CH. IV] STIPA HYGROMETER. 103 



cylinder g across the mouth of which a glass tube is fixed : 



from the centre of the tube a piece of 



Stipa-ayvji s projects at right angles 



and bears at its end an index i, 



which may conveniently be made of 



thin iron wire. The awn is sensitive 



, , ^ . 1 -1 . . Fig. 20. Exp. 118. 



to nygrometric change, m damp air it 



untwists, in dry air it twists up again. If the vessel is there- 

 fore placed mouth downwards on damp blotting-paper or on 

 a transpiring leaf, the index ^ will rotate and its movement 

 can be read off on a graduated ring of paper fastened to 

 the bottom of the vessel, or a graduated strip of paper 

 attached round the vessel close to the bottom. If two 

 hygrometers are made, one may be placed on each surface 

 of a leaf and the difference in the movement of their 

 indices compared. Certain precautions are necessary : in 

 the first place, it is difficult to get two pieces of awn^ 

 which behave similarly, so that it is necessary to graduate 

 the two hygrometers to make their reading comparable. 

 Take a filter-paper and damp it carefully, making sure 

 that it is not wetter in one part than the other, place it 

 on a flat glass plate and having marked the position of 

 the index with pencil on the paper rings in both hygro- 

 meters, place them side by side on the wet paper. After 

 from 4 to 8 minutes mark the position of the index again. 

 If, for instance, the movement of hygrometer A is only | 

 of that of B, it is clear that the paper ring on A must be 

 marked out in divisions each of which is J of the unit 



1 The awn should be thoroughly ripe, brown in colour, not yellow, 

 and stiff not weedy in texture. 



