A quantitative approach to nitrogen fixation and the decay of timber in soil contact

Imperial Users onl

Hydraulic flow through a channel contraction: multiple steady states

We have investigated shallow water flows through a channel with a contraction by experimental and theoretical means. The horizontal channel consists of a sluice gate and an upstream channel of constant width $b_0$ ending in a linear contraction of minimum width $b_c$. Experimentally, we observe upstream steady and moving bores/shocks, and oblique waves in the contraction, as single and multiple steady states, as well as a steady reservoir with a complex hydraulic jump in the contraction occurring in a small section of the $b_c/b_0$ and Froude number parameter plane. One-dimensional hydraulic theory provides a comprehensive leading-order approximation, in which a turbulent frictional parametrization is used to achieve quantitative agreement. An analytical and numerical analysis is given for two-dimensional supercritical shallow water flows. It shows that the one-dimensional hydraulic analysis for inviscid flows away from hydraulic jumps holds surprisingly well, even though the two-dimensional oblique hydraulic jump patterns can show large variations across the contraction channel

Coming to terms with the "Border War" in post-apartheid South Africa

If you are a white, male South African between the ages of about 35 and 60 it is very likely that you donned the nutria brown uniform of the South African Defence Force (SADF). Between 1967 and 1994 approximately 300 000 young white males were conscripted by the SADF. As far as most of these conscripts were concerned, there was no option other than heeding the call-up and performing national service or diensplig. Failure to do so meant harsh penalties. The alternatives were to object on conscientious (actually religious) grounds and face a six year jail sentence, or flee the country. And the obligation did not end with national service as conscripts were assigned to citizen force or commando units that were liable for periodical call-ups for camps that might have included deployment in the “operational areas” from 1974 or tours of duty in the black townships from 1984. Those - like myself - belonging to this national service generation were part-time soldiers for much of their adult lives. Most served willingly, some with patriotic fervour. Others did so reluctantly and with little enthusiasm

De La Rey rides (yet) again : Afrikaner identity politics and nostalgia in post-apartheid South Africa

In 2006 a relatively unknown South African artist with the stage name Bok van Blerk released his debut album called “De la Rey”. The album included a music video of the title track that calls upon the legendary Boer War general to save the volk (people) from the wantonly destructive strategies of the British imperial forces: the scorched earth policy and the subsequent internment of women and children in concentration camps. The British justified such extreme – some would say ‘genocidal’ – strategies so as to prevent non-combatants from supporting the irregular Boer soldiers. Although he did not believe that the war could be won on account of the overwhelming odds that the Boer forces faced, De la Rey still fought to the bitter end. Needless to say, he was on the losing side

The New Brighton Advisory Board, c. 1923-1952: its legitimacy and legacy.

Paper presented at the Wits History Workshop: Structure and Experience in the Making of Apartheid, 6-10 February, 1990The historical significance of advisory boards has been downplayed because of their contradictory role in urban African politics. Until the 1940s, the system of Advisory Boards was dominated by the 'most reactionary elements' of the African petty bourgeoisie. This paper contends that, despite the purely consultative functions of the Boards, participation in Advisory Board politics was an important channel of mobilisation in urban African communities until at least the Second World War. Thereafter their legitimacy of was questioned. This paper also studies the New Brighton Advisory Board with particular reference to the question of the Board's legitimacy and its relationship with the local authority in the period between 1923 and 1952. It also evaluates the Board as a locus of activity concerned with wider socio-political issues

The battle for Cassinga : conflicting narratives and contested meanings

Nearly thirty years ago the name Cassinga (or Kassinga) came to the attention of the world. At the time the name evoked a range of responses, from outrage to grief to the celebration of military bravado. It still does so. And Cassinga will continue to elicit such responses as long as participants and witnesses are alive and the events remain part of living memory. Obviously perpetrators and survivors remember the events of 4 May 1978 differently. Memory is, after all, selective. The recollections of participants and witnesses are framed by personal and political agendas. This much is abundantly clear from the conflicting accounts of Cassinga that appear in the media and literature, especially the exchanges that take place between parties with a stake in how the events are remembered. Thus a report headlined “Battle of Cassinga still rages” published on the 29th anniversary suggested that the events are still shrouded in controversy and that there is no agreement about what transpired in the southern Angolan town. The title of this paper reflects my concern with the battle for rather than of Cassinga. The choice of preposition is intended to signify the ongoing contestation over the meaning of Cassinga

Macroscopic and Local Magnetic Moments in Si-doped CuGeO3_3 with Neutron and μ\muSR Studies

The temperature-concentration phase diagram of the Si-doped spin-Peierls compound CuGeO$_{3}$ is investigated by means of neutron scattering and muon spin rotation spectroscopy in order to determine the microscopic distribution of the magnetic and lattice dimerised regions as a function of doping. The analysis of the zero-field muon spectra has confirmed the spatial inhomogeneity of the staggered magnetisation that characterises the antiferromagnetic superlattice peaks observed with neutrons. In addition, the variation of the macroscopic order parameter with doping can be understood by considering the evolution of the local magnetic moment as well as of the various regions contributing to the muon signal

Ordering in weakly coupled random singlet spin chains

The influence of bond randomness on long range magnetic ordering in the weakly coupled S = 1/2 antiferromagnetic spin chain materials Cu(py)2(Cl1-xBrx)2 is studied by muon spin rotation and bulk measurements. Disorder is found to have a strong effect on the ordering temperature TN, and an even stronger one on the saturation magnetization m0, but considerably more so in the effectively lower-dimensional Br-rich materials. The observed behavior is attributed to Random Singlet ground states of individual spin chains, but remains in contradiction with chain mean field theory predictions. In this context, we discuss the possibility of a universal distribution of ordered moments in the weakly coupled Random Singlet chains model

Molecular evolution of sex-biased genes in the Drosophila ananassae subgroup

<p>Abstract</p> <p>Background</p> <p>Genes with sex-biased expression often show rapid molecular evolution between species. Previous population genetic and comparative genomic studies of <it>Drosophila melanogaster </it>and <it>D. simulans </it>revealed that male-biased genes have especially high rates of adaptive evolution. To test if this is also the case for other lineages within the <it>melanogaster </it>group, we investigated gene expression in <it>D. ananassae</it>, a species that occurs in structured populations in tropical and subtropical regions. We used custom-made microarrays and published microarray data to characterize the sex-biased expression of 129 <it>D. ananassae </it>genes whose <it>D. melanogaster </it>orthologs had been classified previously as male-biased, female-biased, or unbiased in their expression and had been studied extensively at the population-genetic level. For 43 of these genes we surveyed DNA sequence polymorphism in a natural population of <it>D. ananassae </it>and determined divergence to the sister species <it>D. atripex </it>and <it>D. phaeopleura</it>.</p> <p>Results</p> <p>Sex-biased expression is generally conserved between <it>D. melanogaster </it>and <it>D. ananassae</it>, with the majority of genes exhibiting the same bias in the two species. However, about one-third of the genes have either gained or lost sex-biased expression in one of the species and a small proportion of genes (~4%) have changed bias from one sex to the other. The male-biased genes of <it>D. ananassae </it>show evidence of positive selection acting at the protein level. However, the signal of adaptive protein evolution for male-biased genes is not as strong in <it>D. ananassae </it>as it is in <it>D. melanogaster </it>and is limited to genes with conserved male-biased expression in both species. Within <it>D. ananassae</it>, a significant signal of adaptive evolution is also detected for female-biased and unbiased genes.</p> <p>Conclusions</p> <p>Our findings extend previous observations of widespread adaptive protein evolution to an independent <it>Drosophila </it>lineage, the <it>D. ananassae </it>subgroup. However, the rate of adaptive evolution is not greater for male-biased genes than for female-biased or unbiased genes, which suggests that there are differences in sex-biased gene evolution between the two lineages.</p