<span>Handwritten letter from Mary C. Hatch to Charles Brush, Sr., acknowledging receipt of Brush's third paper on Gravitation, which she enjoyed, </span><span>although she did not understand all of it.Charles F. Brush, Sr., PapersSeries 1: CorrespondenceSubseries 2: Acknowledgments (1921-1929)Box 3Folder 1</span>