Coleg Gwyddoniaeth a Pheirianneg

  1. Pennod › Ymchwil › Heb ei adolygu gan gymheiriaid
  2. Cyhoeddwyd

    Wood as a biobased building material: Wood plastic composites.

    Ormondroyd, G. & Kers, J., 2017, Performance of Bio-based building materials . 1 gol. Cambridge: Woodhead Publishing Ltd, t. 61-67 7 t.

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennod

  3. Cyhoeddwyd

    Wood polymer composites

    Spear, M. J., Eder, A. & Carus, M., 1 Awst 2015, Wood Composites. Ansell, M. P. (gol.). 2015 gol. Woodhead Publishing Ltd, t. 195-249

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennod

  4. Cyhoeddwyd

    Workshop on the Celtic Seas Ecoregion Aquaculture Overview

    O'Biern, F. (gol.), Ojaveer, H. (gol.), Boyd, A., Capuzzo, E., Devine, G., Moore, H., O'Carroll, J., Dennis, J., King, J., Falconer, L., Ellis, T. & Telfor, T., 5 Hyd 2022, ICES Scientific Reports. 57 gol. Copenhagen, Cyfrol 4. 126 t. (ICES Scientific Reports; Cyfrol 4, Rhif 57).

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennod

  5. Cyhoeddwyd

    Zimbabwe

    Kamete, A., Kamete, A. Y., Levinson, D. (gol.) & Ross, M. (gol.), 1 Ion 2007, Homelessness handbook. 2007 gol. Berkshire Publishing Group, t. 414-418

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennod

  6. Cyhoeddwyd

    Zimbabwe.

    Kamete, A., Kamete, A. Y., Mehler, A. (gol.), Melber, H. (gol.) & Van Walraven, K. (gol.), 1 Ion 2010, Africa Yearbook 6: Politics: economy and society south of the Sahara in 2009. 2010 gol. Brill, t. 537-547

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennod

  7. Pennod › Ymchwil › Adolygwyd gan gymheiriaid
  8. Cyhoeddwyd

    A bioassay study of soils in the Blue Mountains of Jamaica

    Healey, J., 1989, Mineral Nutrients in Tropical Forest and Savanna Ecosystems : Special publication no. 9 of the British Ecological Society. Proctor, J. (gol.). Oxford: Blackwell Scientific, t. 273-288

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  9. Cyhoeddwyd

    A conceptual methodology for estimating stored sequestered carbon in the built environment

    Spear, M., Hill, C. & Price, C., Ion 2021, Smart Innovation, Systems and Technologies, Vol 23, Sustainability and energy in buildings 2020. Littlewood, J. (gol.). Cyfrol 23. t. 381-393 12 t.

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  10. Cyhoeddwyd

    ASSESSMENT AND MONITORING OF AESTHETIC APPEARANCE OF BUILDING BIOMATERIALS DURING THE SERVICE LIFE

    Sandak, A., Sandak, J., Dimitriou, A., Burud, I., Thiis, T., Gobakken, L. R., Ormondroyd, G. A. & Kraniotis, D., 2018, SUSTAINABLE DEVELOPMENT AND PLANNING IX. Cyfrol 226. t. 527-536 (WIT Transactions on Ecology and the Environment).

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  11. Cyhoeddwyd

    Acidophile microbiology in space and time

    Johnson, D. B. & Quatrini, R., 2016, Acidophiles: Life in Extremely Acidic Environments. Johnson, D. B. & Quatrini, R. (gol.). Caister Academic Press Limited, t. 3-16

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  12. Cyhoeddwyd

    Aerobic Hydrocarbon-Degrading Gammaproteobacteria: Oleiphilaceae and Relatives

    Golyshin, P., Jaeger, K-E., Toshchakov, S. V., Golyshina, O., Chernikova, T., Yakimov, M. M. & Ferrer, M., 27 Tach 2019, Handbook of Hydrocarbon and Lipid Microbiology Series. Taxonomy, Genomics and Ecophysiology of Hydrocarbon-Degrading Microbes. McGenity, T. J. (gol.). Springer, t. 153-166

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  13. Cyhoeddwyd

    Agri-Food Wastes and Biomass Valorization: Establishing Experimental Conditions to Produce Lignin-Degrading Enzymes on Wheat Bran by Trametes versicolor CM13 Using Solid State Fermentation

    Charlton, A. & Baker, P., Rhag 2023, Agri-Food Wastes and Biomass Valorization: Establishing Experimental Conditions to Produce Lignin-Degrading Enzymes on Wheat Bran by Trametes versicolor CM13 Using Solid State Fermentation. MDPI AG, t. 53-66 13 t.

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  14. Cyhoeddwyd

    Amazon Unbound: Utopia and Emancipation under Conditions of Planetary Urbanization

    Wilson, J., Ion 2018, Public Space Unbound: Urban emancipation and the post-political condition. Routledge

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  15. Cyhoeddwyd

    Amazon Unbound: Utopian Dialectics of Planetary Urbanization

    Wilson, J., Maw 2018, Public Space Unbound. Routledge

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  16. Cyhoeddwyd

    Anamorphosis of Capital: Black Holes, Gothic Monsters, and the Will of God

    Wilson, J., 1 Medi 2018, Psychoanalysis and the GlObal. Kapoor, I. (gol.). University of Nebraska Press

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  17. Cyhoeddwyd

    Bacterial biota of forest trees

    Pettifor, B. & McDonald, J., Awst 2021, Forest Microbiology: Tree Microbiome: Phyllosphere, Endosphere and Rhizosphere. Asiegbu, F. O. & Kovalchuk, A. (gol.). 1 gol. Academic Press, Elsevier, Cyfrol 1. t. 161 - 173 12 t.

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  18. Cyhoeddwyd

    Batch Foaming of Amorphous Poly (DL-Lactic Acid) and Poly (Lactic Acid-co-Glycolic Acid) with Supercritical Carbon Dioxide: CO2 Solubility, Intermolecular Interaction, Rheology and Morphology

    Tai, H., 7 Maw 2012, Rheology. De Vicente, J. (gol.). INTECH, t. 133-148

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  19. Cyhoeddwyd

    Best Practices Toward Sustainable Ecotourism

    Shannon, G., 27 Mai 2017, Ecotourism’s Promise and Peril. Germany: Springer International Publishing, t. 153-178 26 t.

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  20. Cyhoeddwyd

    Biological removal of sulfurous compounds and metals from inorganic wastewaters

    Johnson, B. & Santos, A. L., Hyd 2020, Environmental Technologies to Treat Sulfur Pollution: Principles and Engineering. Lens, P. N. L. (gol.). 2nd gol. IWA Publishing, t. 215-246

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  21. Cyhoeddwyd

    British red squirrel strongholds: challenges for conservation

    Gurnell, J., Blackett, T., Butler, H., Lurz, P. W. W., Magris, L. & Shuttleworth, C., 2015, Red squirrels: Ecology, Conservation & Management in Europe. Shuttleworth, C. M., Lurz, P. W. W. & Hayward, M. W. (gol.). European Squirrel Initiative, t. 211-232

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  22. Cyhoeddwyd

    Can In-Situ Soil Nitrate Measurements Improve Nitrogen-Use Efficiency in Agricultural Systems?

    Jones, D. L., Chadwick, D. R., Rengaraj, S., Wu, D., Williams, A. P., Hill, P. W., Miller, A. J., Lark, R. M., Rosolem, C. A., Damin, V. & Shaw, R., 1 Rhag 2018, Proceedings 1466-1314. International Fertiliser Society, Cyfrol 825. t. 1-32 32 t. 825

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  23. Cyhoeddwyd

    Changing Hydrology: A UK Perspective

    Robins, P. & Lewis, M., 2019, Coasts and estuaries: The future. Wolanski, E., Day, J., Elliott, M. & Ramachandran, R. (gol.). Burlington: Elsevier, t. 611-617 6 t.

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  24. Cyhoeddwyd

    Climate change impacts on non-human primates: what have we modelled and what do we do now?

    Winder, I. C., Mace, B. D. & Korstjens, A. H., 2 Ion 2023, Primates in Anthropogenic Landscapes: Exploring Primate Behavioural Flexibility Across Human Contexts. McKinney, T., Waters, S. & Rodrigues, M. A. (gol.). Springer Nature

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  25. Cyhoeddwyd

    Composition, structure and regeneration of miombo forest at Kitulangalo, Tanzania

    Obiri, J., Hall, J. & Healey, J., 4 Awst 2010, Degraded forests in Eastern Africa: Management and Restoration. Bongers, F. & Tennigkeit, T. (gol.). London: Earthscan, t. 109-122 (The Earthscan Forest Library).

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  26. Cyhoeddwyd

    Compounds Containing the Boron–Chalcogen B–E (E[double bond, length half m-dash]S, Se, Te) Bond

    Beckett, M., 2007, Handbook of Chalcogen Chemistry: New Perspectives in Sulfur, Selenium and Tellurium . Royal Society of Chemistry, t. 1-30

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  27. Cyhoeddwyd

    Confusion reigns? A review of marine megafauna interactions with tidal stream environments

    Benjamins, S., Dale, A. C., Hastie, G., Waggitt, J. J., Lea, M-A., Scott, B. & Wilson, B., 2015, Oceanography and Marine Biology: An Annual Review. Cyfrol 53. t. 1-54

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  28. Cyhoeddwyd

    Contour hedgerows of Calliandra calothyrsus Meissn. for soil and water conservation in the Blue Mountains of Jamaica

    McDonald, M., Stevens, P. A. & Healey, J., 2001, Sustaining the Global Farm – Selected Papers from the 10th International Soil Conservation Organization Meeting. Stott, D. E., Mohtar, R. H. & Steinhardt, G. C. (gol.). West Lafayette, Indiana, USA: International Soil Conservation Organization , t. 395-402

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  29. Cyhoeddwyd

    Coordinated Multiple Views for Exploratory GeoVisualization

    Roberts, J. C., 2008, Geographic Visualization. Dodge, M., McDerby, M. & Turner, M. (gol.). John Wiley & Sons, Ltd, t. 25-48 24 t.

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  30. Cyhoeddwyd

    Current approaches and future trends in compost quality criteria for agronomic, environmental and human health benefits

    Bernal, M. P., Sommer, S., Chadwick, D., Qing, C., Li, G. & Michel Jr, F., 2017, Advances in Agronomy. Cyfrol 144. t. 143-233

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  31. Cyhoeddwyd

    Degradation, species invasion and management in Tanzanian dry forests: what do local people say?

    Obiri, J., Hall, J. & Healey, J., 4 Awst 2010, Degraded forests in Eastern Africa: Management and Restoration. Bongers, F. & Tennigkeit, T. (gol.). London: Earthscan, t. 305-321 (The Earthscan Forest Library).

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  32. Cyhoeddwyd

    Design considerations for eDNA metabarcoding surveys

    Perry, W. B., Pillay, K., George, P., Brennan, G., Lowe, A., Jones, L., Holman, L., Gibson, T., de Vere, N. & Creer, S., 19 Ion 2024, Applied Environmental Genomics. Berry, O. F., Holleley, C. E. & Jarman, S. N. (gol.). CRC Press

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  33. Cyhoeddwyd

    Developing integrated and applied red squirrel conservation programmes: What lessons can Europe learn from a regional grey squirrel eradication programme in North Wales?

    Shuttleworth, C., Schuchert, P., Everest, D. J., McInnes, C. J., Rushton, S. P., Jackson, N. & Kenward, R. E., 2015, Red squirrels: Ecology, Conservation & Management in Europe. Shuttleworth, C. M., Lurz, P. W. W. & Hayward, M. W. (gol.). European Squirrel Initiative, t. 233-250

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  34. Cyhoeddwyd

    Developing integrated red squirrel conservation in Europe

    Shuttleworth, C., Lurz, P. & Hayward, M., Mai 2016, Red Squirrels: Ecology, Conservation and Management in Europe. Shuttleworth, C. M., Lurz, P. & Hayward, M. W. (gol.). European Squirrel Initiative, t. 319-330

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  35. Cyhoeddwyd

    Diversity and Physiologies of acidophilic Archaea

    Golyshina, O., 1 Ebr 2016, Acidophiles: Life in Extremely Acidic Environments. Quatrini, R. & Johnson, D. B. (gol.). Caister Academic Press Limited, t. 93-07

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  36. Cyhoeddwyd

    Ecological Consequences of Climate Extremes for Lakes

    Jennings, E., de Eyto, E., Jones, I. D., Ibelings, B., Adrian, R. & Woolway, I., 1 Meh 2021, Encyclopedia of Inland Waters: Reference Module in Earth Systems and Environmental Sciences. Elsevier

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  37. Cyhoeddwyd

    Ecosystem impacts of an alien invader in Europe, the grey squirrel Sciurus carolinensis

    Gurnell, J., Lurz, P. W. W. & Shuttleworth, C., 2016, The grey squirrel: ecology and management in Europe. Shuttleworth, C. M., Lurz, P. W. W. & Gurnell, J. (gol.). European Squirrel Initiative, t. 307-329

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  38. Cyhoeddwyd

    Ecosystem service flows across the rural-urban spectrum

    Lewis, S., Devenish, K., Dolan, R., Garraty, T. A. ON., Okosun, O. S., Scowen, M., Welivita, I. & Willcock, S., 1 Gorff 2022, Exploring the Multiple Values of Nature - Connecting Ecosystems and People across Landscapes. Springer Nature, 8 t.

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  39. Cyhoeddwyd

    Effective conservation depends upon understanding human behaviour: Chapter 19

    St John, F. A. V., Keane, A. M. & Milner-Gulland, E., 5 Ebr 2013, Key Topics in Conservation Biology 2. Macdonald, D. W. & Willis, K. J. (gol.). Wiley-Blackwell, t. 344-361

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  40. Cyhoeddwyd

    Electromagnetic Fields in Meta-Media with Interfacial Surface Admittance

    Christie, D. & Tucker, R., 2018, Generalized Models and Non-classical Approaches in Complex Materials. Cyfrol 1. t. 155-178 (Advanced Structural Materials).

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  41. Cyhoeddwyd

    Encounters in Geography Field-Based Learning: Wales

    Lane, E., Jones, R. & Andrews, W., 2021, Experiential Learning in Geography: Experience, Evaluation and Encounters. Wessell, J. (gol.). 1 gol. Springer, t. 167-193

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  42. Cyhoeddwyd

    Enjoying inequality

    Wilson, J., 26 Medi 2023, Clickbait capitalism: Economies of Desire in the Twenty-First Century. Samman, A. & Gammon, E. (gol.). Manchester University Press

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  43. Cyhoeddwyd

    Environmental Impacts of Tidal and Wave Energy Converters

    Hutchison, Z. L., Lieber, L., Miller, R. G. & Williamson, B. J., 2022, Comprehensive Renewable Energy. Elsevier, t. 258-290

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  44. Cyhoeddwyd

    Environmental Issues for Offshore Renewable Energy

    Wolf, J., Dominicis, M. D., Lewis, M., Neill, S., Murray, R. OH., Scott, B., Zampollo, A., Chapman, J. & Declerck, M., 2021, Reference Module in Earth Systems and Environmental Sciences. Elsevier

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  45. Cyhoeddwyd

    Environmental issues for offshore renewable energy

    Wolf, J., De Dominicis, M., Lewis, M., Neill, S., O'Hara Murray, R., Scott, B. E., Zampollo, A., Chapman, J. & Declerck, M., 1 Chwef 2022, Comprehensive Renewable Energy. 2nd gol. Elsevier

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  46. Cyhoeddwyd

    Enzymes from Extreme Environments

    Cowan, D. A., Kirby, B. B., Ferrer, M., Guazzaroni, M-E., Golyshina, O. & Golyshin, P., 25 Maw 2010, Manual of Industrial Microbiology and Biotechnology, 3rd Ed. ASM Press, t. 43-61

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  47. Cyhoeddwyd

    Experiences of grey squirrel management in an upland conifer forest

    Halliwell, E. C., Shuttleworth, C., Cartmel, S., Lloyd, I. & Jenkins, R., 2016, The grey squirrel: ecology and management in Europe. Shuttleworth, C. M., Lurz, P. W. W. & Gurnell, J. (gol.). European Squirrel Initiative, t. 453-474

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  48. Cyhoeddwyd

    Fluxes of Carbon Compounds

    Farrar, S., 1985, British Plant Growth Regulator Group. Cyfrol 12.

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  49. Cyhoeddwyd

    For a world without boundaries: connectivity between marine tropical ecosystems in times of change

    Earp, H., Prinz, N., Cziesielski, M. & Andskog, M., 2018, Oceans across boundaries: Learning from each other - The 2017 conference for YOUng MARine RESearchers in Kiel. Jungblut, S., Liebich, V. & Bode, M. (gol.). Berlin: Springer Science and Business Media Deutschland GmbH, t. 122-144

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  50. Cyhoeddwyd

    Genetic evidence of the effectiveness of grey squirrel control operations: lessons from the Isle of Anglesey

    Signorile, L. & Shuttleworth, C., 2016, The grey squirrel: ecology and management in Europe. Shuttleworth, C. M., Lurz, P. W. W. & Gurnell, J. (gol.). European Squirrel Initiative, t. 439-452

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  51. Cyhoeddwyd

    Geografski aspekti vrednovanja prirodnih resursa Općine Polača

    Barada, M. & Perić, Z., 2020, Polački kraj u prošlosti i sadašnjosti. 1 gol. Zadar, Croatia: Matica Hrvatska ZADAR, t. 591-620

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid

  52. Cyhoeddwyd

    Governing like a forest: achieving diachronic integrity or emergency carbon sequestration through post-Brexit forest policy?

    Wynne-Jones, S., Dandy, N., Bodner, T. & Healey, J., 3 Tach 2022, Rural Governance in the UK: Towards a Sustainable and Equitable Society. Attrop, A., McAreavey, R. & Heron, S. (gol.). Routledge, (Routledge Studies in Sustainability).

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennodadolygiad gan gymheiriaid