[Footnote B: This boiler power is required for continuous running at full speed and pressure. It is, however, often best to put in a larger pump than the existing boilers could drive at full capacity, as a _small boiler will drive a 750-gallon pump at the 500-gallon speed with very nearly as good economy as it can drive a 500-gallon pump at full speed_. The pump then does not have to be changed when the plant is enlarged and the boiler power increased. A steam piston relatively larger than necessary is a source of weakness. It takes more volume of steam, and gives more power with which to burst the pipes if the throttle is opened wide suddenly during excitement. It has been common to make all fire-pumps with water plunger of only one-fourth the area of steam piston, with the idea that pump could thereby be more readily run at night, when steam was low. The capacity in gallons is thus reduced 25 per cent. as compared with a 3 to 1 plunger on the same steam cylinders.] _b._ The above sizes of steam and water cylinders and length of stroke have given general satisfaction and will now be considered as standard. Often, especially with large pumps, “4 to 1” construction is a mistake, and gives no additional security, although the pump might start and give a few puffs with 30 lbs. of steam on banked fires; because, if any pump of whatever cylinder ratio draws 50 or 100 horse-power of steam from boilers with dead fires, it can run effectively only a very short time (ordinarily, perhaps, 3 to 5 minutes), unless fires are first aroused to make fresh steam to replace that withdrawn. Steam pressures stated above must be maintained _at the pump_, to give full speed and 100 lbs. water pressure. Pressure at boilers must be a little more, to allow for loss of steam pressure between boiler and pump. Pumps in poor order, or too tightly packed, will require more steam. The boiler horse-powers above are reckoned on the A. S. M. E. basis of 34-1/2 lbs. of water evaporated from and at 212 degrees Fahrenheit as the unit of boiler horse-power. From 12 to 15 square feet of water-heating surface in the boiler is commonly assumed necessary for the generation of one horse-power. Smaller boilers than called for above, if favorably set, and having excellent chimney draft, can sometimes be forced to nearly double their nominal capacity for a short run, as for fire service. _c._ _250 gallons per minute is the standard allowance for a good 1-1/8-inch (smooth nozzle) fire stream._ A so-called “Ring Nozzle” discharges only three-fourths as much water as a smooth nozzle of the same bore, and is not recommended. From fifteen to twenty automatic sprinklers may be reckoned as discharging about the same quantity as a 1-1/8-inch hose stream under the ordinary practical conditions as to pipes supplying sprinkler and hose systems respectively. 4. CAPACITY.