AN AUTOGRAPHIC AIR-FLOW RECORDER. 261 



The discharge holes (see Plate XII) are in a vertical row, 

 V spacing, and about - 3 V diameter. They are counter- 

 sunk to reduce the tendency for a film to remain over them 

 as they rise from the liquid. To further reduce this trouble 

 arising from the surface tension of the liquid, it has been 

 found desirable to use a mixture of alcohol and water. 

 Alcohol would be satisfactory but for its rapid evaporation. 

 A half and half mixture is found satisfactory, and does not 

 evaporate to any appreciable extent during a test. Its 

 density is kept constant by the aid of a hydrometer. The 

 height of the liquid is adjusted to a mark on a gauge glass 

 (see Plate XII) in front of the containing vessel, when the 

 instrument is out of action, and the air space at atmos- 

 pheric pressure. The instrument is emptied after each 

 test through a drain cock in the base. 



The recording glass tube pen (Fig. 1 and Plate XII) is 

 attached to the end of a plunger working in a horizontal 

 barrel across the top of the float. It is kept in contact 

 with the recording drum by a spring in the barrel, whose 

 compression is regulated by means of a screw. A catch 

 enables the pen to be held out of action, and it is also 

 adjustable as to height. A fixed pen gives a base line from 

 which the height of the diagram is taken. The two pens 

 are set to the same line before the instrument is put into 

 action. 



The recording drum is 4" in diameter and is clock-driven, 

 making one revolution per hour. It unwinds a roll of paper 

 9" wide from another drum 3" in diameter. The tempera- 

 ture of the air is read at entrance to the instrument where, 

 also, a valve enables the air to be by-passed to atmosphere. 



The instrument at 7" height of diagram is passing 1*41 

 cubic feet of air at the barometric pressure at the time of 

 use. The measuring capacity of the instrument is increased 

 without affecting its sensitiveness by fitting diaphragm 



