_Answer._ FIND, AS BEFORE, THE CENTRES _C_, _C′_, _C″_, _C‴_, FIG. 197, OF THE SEGMENTS; DRAW _C″ C′_ AND _C‴ C_, CUTTING THE LINE _A B_; BISECT THE INTERVAL SO INCLOSED AT _E_ FOR THE COMMON CENTRE OF GRAVITY. [Illustration: _Fig. 197._] QUESTION 321. _How is the counterweight for outside-cylinder engines with a single pair of driving-wheels calculated?_ _Answer._ FIND THE TOTAL WEIGHT, IN POUNDS, OF THE REVOLVING AND RECIPROCATING MASSES FOR ONE SIDE, NAMELY, THE PISTON AND APPENDAGES, CONNECTING-ROD, CRANK-PIN AND CRANK-PIN BOSS; MULTIPLY BY THE LENGTH OF CRANK IN INCHES, AND DIVIDE BY THE DISTANCE IN INCHES OF THE CENTRE OF GRAVITY OF THE SPACE TO BE OCCUPIED BY THE COUNTERWEIGHT. THE RESULT IS THE COUNTERWEIGHT IN POUNDS, TO BE PLACED EXACTLY OPPOSITE TO THE CRANK. Question 322. _How is the counterweight for outside-cylinder engines with coupled driving-wheels calculated?_ _Answer_. FIND THE SEPARATE REVOLVING WEIGHTS, IN POUNDS, OF CRANK-PIN, CRANK-PIN BOSS, COUPLING-RODS AND CONNECTING-ROD, FOR EACH WHEEL, ALSO THE RECIPROCATING WEIGHT OF THE PISTON AND APPENDAGES, AND HALF THE CONNECTING-ROD; DIVIDE THE RECIPROCATING WEIGHT EQUALLY BETWEEN THE COUPLED WHEELS, AND ADD THE PART, SO ALLOTTED, TO THE REVOLVING WEIGHT ON EACH WHEEL; THE SUMS SO OBTAINED ARE THE WEIGHTS TO BE BALANCED AT THE SEVERAL WHEELS, FOR WHICH THE NECESSARY COUNTERWEIGHT MAY BE FOUND BY THE PRECEDING RULE. Question 323. _How are the counterweights constructed?_ _Answer_. They are usually made of cast iron, so as to fill or partly fill the spaces between the spokes of the wheel, as shown at _A A_ in fig. 161. Each of the segments consists of two pieces, one of which is put in from the outside of the wheel and the other from the inside. The two are then bolted together with bolts, which are shown in the above figure. In some cases the counterweights are cast solid with the wheels, and in others, cavities are cast in the wheels which are filled with lead, which is poured in when melted. Some locomotive builders make the rims and spokes of the wheels hollow and fill them with lead opposite to the crank-pins. It is difficult, however, to get the required weight in such wheels unless they are of large diameter. QUESTION 324. _What effect has the strain on the draw-bar, when the engine is pulling a load, upon the distribution of the weight on the wheels?_ _Answer_. It has the effect of pulling the back end of the engine down, and, as it is balanced on the fulcrums of the equalizing levers, it at the same time lifts up the front end. In this way the harder a locomotive is pulling the greater will be the weight which is thrown on the driving-wheels, and that on the truck will be correspondingly diminished. The higher the draw-bar is above the rails the greater will be the tendency to pull the engine down behind and up in front. PART XV. MISCELLANEOUS. QUESTION 325. _What is the sand-box, V, plates I and II for, and how is it constructed?_