VELOCITY OF THE DISCHARGE. 615 



knob downwards above the disc, and discharged with the 

 common discharger. Each of the white and black 

 quadrants on the disc appears by the light of the, spark 

 sharply defined, however fast the apparatus may, be* 

 turned. 



We may assume that the whirling table may be 

 worked so that the disc makes about ten revolutions in 

 one second, and that hence the time required for the 

 disc to move through the twelfth part of a circle is about 

 O008 second. If, therefore, the duration of the spark 

 were 0*008 second, a particular quadrant would not be 

 seen in its true magnitude, but would, in consequence of 

 the persistence of the visual impression, appear to cover 

 an additional space equal to that through which it moves 

 in that time ; in other words, each quadrant would 

 appear broader by r ^-th of the circle or -J-rd of its own 

 magnitude, and the result would be that black and white 

 would partly merge into one another. But since each 

 quadrant appears perfectly sharply defined, the disc 

 cannot have moved through any appreciable distance 

 during the time that it was illuminated by the spark, 

 that is, the duration of the spark must have been much 

 less than 0*008 -second. 



Our atmosphere nearly always contains free electri- 

 city. Nothing certain is known about the origin of 

 atmospheric electricity, but its existence may nearly 

 at all times be easily proved by an electroscope provided 

 with a collecting apparatus. The observer should be- 

 take himself into the open air t and station himself in a 

 commanding position, such as the top of a hill, but 

 avoid the vicinity of trees, shrubs, houses, etc.; it is 

 then only necessary to raise the electroscope above the 



