The spin structure of protons and neutrons has been an open question for nearly twenty-five years, after surprising experimental results disproved the simple model in which valence quarks were responsible for nearly 100% of the nucleon spin. Diverse theoretical approaches have been brought to bear on the problem, but a shortage of precise data -especially on neutron spin structure -has prevented a thorough understanding.Experiment E06-014, conducted in Hall A of Jefferson Laboratory in 2009, presented an opportunity to add to the world data set for the neutron in the poorly covered valence-quark region. Jefferson Laboratory's highly polarized electron beam, combined with Hall A's facilities for a high-density, highly polarized 3 He target, allowed a high-luminosity double-polarized experiment, while the large acceptance of the BigBite spectrometer gave coverage over a wide kinematic range: 0.15 < x < 0.95. In this work, we present the analysis of a portion of the E06-014 data, measured with an incident beam energy of 4.74 GeV and spanning 1.5 < Q 2 < 5.5 (GeV/c) 2 . From these data, we extract the longitudinal asymmetry in virtual photon-nucleon scattering, A 1 , on the 3 He nucleus. Combined with the remaining E06-014 data, this will form the basis of a measurement of the neutron asymmetry A n 1 that will extend the kinematic range of the data available to test models of spin-dependent parton distributions in the nucleon. AcknowledgmentsNo one in this age can earn a physics PhD based solely on her own work and study; the making of a doctor, like the making of a modern particle-physics experiment, requires the goodwill and support of many. In my own life and studies, I have been profoundly blessed by the presence of my family, friends, and colleagues. It is traditional to thank these wonderful people in these pages, as though words were enough. They aren't, but they are a start.My parents, Christine Marwick and Richard Seymour, brought me up to have both a deep curiosity about the world and the work ethic I needed to answer my own questions. The love of language my mother gave me made writing this dissertation far less painful than it might have been; my father urged me to take my first voluntary science course and then kindly refrained from saying "I told you so" at the sudden fascination with physics that resulted. My not-so-little brother, Matthew, kept me honest with everything I did.The love and support of my extended family has been invaluable. My grandparents, Bob and Dolores Marwick, generously funded my undergraduate education. They and my aunts, uncles, and assorted cousins, scattered around the continent, taught me that I could find something of home wherever I am. To Rob and Ann Marwick, Mike and Monica Marwick, Lorrie Marwick and Guy Marsh, Nancy and Don DeMuth, and Josephine Marwick, thank you for the delicious meals, the fascinating conversations and your willingness to talk physics. To my beloved cousins, thank you for everything. Growing up, I was also lucky to enjoy the love of many peo...