"You have struck out on original and alluring lines of thought and investigation in which you stand alone. It must be a thrilling sensation to blaze a new trail for the coming march of men. The great highways of present knowledge were once obscure and hidden paths."—Lord Salisbury.
"He has the scientific knowledge of Flammarion combined with the fancy of Jules Verne."—Nicola Tesla.
"I am reading with much interest your remarkable book. I am deeply interested and profited by its study. It is grand, cheering, helpful."—Rev. Dr. R. S. MacArthur.
"Your book has impressed me very greatly for its learning, research and intellectual force."—Gen. John W. Noble (Ex-Secretary of the Interior.)
"Its arguments are carefully worked up on scientific lines and it is interesting reading from cover to cover."—New York Tribune.
"Few writers have ventured such boldly frank opinions. His style is polished and sometimes brilliant, while his thoughts are presented with lucid directness."—N. Y. Evening Telegram.
"It is a fascinating work which goes intelligently and with scientific plausibility into speculations concerning the destinies of Men and the Universe."—The New York World.
"So much has been accomplished by electricity in the last few years that no person can come forward and say with reason that the electric theory of Col. Warder is not correct."—Denver Republican.
"Your books are intensely interesting and instructive. I accept their theories and believe they will finally prevail and revolutionize scientific thought. Have read them three times."—Dr. L. M. Taylor (Scientist, Washington, D. C.).
"I know no greater favor that I can bestow on my friends than to present them your book, 'The Cities of the Sun.' It will give them new thoughts and teach them to think. Send me fifty volumes to-morrow."—Col. Henry H. Adams, 177 Broadway, New York.