GLASS-MAKING. 



629 



it must displace its own weight of the supporting fluid. If, then, 

 a float of invariable weight be immersed in a liquid, the depth to 

 which it sinks will be a measure of the specific gravity of the 



Fig. 7. Marking Thermometers. 



liquid. The hydrometer is simply such a float as this. It is made 

 in different forms and styles, according to the use for which it is 

 intended ; but in all cases it is essentially a cylinder or bulb of 



VOL. XXXIX. 45 



