[D9246AAB], Letter from Louis Glass to Thomas Alva Edison, February 19th, 1892
https://edisondigital.rutgers.edu/document/D9246AAB
Transcription
San Francisco, CAL., February 19th, 1892 Thomas Edison, Esq., Orange, New Jersey Dear Sir: When engaged in Hydraulic Mining, I sometimes ran long [preliminary gulch?] lines, sighting from hill to hill, and a traveling engineer gave me the following ready formula for determining the approximate curvature of the earth. I have never seen it in print, and thought it would be useful to you in connection with your induction telegraph system at sea. It is 1/7 of the square of the distance in miles gives the curvature in feet. Example: Curvature for miles: 10 x 10 = 100 / 7 = 14.3 x 4 = 57.2 Pt. Curvature for miles: 20 x 20 = 400 / 7 = 57.1 x 4 = 228 Pt. Very truly yours, [Signed Louis Glass]