Sheep Meadow is quiet on this Wednesday morning in June. The guitar-strummers and sunbathers who lounged on its lawn yesterday are gone, and now its only occupants are the elms and sycamores that cluster along its edge, where I now stand with the Central Park Conservancy’s historian of the past forty years, Sara Cedar Miller. I have come to the leafy center of Manhattan on a quest to find traces, however faint, of parkmaker Frederick Law Olmsted’s 1854 horseback ride across Texas. I may be 1,800 miles from the heart of Texas, but I’m drawn to the alluring, if unprovable, theory that this famous green sward—borrowing the word Olmsted and his partner Calvert Vaux used when they designed this park in 1858—could be inspired by…