188 OF THE CONSTRUCTION OF [SECT. vi. 



the cistern ; and therefore in order that the injection be sufficiently powerful at 

 first, the head should be about three times the height of the cylinder ; and making 

 the jet apertures square, the area should be the 850th part of the area of the 

 cylinder, or its side should be A of the diameter of the cylinder. The conducting 

 pipe should be about four times the diameter of the jet. 



396. To determine the power of an atmospheric engine. The moving force is 

 the pressure of the atmosphere, from which the whole of the friction and the force 

 of the uncondensed steam is to be deducted. 



The moving force is the pressure of one atmosphere or 1-00 



The loss of force measured in atmospheres consists of, 



1. The uncondensed steam corresponding to the tem- 



perature of condensation (usually about 160) '34 



2. The force to expel it and the air from the cy- 



linder (art. 154.) - - -007 



3. The friction of the piston (art. 474.) - -050 



4. The force required to open and close the valves, 



raise injection water, and overcome the friction of 



the axes - -093 



Making total loss of force O49 



The portion of the pressure of the atmosphere equal the 



effective pressure is - O51 



or 5'9 Ibs. per circular inch. 



397. RULE for the power of the common atmospheric engine. Multiply 

 5 - 9 times the square of the diameter of the cylinder in inches, by half the velocity 

 of the piston in feet per minute ; and the product is the effective power of the 

 engine in Ibs. raised 1 foot high per minute. 



To find the horse power, divide by 33000. 



Example. Let the diameter of the cylinder be 72 inches, and the length of the 

 stroke 9 feet, making 9 strokes per minute : in this case, half the velocity is 

 9x9 = 81 feet per minute ; consequently 5'9 x 72 2 x 81 = 2477433-6 Ibs. 

 raised 1 foot per minute, or 



2477433-6 ,. , 



-33000- = 75 hOTSe P Wer ' 



This example is the size of the Chase Water engine, designed by Smeaton, (see 

 art. 24. p. 23.) His estimate of the equivalent horse power differs from this chiefly 

 through using a different measure of that power ; but he also estimated on con- 



