The style which I have chosen for all the books I have written for use in teaching is what may be called the _lecture style_. There are three other kinds of style which are more commonly used in school-books. The most common is what I term the _formal statement style_. In this principles and rules are stated, and then illustrations are given. This makes a formal and uninviting book. The bare skeleton of the science is generally for the most part presented, and the young pupil is apt to learn the statements by rote without understanding them. It is a style fitted only for books intended for advanced scholars. Another style is the _catechetical_. This is an unnatural mode of communicating knowledge; and besides, it encourages learning by rote as the formal statement style does. In the third style, the _dramatic_, conversations are held between the teacher and some learners. The chief objection to this is that it undertakes to put in permanent shape what should be extemporized in the recitation. What is needed in the book is simply clear and concise statement in an interesting style, and the living teacher and his scholars can best furnish the conversational element as the recitation goes on. In the lecture style there may be and should be as much precision of statement as in the formal statement style, while it is more interesting, because it is the natural mode of communicating knowledge. In this style the facts are ordinarily so stated as to develop principles; while in the other the order is reversed, the principles being first stated and the facts given afterward. One of the most successful books ever used in our colleges--"Paley's Natural Theology"--is in the lecture style, and it is a matter of surprise that this fact has had so little influence with those who have prepared books for instruction. Whatever may be true of advanced scholars, in teaching the young student in science bare, dry statement should be avoided, and the subjects should be presented in all their attractive features. I would not be understood as advocating the dressing up of science in adventitious charms. This is not necessary. Science possesses in itself an abundance of charms, which need only to be properly developed to attract the young mind; and the lecture style furnishes the best vehicle for such a development. One grand essential for giving interest to any study is the presentation of the various points in the _natural order_ in which they should enter the mind. _They should be so presented that each portion of a book shall make the following portions more interesting and more easily understood._ This principle, which is so commonly transgressed, I have endeavored to observe strictly in the preparation of these volumes. Questions are put at the end of this book for those teachers who desire to use them. There is also an Index. W. HOOKER. _January_, 1863. CONTENTS. CHAPTER PAGE