ANEMOMETER 



171 



second wheel which has fifty teeth. At each revolution of the first wheel with the 

 hundred teeth, the cam starts the second wheel with fifty teeth by one tooth. The 

 method of using this instru- 

 ment will be understood 

 from this concise descrip- 

 tion. The wings and first 

 wheel are adjusted at zero, 

 and kept immovable until 

 the moment of commencing 

 the observation. Then for 

 every complete revolution 

 of the wings the first wheel 

 is advanced one tooth, and 

 when this wheel has com- 

 pleted its revolution, or 

 advanced a hundred teeth, 

 one tooth is moved forward 

 on the wheel of fifty teeth. 

 An index-pointer fixed upon 

 light uprights indicates tho 

 number of revolutions of 

 the axle of the wings. The 

 manner of using this in- 

 strument is easily under- 

 stood. The limbs are 

 placed at zero, and the in- 

 strument in the axis of the 

 air-tubes, keeping the limbs 

 immovable by means of a 

 catch, which is loosened at 

 the moment of commencing 

 the observations, and fas- 

 tened at the end of the 

 same. The division of the 

 limbs does not admit of 

 counting over 5,000 turns, 

 which for a velocity of air 

 at 9'84 feet per second, 

 would correspond with a 

 duration of nearly 3 

 minutes. 



The anemometer of Dr. 

 Robinson is constructed on 

 the assumption that the 

 force of impact of the air 

 against hollow hemisphe- 

 rical cups is twice as great on the concave as on the convex side of the cups, and that 

 the vanes revolve at the rate of one-third the velocity of the current, except in so far 

 as the velocity of revolution is modified by friction. 



The mechanism of this instrument is very strong, and allows of the revolutions 

 being recorded throughout the whole day ; it would, therefore, bo a very suitable 

 anemometer to have near a furnace, or in the principal intake or return of a mine. 



Pressure Anemometer. Perhaps the best known of the pressure anemometers are 

 M. Bongui's, Dr. Lind's, that of Henaut, described by Ponson, and Dickinson's, one of 

 her Majesty's Inspectors of Mines. The anemometer of Bongui consists of an appa- 

 ratus like a spring balance, furnished with a float-board, or plain surface of given 

 area, and the pressure or impulse is indicated by marks on the sliding-rod of tho 

 spring ; it is figured and described in the ' Edinburgh Encyclopedia.' 



The anemometer most generally used in the coal mines of England is that intro- 

 duced by the late Mr. Biram. It consists of a series of vanes, which revolve with the 

 action of the air-current the number of revolutions, or, rather, numbers proportional 

 to the revolutions, being registered by pointers on the face of a dial forming a part of 

 the instrument itself. It is made of three sizes, 4, 6, and 12 inches ; is very portable ; 

 and is not, with proper care, liable to get out of order, especially the smaller size. A 

 certain force of current is required to overcome tho friction, and put the instrument 

 into motion. Some of these instruments will continue to revolve in a current as low 

 as 30 feet per minute, but with the most of them a velocity of about 50 feet is required. 



