146 



CALORIFIC POWER OF FUELS. 



diameter and 59 inches long. A tube leads from the front 

 to the apparatus E, devised by Hirn, placed on a table or 

 against the boiler-wall. This apparatus consists of a little 

 gas holder whose upper surface is just one decimeter (3.9 



c/V/ 



\V o 



/ 





FIG. 35. 



inches) on a side. Inside this and above the water level the 

 tube A opens. The bell dips into a vessel of water and is 

 suspended from a balance arm. 



The balance being in equilibrium when the atmospheric 

 pressure acts on both sides of the bell, if the interior is con- 

 nected with the ash-pit the weight needed to restore equili- 

 brium will give a measure of the difference in pressure. The 

 weight of half a gram (7.7 grains) represents one-twentieth 

 millimetre (0.002 inch) of water. 



The formula adopted by Hirn is 



lh 



-9\/ 2 <-- 



X 0.76(1 + 0.0037^) 



0.0013^5 



in w 



hich 



V = volume of air introduced under the grate in cubic 



metres ; 

 5 = section in square metre of pipe-opening leading air to 



the ash-pit ; 

 0.9 = coefficient of reduction; 



