The front bar of the frames also has usually two lugs or projections forged on it, between which the cylinders are attached. The latter are securely held in their position by wedges, which are driven in between the lugs and the cylinder castings. The frames, as already stated, are made of wrought iron and are accurately planed off over their whole surface. QUESTION 276. _How are the frames fastened to the boiler?_ _Answer._ As already stated, they are fastened to the cylinders with wedges and bolts, and as the cylinders are bolted to the smoke-box the frames are thus rigidly attached to the front end of the boiler. In order to strengthen those portions of the frames which extend beyond the front of the smoke-box and to which the bumper-timber is attached, diagonal braces, _r′ r′_, plates I, II and III are bolted both to the timber and to each of the frames at their lower ends. The upper ends are bolted to the smoke-box. Other braces, _d′_, plate II, are also fastened to the frames and to the barrel of the boiler. The frames are fastened to the fire-box by clamps, _I_, _I_, plate I, called _expansion clamps_. These clamps embrace the frames so that the latter can slide through the former longitudinally. There are also usually two diagonal braces not shown in plate II, the upper ends of which are fastened to the back end of the shell of the fire-box at about the level of the crown-sheet, and the lower ends to the back ends of the frames. There are also usually transverse braces attached to the lower part of the frames, thus uniting the two together. The guide-yokes, _j j_, plate I, are also usually bolted to the frames and to the boiler. In many cases one only is used, which extends across from one frame to the other and is fastened to the boiler. QUESTION 277. _Why are the frames attached to the shell of the fire-box so as to slide longitudinally through the fastenings?_ _Answer._ Because when the boiler becomes heated it expands, and if it could not move independent of the frames its expansion would create a great strain on both itself and the frames. The fastenings to the fire-box are therefore made so that the frames can move freely through them lengthwise, but in no other direction. QUESTION 278. _How much more will the boiler expand than the frames in getting up steam?_ _Answer._ From ¹⁄₄ to ⁵⁄₁₆ of an inch. QUESTION 279. _Why is it necessary to support the engine on springs?_