192 OF THE CONSTRUCTION OF [SECT. vi. 



motion is given to the rod O, which shuts the valves a and c, and opens the valve 

 b ; there is then a communication open by the pipe E, between the top and bottom 

 of the cylinder, and the pressure of the counter weight must be sufficient to over- 

 come the friction of the piston, and expel the steam from the upper to the lower 

 side of the piston : the action of the counter weight has also to expel the air and 

 water of condensation through the valve Q by means of the air pump. The mode 

 I have shown of placing the valves and moving them by a single motion is not 

 Messrs. Boulton and Watt's, but is one intended to render the motion of the steam 

 from the upper to the under side of the cylinder more quick, by the pipe E being 

 exhausted : the motion of the valves is simple and easily balanced. The valves of 

 Messrs. Boulton and Watt are similar to Fig. 5. but they move them independently 

 of one another ; and this ought to be the case for an engine to work expansively, 

 unless a separate valve acted on by a regulator be used to cut off the steam, (see 

 Sect, vin.) An elevation of Boulton and Watt's single engine is represented in 

 Plate xn. as applied to raising water. 



407. The proportions of the parts. The length of the cylinder should be twice 

 its diameter, (art. 329.) The velocity of the piston in feet per minute should be 98 

 times l the square root of the length of the stroke, (art. 342.) The area of the steam 

 passages should be equal to the area of the cylinder, multiplied by the velocity of 

 the piston in feet per minute, and divided by 4800, (art. 154.) The air pump 

 should be ith of the capacity of the cylinder, or I the diameter and i the 

 length of the stroke of the cylinder, (art. 351.) and the condenser should be of 

 the same capacity. The quantity of steam will be found by multiplying the 

 area of the cylinder in feet, by half the velocity in feet ; with an addition of 

 TVth for cooling and waste (art. 160.) ; and this divided by the column of the 

 steam corresponding to its force in the boiler (art. 121.) gives the quantity of 

 water per minute required for steam, from whence the proportions of the boiler 

 may be determined, (see Sect. in. art. 224 and 227.) At the common pressure 

 of 2 Ibs. per circular inch on the valve, the divisor will be 1497. The quantity 

 of injection water should be 24 times that required for steam, (art. 284.) and 

 the diameter of the injection pipe sVth of the diameter of the cylinder. The 

 valves in the air pump bucket should be as large as they can be made, and 

 the discharge and foot valves not less than the same area. For the proportions 

 of the beams and other parts for strength, see Sect. vn. ; and for the modes of 

 regulation and management, see Sect. vin. 



408. The power of the single engine may be ascertained as follows: 



i See note p. 190. 



