04 Farm Mechanics. 



In this equation 461 is the number of degrees F. which 

 the zero of the Fahrenheit scale is above absolute zero, and 

 in such problems as these it is necessary to express the tem- 

 perature in absolute degrees. When this is done 300 F. 

 becomes 761 F and 212 F. becomes .673 F., and the 

 above equation becomes 



761 - 673 



-^TJ- = 115 per cent. 



From the results of this problem it is clear why it is 

 not possible for the steam engine to utilize a very large per 

 cent, of the total energy which the steam carries with it 

 into the cylinder. Even if the steam could be carried into 

 the cylinder at 1,000 F. and could do work on the piston 

 until its temperature fell to 100, the maximum efficiency 

 would only be 



(1000 + 461) - (100 + 461) 



1000 + iu =61. 6 per cent. 



628. Pressure of Steam at Different Temperatures. The 

 temperature at which water is changed from a liquid into 

 steam or invisible vapor varies with the pressure to which 

 the water is subjected as stated in the table below: 



Table showing the pressure of steam or water vapor at differ- 

 ent temperatures. ' 



Temperature of water. Pressure of steam. 



102 F 1 Ib. per sq. inch. 



162 5 Ib. per sq. incl 



194 lOlb. per sq. inch. 



212 14.73 persq.incli. 



228 20lb persq.incl). 



328 100 Ib. persq.incl). 



432 3501b. per sq. inch. 



546 1000 Ib. per sq. inch . 



629. Dry and Wet Steam When steam contains no 

 water held mechanically in suspension it is known as dry 

 steam, but it is seldom possible to develop absolutely dry 

 steam because as it escapes from the surface of the water 

 in the boiler there is a tendency to carry away with it 



