CHAP. ii. J AREOMETERS OR HYDROMETERS. 41 



mixture subjected to examination, to reduce it to proof-spirit. Thus 

 if the instrument indicates 10 over-proof, 10 volumes of water must 

 be added to bring the liquid down to proof strength, and 1 00 gallons 

 of such strength would be reckoned as 110 ; in the same way 100 

 gallons at 10 under- proof would in the same way be charged at 90 

 The indications of the instrument referred to are of a perfectly 

 arbitrary character, and reference must be made to the tables to as- 

 certain the proportion of spirit they represent. It may be remarked 

 generally, however, that these indications commence with zero at the 

 highest strength, and that, upon an average, every subdivision of the 

 scale shows a diminution of three-tenths per cent, of proof-spirit. 

 This instrument is therefore greatly more exact than the continental 

 one, which indicates only differences of one per cent. Corrections on 

 account of temperature are provided for by tables wherein Sikes has 

 made the correction for each degree between 30 and 80 Fahrenheit. 



The centesimal alcoholometer is officially adopted in France for 

 testing brandies, spirits, and all alcoholic liquors. In Germany 

 Trelle's alcoholometer, which only differs from that of Gay-Lussac's 

 by the temperature of the graduation (60 Fahr. or 15 0- 5 C.), is 

 employed. 



It is important to remark that the different instruments described 

 here enable us to determine the density of the liquid mixtures in 

 which they are immersed only indirectly. Tables however have 

 been calculated giving the density for each degree. But they give 

 no information as to the composition of the mixture which may be 

 changed from its normal composition by the introduction of foreign 

 substances. 



III. WATER-LEVELS. SPIRIT-LEVELS. 



The free surfaces of liquid in communicating vessels when in 

 equilibrium lie in the same horizontal plane. This fundamental 

 property of liquids has been utilized for making a very simple instru- 

 ment, used in levelling operations. This is called the water-level. 

 It is composed of a long metal tube bb, the two ends of which 

 are bent at a right angle, vertically supporting two glass vessels open 

 at the top. To use it, tlie tube is filled with water, so that the 

 liquid nearly fills the vessels, when the tube is arranged horizontally. 



