One of the difficulties that persons unfamiliar with certain branches of science encounter in reading about them arises from the excessive use of technical terms, the lack of simple illustrative examples, and also, sometimes, a lack of sympathetic appreciation of the reader's difficulties. It has been a special object of this series to avoid this trouble. Ordinary textbooks are prepared for students in school and are intended to be supplemented by the personal instruction and guidance of a teacher, standing at the pupil's elbow, or readily approachable. But the reader who wishes to inform himself upon some progressive branch of science after his school days are over needs to have the teacher included in the book itself. Then, too, there are many persons who have no comprehension of the great and gratifying power that a knowledge of some of the elementary principles and formulas of science bestows upon anybody who may take the little trouble necessary to master them, a trouble that does not imply a long course of scientific study. The "man in the street," if he possesses these easy-working keys to knowledge, can verify for himself some of the calculations of scientists which, if he did not know how they were done, would always remain for him in the category of the mysterious achievements of genius. To illustrate, let us take a simple example--that of the Newtonian law of falling bodies. Many persons would assume on the face of it that there was nothing in this law that could have a particular interest for them. But let us see. You will find in the volume on physics that the law is stated thus: S = ½gt², i. e., "S equals one-half of the product of g multiplied by t squared." As you look at it you would, perhaps, as soon think of picking up a complicated tool and trying to use it for some ordinary purpose. Nevertheless, let us try. "S" in the formula means the space or distance traversed by the falling body, "g" means the velocity that the force of gravity imparts in each successive second to the body, and "t" means the time elapsed during the fall. What the formula tells us, then, is that if we observe the time during which the body is falling, and then square the number of seconds involved (multiply the number by itself), multiply this square by "g," which is represented practically everywhere on the face of the earth by the number 32, and finally divide the whole by 2, we shall have the distance that the body fell. This distance will be in feet, since the number 32, representing "g," is in feet. Now, it might be a matter of life and death, or at any rate of mental discomfort against quietude of mind, to have that rule in memory and to be able to apply it. For instance, you are on your vacation and stopping in a strange hotel, where they have put you in the top story. On looking out of the window you are dismayed at finding no fire escape, or other appliance of safety, so that your only resource in case of fire would be to make