26 M.J.BLACKWELL 



In the Cambridge version, movement of the secondary lever interrupts 

 the illumination of a photo-transistor which in turn operates the relay 

 controlling the drive motor. This scheme was originally developed at 

 N.I. A.E. and gives a saw-tooth type of record which can be read to about 

 ± 10 g. As originally developed the soil-balance used a pen-recorder, with 

 consequent loss of sensitivity through friction. To overcome this the pen 

 and chart system can be removed and a remote-recording system instituted. 

 A drive taken off one of the moving weight pulleys is used to turn a hehcal 

 potentiometer which feeds a variable voltage to a recorder. The system has 

 the advantage of requiring less frequent removal of the guard plants for 

 chart changing. 



For the derivation of £ and Q by the aerodynamic approach (historically 

 the first major problem of the group), wet- and dry-bulb fine wire thermo- 

 couples were finally adopted for measurement of temperature and huirddity 

 gradients or differences. These were chosen partly to facihtate future 

 investigations in and above growing crops where forced ventilation would 

 disturb the local regime, and partly to reduce radiation errors to a value 

 which is not only very small but also constant for each level. One new 

 disadvantage is the efficiency with which the thermocouples respond to 

 rapid temperature fluctuations. For wind profiles, sensitive cup contact 

 anemometers are used. 



4. THE AERODYNAMIC METHOD 



At this stage, we must return to the basic concepts of turbulent transfer 

 theory already touched upon in section 2. It follows from eq. 2 and the 

 definitions o£ Km and Kiv in eqs. 4 and 5 that: 



provided that Kw = Km- This result was first apphed by Thornthwaite and 

 Holzman (1942). The analogous relation for Q: 



Q_ ^V^p("2-"i)(^i-'^2) (^^) 



KS 



depends on the assumption that Kh = Km but has not been tested as 

 effectively. 



For conditions when vigorous buoyancy forces are absent, the aero- 



