HIRN'S METHOD. 



145 



under the 



FIG. 34. 

 SEGUR GAUGE. 



grate. Whatever passes in by the doors or 

 through cracks escapes accounting. On account 

 of this it is certain that the calculations based on 

 anemometer readings are lower than th al 



air supply. 



SEGUR'S DIFFERENTIAL GAUGE. 

 This gauge (Fig. 34) consists of a U-tubeof 

 |-inch glass, surmounted by two chambers of 2 \ 

 inches diameter. Two non-miscible liquids of 

 different colors, usually alcohol and paraffin oil, 

 are put into the two arms, one occupying the 

 portion AB y the other the portion BCD. The 

 movement of the line of demarcation is pro- 

 portional to the difference in area of the chambers 

 and the tube adjoining. A movement of 2 

 inches in the column represents J-inch difference 



pressure or draft. 



HIRN'S METHOD. 



The apparatus used by Burnat as a check on his own 

 calculations was devised by Him, and is based on the formula 

 of the rate of flow of compressed gases from a reservoir, 

 friction being neglected. The coefficient of reduction used 

 is 0.9, the one given by Dubuisson in his treatise on hydraulics. 



The main difficulty consists in measuring the difference of 

 pressure of the atmosphere in the ash pit and that outside, 

 for the depression in the flues in some cases does not exceed 

 a few millimetres of water. Hirn's apparatus removes this 

 difficulty. 



Burnat describes it as follows : 



When making a test the doors of the ash pit are removed 

 and replaced by a piece of sheet iron, A (Fig. 37), which com- 

 pletely shuts out all access of air except through the opening 

 in the middle, to which is fitted the pipe CD, 13.8 inches 



