In its literary character, this periodical aims to be popular, without being superficial, and appeals to the intelligent reading-classes of the community. It seeks to procure authentic statements from men who know their subjects, and who will address the non-scientific public for purposes of exposition and explanation. It will have contributions from HERBERT SPENCER, Professor HUXLEY, Professor TYNDALL, Mr. DARWIN, and other writers identified with speculative thought and scientific investigation. _THE POPULAR SCIENCE MONTHLY is published in a large octavo, handsomely printed on clear type. Terms, Five Dollars per annum, or Fifty Cents per copy._ OPINIONS OF THE PRESS. "Just the publication needed at the present day."--_Montreal Gazette._ "It is, beyond comparison, the best attempt at journalism of the kind ever made in this country."--_Home Journal._ "The initial number is admirably constituted."--_Evening Mail._ "In our opinion, the right idea has been happily hit in the plan of this new monthly."--_Buffalo Courier._ "A journal which promises to be of eminent value to the cause of popular education in this country."--_N. Y. Tribune._ IMPORTANT TO CLUBS. THE POPULAR SCIENCE MONTHLY will be supplied at reduced rates with any periodical published in this country. Any person remitting Twenty Dollars for four yearly subscriptions will receive an extra copy gratis, or five yearly subscriptions for $20. THE POPULAR SCIENCE MONTHLY and APPLETONS' JOURNAL (weekly), per annum, $8.00. _Payment, in all cases, must be in advance._ Remittances should be made by postal money-order or check to the Publishers, D. APPLETON & CO., 549 & 551 Broadway, New York. Transcriber's Note Variant and obsolete spellings were not changed. Footnotes were renumbered sequentially and moved to the end of the chapter in which the related anchor occurs. Illustrations that fell within paragraphs were moved to precede or follow the paragraph, and in some cases no longer occur on the page number listed in the List of Illustrations. Words in italics are surrounded by underscores, _like this_. A few index entries are not listed in alphabetical order. Alterations: