[3] Mr. David A. Molitor has a chapter, copiously illustrated, on the esthetic design of bridges, beginning page 11 in the “Theory and Practice of Modern Framed Structures,” by Mr. J. B. Johnson and other authors, New York, John Wiley & Sons. Eighth edition, revised and enlarged. $10.00. CHAPTER IV FORM--_Continued_. WEIGHT AND FRICTION DIMINISHED. Why supports are made hollow . . . Advantages of the arch in buildings, bridges and dams . . . Tubes in manifold new services . . . Wheels more important than ever . . . Angles give way to curves. Having glanced at methods by which forms, judiciously chosen, economize the materials of buildings and rails, of bridges diverse in type, we pass to further consideration of these and like shapes, to find that they effect a saving in material while they make feasible a new boldness of plan, and introduce new elements of beauty. We will also remark that judicious forms prevent waste of energy as structures are either set in motion, or serve to convey moving bodies. Incidentally we shall see that well chosen shapes insure a structure against undue hurt and harm. [Illustration: Square Octagonal 16-Sided Round Girder sections.] Hollow Columns and Tubes. In lofty structures, the box girder is frequently employed as a column or a beam because it has even greater rigidity than the I-beam; usually it has four sides, but it may have eight, sixteen, or more, and thus step by step we come to a hollow cylindrical column which has, indeed, the best form that can be bestowed on supporting material. Chinese builders learned its economy on the distant day when they adopted the bamboo for their walls and roofs. Comparison with a solid stick of timber of like weight and substance will show that an equal length of bamboo is decidedly preferable. The inner half of a round solid stick does comparatively little in holding up a burden; to remove that half is therefore as gainful as to strip from a joist the timber surrounding its working skeleton. At first the journals or axles of engines and large machines, as well as the axles of railroad cars and the shafts of steamships were solid; to-day, in a proportion which steadily increases, they are hollow. The advantage of this form comes out when we take two cylinders of rubber, alike in length and weight, one solid, the other hollow. Supporting both at their ends, the hollow form sags less than the solid form, proving itself to be the more rigid of the two. [Illustration: Solid rubber cylinder sags much. Hollow rubber cylinder sags less.] [Illustration: Handle-bar of bicycle in steel-tubing.] [Illustration: A sulky in steel tubing.] [Illustration: A pneumatic hammer, steel tubing.] [Illustration: Fishing-rod in steel tubing.] [Illustration: Bridge of steel pipe.]