176 NEGEETTI AND ZAMBEA, HOLBOEN VIADUCT, E.G. 



HYDEOMETEES, &c. 



418 HYDROMETERS, or Areometers, are instruments constructed to determine the 

 specific gravity of fluids. Their use has been traced back to a date about 300 

 years before Christ, the invention being ascribed to Archimedes, the Sicilian 

 philosopher. Their action is dependent upon the law " that a body immersed in 

 any liquid sustains a pressure from below upwards equal to the weight of the 

 volume of liquid displaced by such body." 



First on our list of Hydrometers we place those showing Specific Gravity, 

 because all other Hydrometer scales are referable to it ; and as the figures 

 indicated are absolute and definite quantities, or values without possibility of 

 dispute, it is the best both for scientific and manufacturing purposes. 



The Specific Gravity of Fluids may simply be described in a few words. 



A very light glass flask is accurately adjusted and stoppered to hold 

 exactly 1,000 grains of pure distilled water at a temperature of 60 Fahrenheit. 

 If this flask be filled with highly rectified asther, and then carefully weighed in 

 a delicate balance, it will be found that the flask instead of holding 1,000 

 grains will only weigh say 713 or 715 grains at 60 degrees of temperature. 

 This is the specific gravity of pure a9ther, or as written in chemical language, 

 0*713 or 0*715. On the contrary, if the flask be filled with concentrated sul- 

 phuric acid it will be found to hold 1842 or 1845 graine, or specific gravity, 

 1-842 or 1*845, at 60 degrees temperature. In these readings water is repre- 

 sented by one thousand, I'OOO. 



All other fluids (save Mercury) will be found to be of intermediate specific 

 gravity, say between 600 and 2*000. 



Our list embraces the whole of the Hydrometers in use in the United 

 Kingdom and most of the Foreign instruments. The comparative value of 

 these may be ascertained by reference to a valuable series of carefully compiled 

 tables described at the end of Hydrometer Section. 



As a rule all Hydrometers made in England are adjusted to a temperature 

 of 60 Fahrenheit, but if they are required for use in the East or West Indies, 

 they must be specially adjusted at 84 Fahrenheit, and should be ordered 

 accordingly. 



