Wavelengths of bioconvection patterns

Bioconvection occurs as the result of the collective behaviour of many micro-organisms swimming in a fluid and is realised as patterns similar to those of thermal convection which occur when a layer of water is heated from below. A methodology is developed to record the bioconvection patterns that are formed by aqueous cultures of the single-celled alga Chlamydomonas nivalis. The analysis that is used to quantify the patterns as a function of cell concentration, suspension depth and time is described and experimental results are presented

Linear bioconvection in a suspension of randomly swimming, gyrotactic micro-organisms

We have analyzed the initiation of pattern formation in a layer of finite depth for Pedley and Kessler’s new model [J. Fluid Mech. 212, 155 (1990)] of bioconvection. This is the first analysis of bioconvection in a realistic geometry using a model that deals with random swimming in a rational manner. We have considered the effects of a distribution of swimming speeds, which has not previously received attention in theoretical papers and find that it is important in calculating the diffusivity. Our predictions of initial pattern wavelengths are reasonably close to the observed ones but better experimental measurements of key parameters are needed for a proper comparison

Millers and millwrights in antiquity and the early middle ages

The study of early watermills has, from its very beginning, concentrated on two issues: their diffusion (in time and space) and their technical construction. Very little interest has been devoted to the persons who built and managed them—the millwrights and the millers. This tendency has been even more manifest from the 1980s onward, when interest has focused more and more on the increasing number of archaeological finds. The written evidence, our almost sole source for people connected with the mills, plays a quite insignificant part in modern scholarship. This short article does not aim at far-reaching conclusions concerning the socioeconomic conditions of the two professions involved. Its main purpose is to show the extent of the evidence actually at hand. The varied nature of this evidence, as well as the uncertain authenticity of parts of it, complicates the study. After a short presentation of the terminology, I start my investigation by presenting the millers according to the three basic socioeconomic areas within which they were working, and end up with a discussion of the millwrights, in order to show how the two occupations were at least partly related