PHYSICAL PARADOXES. 



indicated by the thermometer B, of which the 

 bulb is covered by the water. 



But the bulb of the thermometer A is 

 covered by absorbent material which capillary 

 attraction keeps moistened from the cistern. 

 With so large a surface exposed to the air, it 

 affords great opportunity for vaporisation, and 

 in dry weather a good deal of water vaporises 

 from the moist covering of the bulb. The per- 

 formance of this vaporisation requires heat, 

 which is supplied by the water itself in the act 

 of vaporising, just as it was in the last experi- 

 ment by the water boiling without a flame. 

 Consequently the film of water in the porous 

 covering all over the bulb is considerably cooled 

 by having its heat used up in this way ; and 

 so this thermometer indicates a lower tempera- 

 ture than the other. 



When the air is already well saturated with 

 water vapour, or, as we commonly say, quite 

 damp, drying from the bulb takes place more 

 slowly, less heat is used up in the process, and 

 so the water in the bulb-covering is less cooled, 

 and thermometer A does not show a much lower 

 temperature than B. 



Thus the difference between the tempera- 

 tures of the two thermometers indicates the 

 dryness of the air : when the difference is greater 

 the air is drier, when the difference is less the 

 air is damper. 



Such an arrangement of two thermometers 

 150 



