The influence of vegetation structure on saltation threshold was investigated using uniformly spaced arrays of non-erodible roughness elements on a bed of erodible sand in an wind tunnel. Structural variables tested using arrays of solid cylinders included element aspect ratio (height/diameter) and lateral cover (total frontal-silhouette area per unit ground area). In agreement with previous studies, increase in saltation threshold above the value for bare sand was strongly related to lateral cover. Increasing aspect ratio from 0.25 to 4 tended to enhance the increase in saltation threshold at a given lateral cover. An approximate fit to the results could be obtained Porous elements were constructed as clusters of narrow, vertically oriented cylinders (model 'stems'), forming porous model plant bodies that were cylindrical in overall shape. Low-porosity elements were found to be approximately 50 per cent more effective in increasing saltation threshold than either solid or high-porosity elements. The ratio of plantbody frontal-silhouette area (based on overall dimensions) to total stem frontal-silhouette area was found to be a useful measure of plant-body porosity for wind erosion studies. Some conventional measures of vegetation amount fail to account for changes in vegetation structural attributes that may strongly influence aeolian processes.

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The velocity (v) dependence of the total cross section (Q) has been measured for the scattering of helium in the metastable 2 3S1 state by ground-state helium, argon, and krypton at relative velocities in the range 1000–3300 m/sec. The results are compared with those previously obtained for the scattering of lithium. For each scattering gas, the shape of Q(v) is similar to that of lithium, but the absolute values for Q are approximately 20% higher. The scattering is largely elastic. It is estimated that the inelastic contribution to Q is about 10% for the He*–He case, and that it is less than 3% for the He*–Ar and He*–Kr cases. The Q(v) for scattering by argon and krypton show undulatory behavior. From the extrema velocities, the product εσ is shown to be close to that for Li scattering, where ε is the depth of the interatomic potential well and σ is the interatomic separation at the zero of the potential. The separate parameters ε and σ are also deduced. A minimum to the number of diatom bound states is established for HeAr(3Σ+) and HeKr(3Σ+).

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The total cross section for the scattering of electrons by atomic nitrogen has been measured as a function of electron energy from 1.6 to 10 eV. An electron gun was developed that produced a more intense beam of electrons than was used for similar experiments with atomic hydrogen and atomic oxygen. The number of electrons scattered from a region de6ned by the intersection of an electron beam and a modulated molecular nitrogen beam was compared with the number scattered when the nitrogen beam was partially dissociated. A pulsed dc discharge dissociated about 20~jo of the molecules. The degree of dissociation was measured with a mass spectrometer. From the data, the ratios of atomic to molecular scattering cross sections were obtained. The absolute atomic values were calculated by multiplying these ratios by the molecular nitrogen cross sections obtained by Normand. The results are compared with theoretical estimates of the cross section.