Field Kallop is an artist whose primary tool is gravity. Her exhibition, The Melody of Structures, recently on view at The Tremaine Gallery, was an elegant contemplation of physics, mathematics, and the unseen structure of nature. The work is hard to categorize because it is at once drawing, sculpture, installation, and, during a public event when she constructed the piece, performance. As approximately fifty observers stood around the periphery of the gallery, Kallop calmly presided over the space. A trained dancer, her spare movements were both graceful and functional, but …