THE SECOND BOOK.
The second book is devoted to the investigation of relations between the rectangles contained by straight lines divided into segments in various ways.
When a straight line is divided into two parts, each part is called a segment by Euclid. It is found convenient to extend the meaning of the word segment, and to lay down the following definition. When a point is taken in a straight line, or in the straight line produced, the distances of the poin.t from the ends of the straight line are called segments of the straight line. When it is necessary to distinguish them, such segments are called internal or external, according as the point is in the straight line, or in the straight line produced.
The student cannot fail to notice that there is an analogy between the first ten propositions of this book and some elementary facts in Arithmetic and Algebra.
Let ABCD represent a rectangle which is 4 inches long and 3 inches broad. Then, by drawing straight lines parallel to the sides, the figure may be divided into 12 squares, each square being described on a side which represents an inch in length. A square described on a side measuring an inch is called, for shortness, a square inch. Thus if a rectangle is 4 inches long and 3 inches
broad it may be divided into 12 square inches; this is expressed by saying, that its area is equal to 12 square inches, or, more briefly, that it contains 12 square inches. And a similar result is easily seen to hold in all similar cases. Suppose, for example, that a rectangle is 12 feet long and 7 feet broad; then its area is equal to 12 times 7 square feet, that is to 84 square feet; this may be expressed briefly in common language thus; if a rectangle measures 12 feet by 7 it contains 84 square feet. It must be carefully observed that the sides of the rectangle are supposed to be measured by the same unit of length. Thus if a rectangle is a yard in length, and a foot and a half in breadth, we
