33 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



having precisely the same weight as the weight of the tody ; it is 

 this same principle, discovered and demonstrated by Archimedes, 

 which made the solution of the problem of the crown easy. Other 

 scientific historians have considered the discoverer of areometers to be 

 the beautiful and learned Hypatia, the unfortunate victim of the 

 religious fanaticism of the Alexandrian monks. What is certain is 

 that these valuable little instruments owe their actual form to a 

 modern physicist, Homberg. 



We have described the areometers specially adapted to measure 

 the density of bodies with the most perfect scientific accuracy (see 

 Forces of Nature). It now remains for us to speak of the use 

 made of similar instruments in the arts and manufactures in those 

 cases in which the principle of Archimedes is utilized to determine 

 the composition of certain mixtures. 



They are generally cylindrical glass rods, weighted at the lower 

 end by leaden shots or mercury, enclosed in a globular appendage. 

 The weight of an instrument thus constructed is invariable, hence the 

 name of scale-hydrometer in opposition to weight-hydrometers ; 

 the immersed part sinks lo\ver as the liquid is less dense, because 

 the liquid displaced always has a weight equal to that of the 

 instrument. 



Pure water is the liquid used for comparison : the zero of gradua- 

 tion is made at the point of the stem which touches the surface. 

 Instead of making one graduation only for liquids or mixtures denser 

 or lighter than water, it has been found more convenient to construct 

 two kinds of hydrometers for the two series, the zero being in one 

 case at the top, and in the other at the bottom (see Figs. 16 and 17). 



Fig. 16 represents Baume's hydrometers which, according to the 

 uses to which they are put, are called alcoholometers, sal imeters, acidi- 

 meters, saccharometers, and vinegar hydrometers, because they -are 

 employed to determine the greater or less concentration of these 

 fluids. 



Thus in the salimeter the zero lies at a point at the upper 

 extremity of the stem. Immersed in a solution containing 15 parts 

 by weight of sea-salt and 85 of water, the hydrometer sinks to a 

 lower point, marked 15 ; the division of the interval from to 15 in 

 fifteen equal parts, and continued to the bottom of the stem, furnishes 

 the graduation. 



