ON THE LEVEL OF THE MERCURY. 



69 



The external pressure has no influence on the Thepres- 



sure of the 



amount or the water evaporating in the pores or the air does not 



affect the 



bladder ; that amount depends on the hygrometric amount of 



evapora- 



state of the surrounding air, and on the temperature, tion. 

 In a rarefied air (provided it can take up moisture), 

 evaporation goes on more rapidly than in a denser 

 air ; and hence it is clear, that at certain elevations, 

 the effect of the bladder on the level of the liquid 

 is more quickly produced than at the level of the 

 sea. The amount of water which evaporates is 

 directly proportional to the surrounding space, and 

 to the temperature and corresponding tension of the 

 liquid. 



When the tube, Fig. 10, is filled with water to 

 b, then entirely filled with mercury, and inverted 

 in mercury, the mercury, as we have seen, assumes 

 a fixed level. If now we keep the upper or wide 

 end of the tube, which is closed with bladder, 

 immersed in a vessel of water, 

 Fig. 12, we shall find, after a 

 short time, that the mercury 

 sinks in the narrow tube. If 

 its level has been 12 inches 

 above that of the mercury in 

 the vessel, it sinks when the 

 bladder is put into water, 3 or 

 4 inches for example, and re- 

 mains stationary at 8 or 9 inches, 

 without sinking further for the 



next 12 hours. 



The sinking of 



