Panama canal tolls : Speech of Hon. William Kent, of California, in the House of Representatives May 21, 1912.

Reprinted from the Congressional record.

Memoir of John William Draper. 1811-1882.

"Publications of John William Draper": p. 383-388.

The life of Sir George Pomeroy--Colley, K. C. S. I., C. B., C. M. G., 1835-1881;

Mode of access: Internet.

Letters of William von Humboldt to a female friend.

Mode of access: Internet.