THE STEAM-ENGINE. 257 



„^ ^ ^^ The essential requisites of a steam-boiler are, that it should 



What are the ^ ■ , ■ , , . , 



essential req- possess sufficient Strength to resist the greatest pressure which 



nisites of a jg g^gj. ij^ble to occuT from the expansion of the steam, and 

 Eteam-boUer ? ^ ' 



that it should offer a sufficient extent of surface to the fire 



to insure tlie requisite amount of vaporization. In common low-pressure 

 boilers, it requires about eight square feet of surface of the boiler to be ex« 

 posed to the action of the fire and flame to boil off a cubic foot of water in an 

 hour; and a cubic foot of water in its convertion into steam equals one- 

 horse power. 



The strongest form for a boiler, and one of the earliest which was used, is 

 that of a sphere; but this form is the one which offers least surface to the 

 fire. The figure of a cylinder is on many accounts the best, and is now ex- 

 tensively used, especially for engines of higli-pressure. It has the advantage 

 of being easily constructed from sheets of metal, and the form is of equal 

 strength except at the ends. In such a boiler the ends should be made 

 thicker than the other parts. 



called the "cold-water pnmp," draws from a well or river the cold water to supply the 

 plice of the heated water withdrawn from the condenser by the air-pump. There is also 

 a tliird pump, G Q, which is called the " supply" or " feed-pump," because it pumps into 

 the boiler the hot water which the air-pump withdraws from the condenser, thus econ- 

 omizing the consumption of fuel. 



The various parts of the engine (as shown in Fig. 215) are illustrated in detail by tha 

 following descriptive explanation : — 



A — Piston-rod connected with the walking-beam, and transmitting to it the alternating 

 movement of the piston. 



B, C, D, E — Arrangements of levers and joirits, intended to guide and preserve the pis- 

 ton-rod A in a perfectly rectilinear track during its up-and-down movements. 



F — Arm or rod of the air-pump, which removes the hot water and air from the con- 

 denser. 



G — Rod of the " supply" or " feed-pump," which supplies to the boiler the hot water 

 withdrawn from the condenser. 



H — Rod of the cold-water pump, which supplies the cold water necessary for con- 

 densation. 



I — Connecting-rod, which transmits the motion of the walking-beam, L, to the crank, K. 



M — Cylinder of the air-pump in communication with the condenser, O. 



O — Condenser filled with cold water, in which the steam after acting upon the piston is 

 condensed. 



P — Piston, movable in the cylinder ; it receives directly the pressure of the steam upon 

 the upper and lower surface alternately, and transmits its movements by means of the rod 

 A to the rest of the machinery. 



S — Pipe conducting the hot water withdrawn from the condenser to the boiler. 



T — Pipe discharging the cold water from the cold-water pump into the condenser, O. 



U — Pipe conducting the steam from the cylinder, after it has acted upon the piston, into 

 the condenser. 



V— Fly-wheel. 



Z — Cornecting-rod, which transmits the movements of the eccentric, e, through the 

 lever, Y, to the valves, b. The eccentric is a wheel fixed upon the crank-shaft, as seen 

 at e. It ii called an eccentric from the circumstance of the wheel not being concentric, or 

 having a common center with the crank-shaft upon which it is fixed. It becomes, there- 

 fore, a substitute for a short crank, and transmits a reciprocating movement to the rod 

 Z, which is connected with the valves at 6 by the lever Y. These valves being alternately 

 opened and closed by the movemeat of the rod Z, admit the steam alternately above or 

 below the pibton. 



