Scale 1¹⁄₂ in. = 1 foot.] [Illustration: Fig. 75. Scale 1¹⁄₂ in. = 1 foot.] QUESTION 144. _How is the steam pressure in boilers prevented from exceeding a certain limit?_ _Answer._ By what are called _safety valves_. These consist of circular openings, _a_, fig. 74, about three inches in diameter placed usually on the top of the dome,[36] and covered by a valve, _b_, which is pressed down either by a lever, _c c′_, and spring, _d_, as shown in fig. 74, or by a spring alone, as in fig. 76. Two of these valves are usually placed on the top of the dome, so that if one gets out of order the other one will allow the steam to escape as soon as its pressure exceeds that which, it has been decided, the boiler can safely bear. This pressure, in locomotive boilers, is usually from 100 to 130 pounds per square inch. [36] One of these, _v_, is shown in fig. 41. QUESTION 145. _How is the amount of pressure which must bear on top of a safety-valve determined?_ _Answer._ This pressure is determined BY MULTIPLYING THE AREA OF THE OPENING FOR THE VALVE IN SQUARE INCHES BY THE GREATEST STEAM PRESSURE, IN POUNDS PER SQUARE INCH, WHICH THE BOILER IS INTENDED TO BEAR. Thus, if the opening for a safety-valve is three inches in diameter, its area will be seven square inches, and, therefore, if the greatest steam pressure which it is intended that the boiler shall bear is 100 lbs. per square inch, the valve must be pressed down with a pressure equivalent to 7 × 100 = 700 pounds. If the pressure on the valve is produced by a lever, as in fig. 74,[37] then the total weight of the safety-valve must be MULTIPLIED BY THE SHORT ARM OF THE LEVER, (or the distance _A_ between the centre of the fulcrum _e_ and that of the load _f_,) AND DIVIDED BY _B_, THE TOTAL LENGTH OF THE LEVER. In fig. 74 the short arm of the lever is 3¹⁄₂ inches, and the whole length 35 inches; therefore if the valve is to be pressed down with a pressure of 700 pounds, the pressure on the end of the lever would be calculated as follows: 700 × 3¹⁄₂ ---------- = 70 lbs. 35 [37] The lever is represented in the engraving with a piece broken out, in order to save room.