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When we consider in detail the behaviour of a fluid consisting of two (or possibly more) interpenetrating components, the likelihood of dynamical instabilities induced by coupling between the two fluids cannot be ignored. The phenomenon is generic to all such multi fluid systems and as such is appellated the two-stream instability. Mathematically this class of instability is somewhat akin to the more well known Kelvin Helmholtz instability, but is distinguished by the fluids flowing through each other rather than having a clearly defined interface between them. In this thesis we describe in some detail the mechanisms underlying this instability in a simple linear ow scrutinising in particular the growing (unstable) solutions for small harmonic perturbations. We further consider the application of this genre of instabilities to other physical systems, most conspicuously to that of a rotating super fluid body with rotational lag between the components. This case is of particular interest in neutron star physics, where it offers possibilities for exploring behaviour within the core. There also seems to be the chance of exploring this example in laboratory systems. We also take a tentative first step to extending the application and understanding of the two-stream instability by flirting with the analogous observations in a laboratory realisable binary Bose-Einstein Condensate. This laboratory realisation is a first step towards being able to explore physically issues relating to neutron star dynamics. We further discuss general analogue systems for modelling key features of neutron stars in terrestrial laboratories. The possible applications, along with some of the diffculties in using these analogues, are explored.

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The main focus of this work is to make use of a novel tool in the cosmologist's toolbox when it comes to constraining the parameters of the peculiar velocity fields of the nearby Universe called ROBUST, whose unique properties and lack of reliance on secondary distance indicators sets it apart from other available constraining techniques, rendering it potentially very useful for future upcoming surveys such as the LSST and the SKA. While ROBUST proves itself more than adequate in constraining parameters in a mock controlled environment with the IRAS PSCz survey, it begins to struggle when applied to the real-world 2MRS survey, primarily due to an inherent fault in the survey that causes it to not function properly with the program. These problems persist even when we begin to make use of one of the ancillary tools developed in conjunction with ROBUST, namely relative entropy, despite it once again continuing to function adequately across multiple mock realisations. It is the conclusion of this work that while ROBUST is not successful in recovering values for the cosmological parameters we seek to constrain, this does not necessarily negate its viability for use with upcoming surveys, as it has proven itself successful in determining exclusion intervals on the value of the linear redshift distortion parameter β for real world surveys that are in very good agreement with the generally small values computed by contemporary velocity-velocity constraining techniques such as VELMOD and χ2 minimisation, while also confidently ruling out the results of older density-density constraining techniques such as POTENT that favour values closer to unity.

Both an experimental investigation of the interaction effects within commercial particulate recording media and a numerical investigation of the reversal mechanism of particles within the media have been made. The particle systems investigated comprised three audio y-Fe203 tapes, three audio Cr02 tapes and a video metal particle tape. An absolute measure of the interaction effects within particulate tapes has been investigated by comparing the measured properties of isolated particles taken from the medium with the measured bulk properties of the medium. The results indicated negative interactions for all the samples investigated except the video metal particle tape, which indicated positive interactions. However, Al plots for all the samples however, indicated negative interactions. This is contrary to the absolute interactions measured in the video metal particle tape. A possible explanation for this inconsistency was the presence of highly localised alignment of particles, "a chaining effect", within the metal particle system. This effect allows for increased system coercivity without removing the general negative interactions characteristic of all acicular particulate media. A micromagnetic model was developed to study typical y-Fe203 and Cr02 particles measured experimentally in this study. Simulations were performed as a function of the applied field angle and the results compared to the experimental study. The simulations representing typical 'y-Fe203 particles indicated reasonable agreement at the lower applied field angles, while poorer agreement was observed at larger applied field angles. The simulations representing a typical Cr02 particle indicated reasonable agreement at the higher applied field angles, while poorer agreement was observed at low applied field angles. These inconsistencies for both types of particles investigated were accounted for by assumptions and simplifications within the model, particularly the absence of bulk crystalline imperfections, the degree of surface irregularities and the effect of an oversimplified particle shape. The micromagnetic model developed was also used to investigate the effect of model parameters on the reversal mechanism of the 'y-Fe203 particle simulation. It was found that the reversal mechanism was very sensitive to the size and shape of the model particle.

New UV and EUV observations of off-limb and upper solar corona made by the UltraViolet Coronagraph Spectrometer (UVCS) and the Coronal Diagnostic Spectrometer (CDS) on SOHO (Solar and Heliospheric Observatory) are presented in this thesis. These data were used to establish the physical properties, such as temperature, density and element abundances, of different structures of the off-limb and upper corona. For this project, the UVCS team provided UV spectra of the extended solar corona, which included lines of primary interest for the characterization of this region and the solar wind. CDS data consisted of NIS (Normal Incidence Spectrometer) EUV spectra of the low corona. The data included several density-sensitive and temperature-sensitive lines which enabled diagnostic studies to be made. Absolute element abundances were derived from UVCS data; relative element abundances were derived from CDS data. The Differential Emission Measure (DEM) technique was used (for the first time on UVCS data) as the primary method for abundance study. Electron temperatures along the line of sight were derived applying the line ratio and the DEM techniques to both CDS and UVCS data. From the CDS data, electron densities along the line of sight were derived using the line ratio technique. For the UVCS data, the density was inferred using the 0 VI radiative and collisional components of the line. One equatorial and two mid-latitude streamers were observed with the two instruments, in the low corona and at about 1.6 R®. Electron density and temperature profiles with the solar distance were derived. They appeared to satisfy the hydrostatic equilibrium condition. The equatorial streamer appeared to be cooler than the mid-latitude one. Moreover, while the former tended to isothermal values already in the low corona, the latter was multithermal at the base and isothermal at 1.6 R®. The derived composition at 1.6 R®. showed a depletion with respect to the photospheric values for almost all the elements. The Fe/O ratio found here was consistent with values found in the solar wind. Moreover, a depletion in element composition with increasing solar distance was also found. While observing off-limb in the south polar coronal hole, CDS observations of a macrospicule were obtained. For the first time the background emission was isolated from that of the macrospicule itself, and a diagnostic study of both regions was made. The ambient coronal hole appeared to have a density of = 2 x 108 cm -3 that was about half the density found in the streamer base studied in this thesis. Multithermal components were found at the coronal hole base. The macrospicule spectrum indicated chromospheric and transition region temperatures, and fluctuations in density along the feature were registered. In particular, the data showed the presence of an outward-moving cloud of cool material. Its time evolution and velocity were studied. The maximum (initial) outflow velocity was about 80km s-1 , consistent with the outflow velocity measured in interplume regions. An off-limb hot loop system was observed with CDS at mid-latitude. Its top and base were selected for diagnostic studies. Some newly observed hot lines were identified. The loop system appeared to be composed of multi-thermal plasma. The highest temperature registered was log T=6.4 at the top of the loop. A depletion of element abundances which mainly involves the plasma at the top of the loop was found. A coronal hole bounded by a loop system was observed with CDS. Electron density appeared up to a factor two higher in the boundary than in the coronal hole, and the loop system appeared multi-thermal. The ambient coronal hole temperature appeared to be enhanced by the proximity of this loop arcade. Abundances were derived in both regions, and the coronal hole boundary showed slight depletions relative to photospheric values. New coordinated observations with CDS and UVCS were made during one of the SOlO-Ulysses quadratures. Preliminary results regarding temperatures and densities of the core and boundary regions of streamers were obtained from UVCS data.

The torsion balance is one of the key pieces of apparatus used in experimental searches for weak forces. In the search for an understanding of a Unified Theory, physicists have suggested a number of signatures that are detectable in laboratory measurements. This thesis describes the development of a new torsion balance facility, relocated from the BIPM (Bureau International des Poids et Mesures) [1], which has excellent environmental stability and benefits from a new compact interferometric readout for measuring angular motion which has been characterised and installed onto the torsion balance. The interferometer has sensitivities of 5 \( \times\) 10\( {-11}\) radians\(\char{cmti10}{0x2f}\)\(\sqrt{Hz}\) between 10\( {-1}\) Hz and 10 Hz, an angular range of over \(\pm\)1\( \circ\) and significantly reduces sensitivity to ground tilt. With the new facility the first experiment searching for temporal variations in the Newtonian gravitational constant has been undertaken with a null result for \( \delta\)\(\char{cmti10}{0x47}\)\(\char{cmti10}{0x2f}\)\(\char{cmti10}{0x47}\)\(_0\) for both sidereal and half sidereal signals at magnitudes greater than 5\( \times\)10\( {-6}\). These results have been used to set an upper limit on some of the parameters within the Standard Model Extension framework [2]. The thesis also reports on the design and manufacture of prototype test masses with a high electron-spin density of approximately 10\( {24}\) and negligible external magnetic field \( \leq\) 10\( {-4}\)\(\char{cmr10}{0x54}\). These test masses can be used within the facility to potentially make it sensitive enough to conduct future spin-coupling experiments.