HYGROMETER. 29 



cloud, with a southerly wind, rain may be expected, that 

 night or next day. 



When the swallows fly low and often dip their wing s 

 in the water over which they are flying, when the crow 

 cries louder and more frequently than common, when 

 water-fowl are very noisy and active, when dogs appear 

 unusually dull and sleepy, when pigs run about and look 

 uneasy, when the croaking of frogs is loud and general, 

 when earth worms are seen in great numbers on the 

 surface, some people expect rain. 



90. The principle iipon which the Hygrometer is con 

 structed is the fact that there is always more or less 

 moisture in the air, and that this moisture is absorbed by 

 certain substances, making them heavier, and enters into 

 lines or cords made of other substances, making them 

 thicker and shorter. 



91. A hygrometer may be made of a piece of sponge 

 filled with a solution of some salt, which has an attraction 

 for water. This sponge is suspended to one end of a 

 balance, and, as it grows heavier by the moisture absorbed, 

 causes the other end to rise, and thus indicates the 

 quantity of moisture in the atmosphere. Or it may be 

 made of a cord or string, with a weight attached, placed 

 over a pully, and showing the moisture by its lengthening 

 or shortening. 



92. A still more delicate hygrometer is formed of two 

 thermometers on the same frame, the bulb of one of 

 which is covered with thin gauze, which may be kept 

 continually moist by a contrivance like a wick, communi 

 cating with a cylinder kept full of water. The moisture 

 on the gauze evaporates and cools the bulb within. The 

 amount of evaporation depends upon the dryness of the 



