité d’aréométrie, Paris, 1842).

Dr Boriés of Montpellier constructed a hydrometer which was based upon the results of his experiments on mixtures of alcohol and water. The interval between the points corresponding to pure alcohol and to pure water Boriés divided into 100 equal parts, though the stem was prolonged so as to contain only 10 of these divisions, the other 90 being provided for by the addition of 9 weights to the bottom of the instrument as in Clarke’s hydrometer.

The instrument which has now been exclusively used for revenue purposes for nearly a century is that associated with the name of Bartholomew Sikes, who was correspondent to the Board of Excise from 1774 to 1783, and for some time collector of excise for Hertfordshire.

Sikes’s hydrometer, on account of its similarity to that of Boriés, appears to have been borrowed from that instrument. It is made of gilded brass or silver, and consists of a spherical ball A (fig. 8), 1.5 in. in diameter, below which is a weight B connected with the ball by a short conical stem C. The stem D is rectangular in section and about 3½ in. in length. This is divided into ten equal parts, each of which is subdivided into five. As in Boriés’s instrument, a series of 9 weights, each of the form shown at E, serves to extend the scale to 100 principal divisions. In the centre of each weight is a hole capable of admitting the lowest and thickest end of the conical stem C, and a slot is cut into it just wide enough to allow the upper part of the cone to pass. Each weight can thus be dropped on to the lower stem so as to rest on the counterpoise B. The weights are marked 10, 20, ... 90; and in using the instrument that weight must be selected which will allow it to float in the liquid with a portion only of the stem submerged. Then the reading of the scale at the line of flotation, added to the number on the weight, gives the reading required. A small supernumerary weight F is added, which can be placed upon the top of the stem. F is so adjusted that when the 60 weight is placed on the lower stem the instrument sinks to the same point in distilled water when F is attached as in proof spirit when F is removed. The best instruments are now constructed for revenue purposes of silver, heavily gilded, because it was found that saccharic acid contained in some spirits attacked brass behind the gilding.

The following table gives the specific gravities corresponding to the principal graduations on Sikes’s hydrometer at 60° F. and 62° F., together with the corresponding strengths of spirits. The latter are based upon the tables of Charles Gilpin, clerk to the Royal Society, for which the reader is referred to the Phil. Trans. for 1794. Gilpin’s work is a model for its accuracy and thoroughness of detail, and his results have scarcely been improved upon by more recent workers. The merit of Sikes’s system lies not so much in the hydrometer as in the complete system of tables by which the readings of the instrument are at once converted into percentage of proof-spirit.

Table showing the Densities corresponding to the Indications of Sike’s Hydrometer.

Sike’s
Indications.		 60° F. 62° F.
Density. Proof
Spirit
per
cent.		 Density. Proof
Spirit
per
cent.
 0 .815297 167.0 .815400 166.5
 1 .816956 166.1 .817059 165.6
 2 .818621 165.3 .818725 164.8
 3 .820294 164.5 .820397 163.9
 4 .821973 163.6 .822077 163.1
 5 .823659 162.7 .823763 162.3
 6 .825352 161.8 .825457 161.4
 7 .827052 160.9 .827157 160.5
 8 .828759 160.0 .828864 159.6
 9 .830473 159.1 .830578 158.7
10 .832195 158.2 .832300 157.8
11 .833888 157.3 .833993 156.8
12 .835587 156.4 .835692 155.9
13 .837294 155.5 .837400 155.0
14 .839008 154.6 .839114 154.0
15 .840729 153.7 .840835 153.1
16 .842458 152.7 .842564 152.1
17 .844193 151.7 .844299 151.1
18 .845936 150.7 .846042 150.1
19 .847685 149.7 .847792 149.1
20 .849442 148.7 .849549 148.1
20B .849393 148.7 .849500 148.1
21 .851122 147.6 .851229 147.1
22 .852857 146.6 .852964 146.1
23 .854599 145.6 .854707 145.1
24 .856348 144.6 .856456 144.0
25 .858105 143.5 .858213 142.9
26 .859869 142.4 .859978 141.8
27 .861640 141.3 .861749 140.8
28 .863419 140.2 .863528 139.7
29 .865204 139.1 .865313 138.5
30 .866998 138.0 .867107 137.4
30B .866991 138.0 .867100 137.4
31 .868755 136.9 .868865 136.2
32 .870526 135.7 .870636 135.1
33 .872305 134.5 .872415 133.9
34 .874090 133.4 .874200 132.8
35 .875883 132.2 .873994 131.6
36 .877684 131.0 .877995 130.4
37 .879492 129.8 .879603 129.1
38 .881307 128.5 .881419 127.9
39 .883129 127.3 .883241 126.7
40 .884960 126.0 .885072 125.4
40B .884888 126.0 .885000 125.4
41 .886689 124.8 .886801 124.2
42 .888497 123.5 .888609 122.9
43 .890312 122.2 .890425 121.6
44 .892135 120.9 .892248 120.3
45 .893965 119.6 .894078 119.0
46 .895803 118.3 .895916 117.6
47 .897647 116.9 .897761 116.3
48 .899509 115.6 .899614 114.9
49 .901360 114.2 .901417 113.5
50 .903229 112.8 .903343 112.1
50B .903186 112.8 .903300 112.1
51 .905024 111.4 .905138 110.7
52 .906869 110.0 .906983 109.3
53 .908722 108.6 .908837 107.9
54 .910582 107.1 .910697 106.5
55 .912450 105.6 .912565 105.0
56 .914326 104.2 .914441 103.5
57 .916209 102.7 .916323 102.0
58 .918100 101.3 .918216 100.5
59 .919999 99.7 .820115 98.9
60 .921906 98.1 .922022 97.4
60B .921884 98.1 .922000 97.4
61 .923760 96.6 .923877 95.9
62 .925643 95.0 .925760 94.2
63 .927534 93.3 .927652 92.6
64 .929433 91.7 .929550 90.9
65 .931339 90.0 .931457 89.2
66 .933254 88.3 .933372 87.5
67 .935176 86.5 .935294 85.8
68 .937107 84.7 .937225 84.0
69 .939045 82.9 .939163 82.2
70 .940991 81.1 .941110 80.3
70B .940981 81.1 .941100 80.3
71 .942897 79.2 .943016 78.4
72 .944819 77.3 .944938 76.5
73 .946749 75.3 .946869 74.5
74 .948687 73.3 .948807 72.5
75 .950634 71.2 .950753 70.4
76 .952588 69.0 .952708 68.2
77 .954550 66.8 .954670 66.0
78 .956520 64.4 .956641 63.5
79 .958498 61.9 .958619 61.1
80 .960485 59.4 .960606 58.5
80B .960479 59.4 .960600 58.5
81 .962433 56.7 .962555 55.8
82 .964395 53.9 .964517 53.0
83 .966366 50.9 .966488 50.0
84 .968344 47.8 .968466 47.0
85 .970331 44.5 .970453 43.8
86 .972325 41.0 .972448 40.4
87 .974328 37.5 .974451 36.9
88 .976340 34.0 .976463 33.5
89 .978359 30.6 .978482 30.1
90 .980386 27.2 .980510 26.7
90B .980376 27.2 .980500 26.7
91 .982371 23.9 .982496 23.6
92 .984374 20.8 .984498 20.5
93 .986385 17.7 .986510 17.4
94 .988404 14.8 .988529 14.5
95 .990431 12.0 .990557 11.7
96 .992468  9.3 .992593  9.0
97 .994512  6.7 .994637  6.5
98 .996565  4.1 .996691  4.0
99 .998626  1.8 .998752  1.6
100 1.000696  0.0 1.000822  0.0