144 



CALORIFIC POWER OF FUELS. 



FLETCHER'S ANEMOMETER. 



Fletcher's anemometer (Fig. 35) is used in England to 

 ascertain the speed of flow in chimneys and flues. In its 

 simplified form it is quite serviceable. It is based on the 

 movement of a column of ether in a U-tube. 



The ends of the glass tubes a, b are placed in the flue a 

 little less than one sixth of its diameter. The straight end a 



FIG. 33. FLETCHER'S ANEMOMETER. 



should be parallel to the direction of the current, the end b 

 being at right angles to this. Hunter proposed bending 

 both ends in opposite directions, to obviate the error caused 

 if the tubes were not so placed. These tubes communicate 

 with the ether tube cd. The draught across the tubes causes 

 the ether to rise in a by aspiration and to fall in b by pres- 

 sure. The difference of level is read, and then the tubes are 

 turned around 180, so as to reverse their positions, and the 

 difference of level read again. The sum of the two differ- 

 ences is called the anemometer reading, and by means of 

 tables the velocity of the current is ascertained. 



The same trouble is common to all anemometer methods. 

 The flue feeding the fire receives only the air passing in 



