CH. Vl] REVOLVING DRUM. 155 



" fall," or vertical distance between the horizontals, is 

 proportional to the growth per hour. The slight shock 

 due to the sudden rotation of the drum is an advantage 

 because it prevents the index sticking on the smoked 

 paper. We use a simple lever to magnify the growth of 

 the plant. It measures 605 mm., so that when arranged to 

 multiply by 10, the short arm is 55 mm. in length. The 

 fulcrum is a knife-edge, and the cord connecting the short 

 arm with the plant is also attached by a knife-edge 

 attachment: the index is a piece of platinum foil or a 

 thin piece of quill. Since the vertical displacement of 

 the end of the short arm equals the growth of the plant, 

 the vertical distance between the horizontal lines on the 

 tracing must be measured, not the distance along the arc 

 described by the index. It will be seen that an error is 

 introduced by this method : since the short arm describes 

 an arc it is obvious that the cord connecting the lever 

 with the plant does not remain absolutely vertical ; but if 

 the cord be of reasonable length, for instance 25' or 30 cm., 

 and the short arm of the lever be also relatively long, e.g. 

 5 cm., the error is insignificant. For some further details 

 in the use of the auxanometer see exp. 186. 



(182) Simpler form of recording auxanometer. 



If the lever attached to a growing plant is allowed to 

 write on a smoked surface, and if to this surface a slight 

 shake is given every hour, a record of the hourly growth 

 is obtained. Such an arrangement however only serves 

 for recording growth, or any change which always varies in 

 one direction. If the recording method is used for geo- 



