Ten subjects performed a numeral reading task under five levels of sinusoidal whole-body vibralion at various frequencies in each of the three translational axes. In the z-axis the vibration frequencies were spaced at half octaves between 2-8 and 63 Hz, and in the x and y-axes at half octaves between 2-8 and 32 Hz. All of the vibration conditions were presented with two sealing conditions: hard flat seat with fixed footrcst and a simulated helicopter seat with moving footrest. With both seats the effect of vibration level on reading accuracy was found to be significant at all but the highest frequencies of z-axis vibration. It was also significant for x-axis vibration with the simulated helicopter seat, but not for x-axis vibralion with the flat seat or for y-axis vibration with either seat. Results are presented in the form of equal performance contours.Measures of translational and rotational head motion were also made for each vibration axis and seat. Very little vibration was found to be transmitted to the head during x-axis vibration with the flat seat or y-axis vibration with either seat, in agreement with the small effect of vibration on reading performance in these cases. These results were combined with performance data to investigate the feasibilily of predicting performance decrements directly from head motions.
An enduring interest in categories (katēgoriai),1 and in Aristotle's Categories in particular, has led readers since antiquity to study the treatise which Porphyry entitled On the Genera of Being (6.1–3).2 Ancient and modern readers broadly agree that: (1) Plotinus understands his own subject matter to be 'the kinds of things that exist' (peri tōn genōn tou ontos); (2) the treatise displays the result of a deep and substantial engagement with Aristotle's Categories; and (3) Plotinus raises important and substantive puzzles (aporiai) about what is said in the Categories.3 Beyond this, plausible interpretations diverge. On one view, Plotinus deploys the resources of earlier Platonist critics to challenge the Categories' ontological prioritization of particular substance, especially as it is treated by earlier Aristotelian commentators.4 On an alternative reading, Plotinus 'purifies' Aristotelian ontology in order to sketch a new taxonomy of the sensible world, complementing his own account of the intelligible world and clearing a trail for Porphyry's integration of Aristotle into a new Platonist curriculum.5
A method developed at the Institute of Sound and Vibration Research, Southampton University, UK, for estimating the transverse metacentric height (GM) of a vessel is described. The method is based upon the identification of natural rolling frequency by spectral analysis. Computer software for waveform analysis, together with a large databank of ship motion recordings, enabled the method to be tested and refined in the laboratory. The method was then incorporated into a prototype portable microprocessor-based stability indicator that was subsequently tested at sea. Commercial manufacture is now planned. There are possibilities for incorporating other useful information into the microcomputer-based system, such as an average list indication and alternative displays and printouts of the continuously-recorded GM estimates. Alarm settings, error detection and trend analysis are also possibilities.
Difference thresholds for seated subjects exposed to whole-body vertical sinusoidal vibration have been determined at two vibration magnitudes [0.1 and 0.5 ms(-2) root mean square (r.m.s.)] and at two frequencies (5 and 20 Hz). For 12 subjects, difference thresholds were determined using the up-and-down transformed response method based on two-interval forced-choice tracking. At both frequencies, the difference thresholds increased by a factor of five when the magnitude of the vibration increased from 0.1 to 0.5 ms(-2) r.m.s. The median relative difference thresholds, Weber fractions (deltaI/I), expressed as percentages, were about 10% and did not differ significantly between the two vibration magnitudes or the two frequencies. It is concluded that for the conditions investigated the difference thresholds for whole-body vibration are approximately consistent with Weber's Law. A vibration magnitude will need to be reduced by more than about 10% for the change to be detectable by human subjects; vibration measurements will be required to detect reductions of less than 10%.
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