crayon itself can not adhere, for the attraction of the earth does not permit it. But small quantities of it can adhere for the same reason that water adheres to surfaces in small quantities. So also dust clings to sides of furniture, though a lump of dirt would not. [Illustration: Fig. 27.] [Illustration: Fig. 28.] 91. =Illustrated in Solid Bodies.=--We can illustrate the limitation of size in solid masses by Figs. 27 and 28. Suppose that _a_ and _b_, Fig. 27, are two projections of timber from a post, _b_ being twice as large as _a_. It is evident that _b_ can not support twice as much weight as _a_, for gravitation is dragging it downward from its attachment to the upright post with twice the force that it does _a_. The case is still stronger when, as represented in Fig. 28 (p. 64), the larger timber is twice as long as the smaller. Here _d_ has four times the bulk of _c_. But it can not support four times as much weight at its end, not only because its own weight presses it downward, but because half of its weight is at a greater distance from the place of attachment than the smaller beam is. Gravitation here operates in opposition to cohesion in such a way that the projecting timber, if carried to a certain size, will fall by its own weight, either breaking in two, or tearing away from its attachment. This tendency is very commonly resisted in buildings and other structures by braces, as represented in Fig. 29. Here the weight of the horizontal timber at some distance on each side of _a_, is made to press upon the upright post instead of directly downward. [Illustration: Fig. 29.]