22 M.J. BLACKWELL 



dence in direct measurements o( R, by means of ventilated net flux radio- 

 meters (e.g. as described by MacDowall, 1955). Previously, solarimeters 

 were used to measure the short-wave components received and reflected 

 back from the surface; grass minimum thermometers were used to 

 estimate the radiative temperature of the surface, from which the upward 

 long-wave component was calculated; and the downward atmospheric 

 radiation was computed from radiation charts. The type of net flux radio- 

 meter referred to above permits measurements to be made under broken 

 cloud but not in rain. Its accuracy can approach + 5%, or a little better 

 with careful checks. Recently, a most promising radiometer, covered with 

 thin hemispherical polythene shells, has been developed by Funk (1959). 

 This removes the need for forced ventilation and is not affected by precipita- 

 tion. Its accuracy is probably of the order of + 2% and impressive measure- 

 ments of radiative flux-divergence have already been made. Over periods of 

 a day or more, the integrated radiation is as large as the total of the other 

 three fluxes. It is therefore of paramount importance to measure it with an 

 accuracy not worse than ± 2%, if energy balance checks of other methods 

 are to be fully effective. In recent work at Cambridge a significant, though 

 apparently simple, step has been made in duplicating all the sensing instru- 

 ments for the four components in the energy balance. In this way, con- 

 sistency of calibrations, efficient working of the data-logging circuits and 

 possible sampUng differences between neighbouring sites can be monitored 

 continuously. Sooner or later in this type of study, most workers are faced 

 with the problem of how representative their measurements are in space 

 and in time. 



The second item in the balance concerns the flux of heat in the ground. 

 Here we are faced with more serious problems of heterogeneity in soil 

 density, specific heat, moisture and other physical parameters. As with the 

 radiation measurements, it has been possible in the last few years to place 

 more confidence in suitably calibrated soil flux-plates to record the varia- 

 tions of heat flux. This avoids the need to compute the flux from the 

 equation : 



S=-Kpc^J^=ks^ (8) 



dz dz 



where K is the thermal diffusivity, and kg the thermal conductivity. 

 However, although the laborious measurement of p and c from soil samples 

 and the difficulty of measuring the temperature gradient near the surface 

 are ehminated, certain precautions must be observed. The first of these is 

 that two or more flux-plates should be used in series to obtain a representa- 



