PHYSIOGRAPHY. 



[chap. 



in the atmosphere, and this determination is effected by 

 instruments termed hygrometers. The simplest but least 

 trustworthy of such instruments depend for their action on 

 the fact that organic structures readily absorb moisture and 

 change their dimensions ; a hair, for instance, is longer when 

 wet than when dry. Taking advantage of this fact De 

 Saussure constructed the simple little 

 instrument represented in Fig. i8. It 

 consists of a human hair free from 

 grease, stretched by a small weight, 

 and furnished with an index, moving 

 over a graduated arc. As the hair is 

 affected by moisture the index moves 

 over the scale, but its indications are 

 not sufficiently exact to be of much 

 scientific value. The instrument, 

 though still used in certain parts of 

 Europe, simply indicates the presence 

 of moisture without accurately measur- 

 ing its amount ; it is, in truth, a hygro- 

 scope rather than a hygrometer} Cruder 

 even than this hair hygroscope is the 

 well-known toy in the form of a little 

 house with two doors, having the figure 

 Fig. i8.— Hair hygrometer, of a man at One door and of a woman 

 at the other. When the air is moist 

 and rain may be expected, the man comes out ; when the 

 air is dry and the weather likely to be fine, the woman 

 makes her appearance. The movements of the figures 



1 Instruments having names terminated in meter {fierpov, meiron, 

 measure) are generally more exact in their indications than those termi- 

 nated in scope {(TKoireu, skopeo, to view). Thus' a 7nicroscope enables us 

 to see very minute objects, whilst a micrometer enables us to measure 

 them. 



