56 



Henry H. Dixon. 



formed by a glass vessel beneath the chamber. In the lower part of 

 the chamber, beneath the wheel, is a small cistern also containing 

 water and from this cistern leads a branching tube which distributes 

 the water to twelve porous pots, some of which are seen in the 

 background. 



Evaporation from the surface of the pots removes the water 

 from the cistern. This diminishes the air pressure in the air-tight 

 chamber, and the atmosphere, pressing on the surface of the water in 



the well beneath, urges 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 as long 

 as there is water in the well 

 beneath. In order to prevent 

 the drops running 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 posi- 

 tion on the edge of the wheel, 

 a camel's hair brush, pro- 

 jecting out of the water in 

 the cistern, is brought suf- 

 ficiently close to the wheel 

 to be able to drink the drops 

 oif 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 cms in diameter and thus the twelve expose a total effective 

 evaporating surface of 3200 sq. cm. With these arrangements the 

 wheel might he 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. 



Fig. 7. Model to illustrate how the energy 



entering at evaporating menisci may be 



utilised in doing' work. 



