In the United States the standard liquor, known as _proof spirit_, contains 92.3 per cent. by weight and 94.9 per cent. by volume, of absolute alcohol; it has a specific gravity of .9186 at 60∞ F. A proof gallon contains by measurement 100 parts of alcohol and 81.5 parts of water. The strength and therefore the value of spirituous liquors is estimated according to the quantity by volume of anhydrous spirit contained in the liquor with reference to this standard. Thus the expression "20 _per cent. overproof_," "20 _per cent. underproof_," means that the liquor contain 20 volumes of water for every 100 volumes over or under this fixed quantity, and that in order to reduce the spirit to _proof_, 20 per cent. of water by volume, must be subtracted or added, as the case may be. Any hydrometer constructed for the measurement of liquids of less density than water may be employed. That known as "Syke's" is most commonly used for alcoholometric purposes. It is shown in Fig. 52 and consists of a spherical brass ball _A_, to which is fixed two stems; the upper one _B_ is also of brass, flat, and about 3-1/2[** fraction] in. in length; it is divided into ten parts, each being subdivided into five, and the whole being numbered as shown in the figure. The lower stem _C_ is conical, and slightly more than an inch long; it terminates in a weighted bulb _D_. A series of circular weights, of the form shown in the figure, accompany the instrument; these are slipped upon the top of the lower stem _C_, and allowed to slip down until they rest upon the bulb _D_. The instrument is used in the following way: It is submerged in the liquor to be tested until the whole of the upper stem is under the surface, and an idea is thus gained of the weight that will be required to _partly_ submerge the stem. This weight is added, and the hydrometer again placed in the liquor. The figure on the scale to which the instrument has sunk when at rest is now observed, and added to the number on the weight used, the sum giving, by reference to the tables, the percentage by volume of absolute alcohol above or below the standard quantity. In exact estimations, the temperature of the liquor tested must be carefully registered, and the necessary corrections made. In Jones's hydrometer, which is an improvement upon Syke's, a small spirit thermometer is attached to the bulb, and by noting the temperature of the liquor at the time of the experiment, and referring to the tables accompanying the instrument, the strength is found at once without the need of calculation. Dica's hydrometer is very similar to Jones's instrument above described. It is of copper, has a stem fitted to receive brass poises, a thermometer, a graduated scale, etc.