I suspect it is a magazine article: The Problem of Increasing Human Energy - The Century Illustrated Monthly Magazine, June 1900 Twenty-First Century Books sells reprints, and has the text online, so you could compare to verify. (www.tfcbooks.com/tesla/1900-06-00.htm) Some self-describing phrases from the article: THE PROBLEM OF INCREASING HUMAN ENERGY WITH SPECIAL REFERENCES TO THE HARNESSING OF THE SUN'S ENERGY. THE ONWARD MOVEMENT OF MAN—THE ENERGY OF THE MOVEMENT—THE THREE WAYS OF INCREASING HUMAN ENERGY. Tesla makes some very insightful observations even though it was just 1900. His genius was formidable. There are however some now known to be incorrect concepts that were held by Tesla, and one needs to keep in mind the manner of speaking will be unusual to us now. You will encounter phrases like "human mass" and "frictional resistances" which are used more broadly than we would use them today.