xi ENERGY AVAILABLE FOR RAISING SAP 207 



water up the glass tube and through the fine nozzle. A 

 series of drops is thus delivered on the edge of the wheel, 

 and keeps it in constant rotation so long as there is water 

 in the well beneath. In order to prevent the drops run- 

 ning round the edge of the wheel, this latter is covered 

 with a thin coating of paraffin wax. By this means the 

 drops remain fixed on the edge of the wheel just like the 

 bucketfuls of water on the rim of an overshot wheel. Lest 

 the drops should not readily detach themselves immediately 

 on reaching their lowest position on the edge of the wheel, 

 a camel's-hair brush, projecting out of the water in the 

 cistern, is brought sufficiently close to the wheel to be 

 able to drink the drops off its edge. 



The arrangements adopted in setting up this model are 

 shown in the accompanying illustration. The pots used 

 were 16 cm. in length and 5 cm. in diameter, and thus 

 the twelve expose a total effective evaporating surface of 

 3200 sq. cm. With these arrangements the wheel might 

 be kept in rotation apparently for an indefinite time, 

 were it not that the paraffin surface on the edge of the 

 wheel ceases to preserve its water-repellent character after 

 a certain amount of wear, and then the drops from the 

 nozzle run down round the wheel without causing its 

 rotation. Notwithstanding this, the wheel may be easily 

 kept in rotation for several days. 



When the pots are replaced by a leafy branch the wheel 

 is kept in lively rotation by the evaporation from the 

 leaves. Variations in the speed of rotation of the wheel 

 mark the variations in the amount of evaporation from 

 the leaves. Thus, exposure to a draught or to a higher 

 temperature, as in the case of the pots, accelerates the rate 

 of rotation. In addition to this, the influence of light upon 

 the stomata may be observed in the increased rate by day; 

 while conversely darkness diminishes the rate of rotation. 



It is interesting to trace in this model how the molar 

 work of maintaining the wheel in rotation is derived from 



