Wind tunnel generation of horizontal roll vortices over a differentially heated surface
Wildland crown fires represent an extreme type of forest fire that advances from top to top of trees or shrubs. Some such fires generate energy comparable to several nuclear explosions. They occur most frequently in coniferous forests found in the United States and Canada, and in broad-leafed eucalyptus forests of Australia. Here we report visualization of wind tunnel experiments in which airflow along the perimeter of a simulated forest fire produced horizontal vortex pairs with axes in the downstream direction. These vortex pairs formed above a differentially heated surface simulating the thermal energy flux from a crown fire. The thermal energy was produced by an electrically heated surface ribbon at a temperature of ∼250°C and also, in a separate experiment, by an electrically heated embedded wire coil at a temperature of ∼825°C. Wind velocities ranged from 0.2 to 3.6 m s−1. Similar vortex action may cause the unburned tree-crown strips that are observed when crown fires, driven by a significant wind, burn over flat terrane.
Spatial and temporal variation of microstimulation thresholds for inhibiting the tail-flick reflex from the rat's rostral medial medulla
Suppression of the tail-flick reflex by microstimulation of the rostral medial medulla in rats lightly anesthetized with barbiturates was studied with regard to spatial and temporal variations in electrical threshold. Trains of constant-current pulses with linearly descending amplitudes (called 'ramps') were passed through the extracellular brain microelectrode during noxious heating of the tail. The pulse amplitude at the time of the reflex, after allowance for conduction and reaction latencies, was taken as the threshold reading. This new method revealed a range of vertical electrode positions corresponding roughly to the nucleus raphe magnus, where the thresholds tended to be lowest (a mean of 4.1 microA for 0.4-ms pulses delivered at 50 Hz). In confirmation of the technique's validity neither the duration of the ramp nor its starting amplitude, within their useful range, significantly affected the measured threshold. Pronounced temporal fluctuation was seen in thresholds measured every 2 min. Spatial variability within the low-threshold region and differences between preparations were statistically much smaller sources of variation. The temporal fluctuation appeared to have a stationary mean for at least 20 min under constant conditions of anesthesia. In some experiments, action potentials from single neurons were recorded through the stimulating electrode, and classified into those inhibited during the tail-flick (off-cells), those excited (on-cells), and those unaffected (neutral cells). The thresholds where off-cells exhibited their maximum action potential were on average significantly lower than corresponding thresholds for on-cells. Short-range (less than 0.2 mm) spatial variations in the threshold appeared however to be uncorrelated with the distance to an individual recorded off-cell or on-cell.(ABSTRACT TRUNCATED AT 250 WORDS)
Die Populationsdynamik Von Canthocampt Us Staphylinus (Jurine) (Copepoda, Harpacticoida) Im Litoral Des Bodensees
The population dynamics of Canthocamptus staphylinus in the littoral zone of Lake Constance are described (fig. 2). Adult animals aestivate as cysts. The main part of the population emerges from the cysts in autumn. During the resting stage and the first spell after emerging, mortality is very low (fig. 3). Reproduction, expressed as numbers of ovigerous females, begins in November and reaches its maximum in January. The new generation becomes adult in May/June and encysts without previous reproduction. According to Sarvala (1979) it may be possible, that in littoral populations dormancy is induced by long day periods and temperatures of over 12° C (figs. 1, 2). In the profundal zone C. staphylinus also forms cysts, however, both light and temperature are not likely to induce this stage in depths of over 60 m.