FACTS ABOUT WATER 261 



Hydrometers. Because of the fact that a body sinks in liquid 

 until the weight of the liquid displaced is equal to the weight of the 

 body, an instrument called the hydrometer may be used for measuring 

 the specific gravity of liquids. The hydrometer is weighted at the 

 bottom with a long stem on which there is a graduated 

 scale. The depth to which the hydrometer sinks gives 

 the specific gravity of the liquid. 



There are three classes of hydrometers: 



1. The specific gravity hydrometer indicates the 

 number of times heavier the liquid is than an equal 

 volume of water. 



2. The per cent hydrometer indicates the per cent 

 of a substance present. This hydrometer is used for 

 determining the per cent of alcohol, water, etc. The 

 amount of water in maple syrup is easily determined FIG. 165. 

 by this hydrometer. The density of maple syrup 



varies from 1.32, with 35 per cent water, to 1.34, with 32 per cent 

 water. 



3. The arbitrary scale hydrometer indicates the concentration of 

 strength of a substance. The lactometer is an example of this hy- 

 drometer, and is used for measuring the specific gravity of milk. 



Look up uses of hydrometer. 

 What does the word mean? 



Use of the Hydrometer. In using the hydrometer a portion of 

 the liquid whose specific gravity is to be measured should be placed 

 in a glass cylinder of such a size that the hydrometer when placed 

 in the cylinder will be free to move up and down without coming 

 in contact with the walls of the vessel. The liquid should be well 

 stirred. For specially accurate work the temperature of the liquid 

 should be observed by means of a thermometer placed directly in 

 the liquid. When the temperature has become fairly constant the 

 readings on the hydrometer may be taken. 



Water Pressure. Since a cubic foot of water weighs about 62| 

 pounds, the greater the amount of water in a reservoir or tank, 

 the greater will be the pressure on the bottom and sides of the 

 container. For every 2f^ feet increase in depth the pressure in- 



