Water-clocks and time measurement in classical antiquity

The water-clock was the first mechanical device for time measurement to be used by the Greeks. Previously they had relied on sundials, or observation of the phases of the moon or the position of the sun in the zodiac to locate points of time within the day, month or year; and by the mid-fifth century BC they had made some progress with the difficult calendar problems due to the fact that the solar day, the lunar month, and the solar year are not commensurable with each other in whole numbers.     

Structural aspects of controllability and observability—I. tensorial aggregation

The classical criteria for controllability and observability are given a tensorial formulation which is then extended into Boolean form directly related to a digraph interpretation of system structure. This is examined in detail with particular reference to group theoretic aspects, invariant properties under transformation, the term rank test and the existence of zero eigenvalues.Within this framework the structural design concepts of potential controllability and observability are presented in generalised terms.     

On Linear Least-Squares Estimators for Continous-Time Stochastic Systems

The problem of least-squares state estimation of stochastic continuous-time linear systems is reconsidered. A concise derivation of the least-squares minimal-order estimator is presented using an innovations approach. An important result is the reinstatement of the problem in a least-squares estimation framework independent of deterministic observer theory. A second result is thus the clarification of previous approaches to the problem, particularly in relation to observer theory.     

The effects of extraneous speech on visual vigilance performance of children

First-grade males performed a 30-min visual vigilance task under 5 conditions of auditory background stimulation. The 5 conditions consisted of (1) continuous conversation, in which subjects listened to a tape of conversation spliced so that there were no intervals of silence lasting longer than 3 sec; (2) continuous reverse, in which subjects heard the continuous conversation tape played in reverse; (3) intermittent conversation, in which subjects heard alternating segments of conversation and silence; (4) intermittent reverse, in which subjects heard alternating segments of reverse conversation and silence; and (5) silence control. Compared with silence or continuous stimulation, intermittent stimulation produced better detection regardless of whether or not it was meaningful. High achievers made more correct detections than low achievers, but only in the second and third time periods. Few subjects made errors of commission.