RULES FOR CALCULATING THE STEAM ENGINE. 205 



XL PRACTICAL RULES FOR CALCULATING THE STEAM ENGINE. 



[The number inserted at the end of each rule refers to the page of the book where information respecting it 



is to be found.] 



PROPERTIES OF STEAM. 



THE additional heat required for the formation of steam may be estimated at 1000 Fahr., the 

 heat of conversion from liquid to vapour. 



1. To find the elastic force of steam when in contact with the liquid from which it is 

 formed, the temperature being given. 



Tredgold's Rule. 



RULE i. Add 100 to the temperature, in degrees, and from the logarithm of this sum 

 subtract 2-24797 j then multiply by 6, and the product will be the logarithm of the force in 

 inches of mercury. (59.) 



Example. Let the proposed temperature be 250 Fahr. 



Temperature 250 

 Add 100 



350 - - - log. 2,54407 ' 



Subtract by the Rule 2,24797 



Difference 0,29610 

 Multiply by 6 



Log. 59-8 1,77660 

 The required force is therefore 59'8 inches of mercury. 



Southern's Rule. 



RULE ii. Add 51'3 to the temperature, and multiply the logarithm of the sum by 5'13 ; 

 from the product deduct 10'94123 and find the natural number answering to the remainder as 

 a logarithm; this number increased by O'l will express the required pressure in inches of 

 mercury. (58.) 



1 When the water is salt, refer to the last column of the table, page 61, for this logarithm. 



Id 



