MINING. 303 



A smaller instrument, termed a new Portable Air Meter, has 

 also been made by Pastorelli on the same principle as the Robin- 

 son cup machine. It will require repeated and careful trials to 

 determine how far this handy little anemometer may prove to rival 

 other kinds which have had the start of it, and also to solve the 

 general problem which of them, seeing that the friction of the 

 air upon the perimeter of the passage renders the results of all of 

 them only approximative, comes nearest to the true average 

 velocity of the current of air. 



A very simple piece of apparatus, which does good service when 

 its indications are duly understood and attended to, is the water- 

 gauge. It is usually made in the form of a simple glass U-shaped 

 tube, containing coloured water. One leg communicates with the 

 fresh air, or intake, on one side of a door or partition ; the other 

 leg, with the impure air, or return. A greater pressure will act on 

 the water on the intake side than on that of the return air, and 

 the vertical distance between the two surfaces is measured by a scale 

 of inches and parts, which may either be pushed up or down by hand 

 or moved by a screw. It is not, however, as a measure of the venti- 

 lation that it is useful, but as an indication of the resistances caused 

 by friction that it becomes valuable. As long as the airways be- 

 tween the two points tested are in good condition, an increase of 

 " water-gauge " would be equivalent to an increase of ventilation, 

 but if an obstruction occurs, as by the fall of shale from the roof, 

 the increase of water-gauge will be an index of increased friction 

 somewhere in the airways, and therefore a call to the exercise of 

 that vigilance which in fiery collieries can never be allowed to 

 rest. 



W. W. SMYTH. 



