174 COALFIKLDo AND C'OLLIKRIES OF AUSTRALIA. 



on one side of the return airway, and connected to it by a gal- 

 vanised iron air drift, so that in case of an explosion this struc- 

 ture being light would be blown away, thus acting as a safety 

 valve and saving the fan : and if destroyed, the drift could be 

 easily and cheaply reconstructed. 



Davis' s water-gauge is used for reading- the pressure of 

 the air. As 1 cub. ft. of water at 62deg. Fahr. and under 

 30in. barometric pressure weighs 62,3551b., the pressure per 

 square foot due to each inch in height equals 62.355 or 



12 



5.1961b., generally taken as 5.21b. The ordinary U-shaped 

 gauge is difficult to read on account of the oscillations of the 

 water columns; also if one limb is broken the gauge is useless. 

 Da vis's gauge consists of two straight pieces of boiler gauge 

 glass sealed into a brass block in such a manner that they 

 can be readily renewed. (Fig. 107.) A small hole in the 



Fig. 107 Davis' Water Gauge. 



brass block connects the two glasses, and in th'is is a thumb 

 tap, with which one can minimise the oscillation of the water 

 column due to the pulsation of the fan, and even stop the 

 connection altogether, so that the height of the water column 

 may be registered. As the pressure of a column is dependent 

 solely on the vertical heisrht, any irregularity in the section 

 of the tube is immaterial. The scale behind the tubes is 



