2. How is power secured from water in a water motor? Or what is the source of power utilized by a water motor? 3. How do you determine the size of pulleys to use on the gasoline engine and on the device it is to operate? 4. What are some indications that a gasoline engine or automobile motor is not running properly? 5. What are the kinds of batteries, and to what uses is each best suited? 6. Do batteries need care? If so, what care? 7. How is acetylene gas made? Describe the device for making it. 8. How is gas for household use made from gasoline? PART XI MEASURING DEVICES CHAPTER XL SCALES FOR WEIGHING =283. Equal-Arm Balances.= Scales are devices for determining the weight of objects. Balances--one form of scales--are made of two arms of equal lengths and supplied with discs of metal of a known weight to be placed on one arm of the balance while the material to be weighed is put on the other. When the two arms are in equilibrium, the weight of the material is equal to the weight of the metal. Since the weight of the metal is known, or can be determined, by adding together the weights of the discs used, the weight of the material is known to be the same. =284. Unequal-Arm Balances.= Equal-arm balances are not convenient for weighing large objects. For this reason, scales are made with one arm of the balance much longer than the other. The metal discs are then marked with the weight of the material on the short arm which they can balance when placed on the long arm. This is the usual form of counter and household balances. On these scales is also a weight which slides along the arm and is used to determine weights smaller than five or ten pounds. The arm of the balance is, therefore, marked at the point where this weight will balance certain amounts of material, such as half ounces, ounces and pounds. =285. Spring Scales.= Spring scales depend on the action of a spring, to which an indicating pointer is attached. When there is no weight on the spring, the place to which the indicator points is marked zero. When these scales are manufactured, a pound weight is placed so that it pulls on the spring and the indicator is pulled down to another place, and this is marked one. Scales are thus marked for the number of pounds they are to weigh. The spaces between the pounds marked are divided into equal divisions, such as sixteenths which indicates ounces. These scales cannot be relied on for accuracy, for springs stretch or become weaker as they are used. Avoirdupois is the weight in common use for marketing, while many tables for calculating dietaries are in the metric system. The housewife can have her balances corrected for weighing by the city or county sealer of weights and measures so that she can ascertain whether or not her food purchases are correctly weighed. TABLE OF WEIGHTS AVOIRDUPOIS METRIC