152 PNEUMATICS. 



The hygrometer is an instrument for measuring 

 the degree of dryness, or dampness, of the atmos- 

 phere. They are of various constructions. One 

 of the most sensible is that made of the beard of 

 wild oats, which, by twisting, moves an index fas- 

 tened to it. Another kind is formed of a thin slip 

 of whalebone, which lengthens in moist, and con- 

 tracts in dry weather. 



The wind-gage (called also aremoscope) is an in- 

 strument for measuring the force of wind ; and the 

 rain-gage ascertains the quantity of rain that falls 

 on each square foot of the earth's surface. 



Plate 7« %• 9. represents one of the best con- 

 structions of rain-gages. It consists of a hollow 

 cylinder, having within it a cork-ball attached to a 

 wooden stem, which passes through a small open- 

 ing at the top, on which is placed a large funnel. 

 When this instrument is placed in the open air in 

 a free place, the rain that falls within the circum- 

 ference of the funnel will run down into the tube, 

 and cause the cork to float ; and the quantity of 

 water in the tube may be seen by the height to 

 which the stem of the float is raised. The stem 

 of the float is so graduated, as to show, by its divi- 

 sions, the number of perpendicular inches of water 

 which fell on the surface of the earth since the 

 last observation. It is hardly necessary to observe, 

 that after every observation the cylinder must be 

 emptied. 



The electrometer will be described under elec- 

 tricity. 



PRACTICAL DIRECTIONS FOR USING THE AIR 

 PUMP. 



The invention and general construction of the 

 air-pump have been already described, p. 129. \ but 



