PHOTOGRAPHING LIGHTNING WITH MOVING CAMERA. 121 



A spring-motor movement (of the kind used to operate revolving 

 stands for exhibiting goods in show windoAvs) was procured and 

 mounted inside a table specially constructed for the purpose, and a 

 stand for suj^porting the cameras was fitted to the central shaft. 



As the speed of the motor was too slow, the fly-vane shaft was re- 

 moved and the vane moved to the next shaft, which was lengthened 

 so as to extend under the table. Thus arranged the fly vane 

 could be made to revolve in a liquid placed in a vessel under 

 the table, thereby preventing much of the vibration and get- 

 ling a more uniform speed. Figure 4 shows the arrangement of 

 the apparatus. The table top is removed in order to show how the 

 motor movement is }daced; the fly vane is seen under the table. The 

 stand is usually revolved at a speed of one revolution in ten sec- 

 onds, which the writer has found to be the most suitable for ordinary 

 purposes. The reason for employing a motor movement with a 

 uniform speed to move the camera is to ascertain the exact duration 

 of a flash or the intervals between the rushes. 



If the angle of the lens and the speed of the camera be known, it 

 is a simple matter of measurement to ascertain the duration of a 

 flash. The formula employed is as follows : 



Call the angle ot tlie leus (In degrees) =A° 



Time for one rotation of stand (in seconds) =T 



Width of plate (in millimeters) =W 



Width of flash measured on plate (in millimeters) =N 



Then time for tiasli (in seconds) = — — — XN 



The measurements should be taken from the middle of the plate, 

 owing to the distortion of the lens. 



A number of i)hotograi)hs have been taken by this method, and 

 about nine out of ten show the multiplicity of a flash. The average 

 number of rushes for each flash is about five or six, and the time 

 varies from an almost instantaneous value up to about half a second 

 for a complete discharge. 



The most interesting discharge obtained is shown in figure 5. It 

 was taken Se])tember 1, 100."). at 9 p. m. The storm during Avhich 

 this flash was i)hotographed began about 7 p. m.. with the Avind 

 northeast, which is something very unusual for Chicago. The wind 

 gradually changed to north and northwest. The temperature during 

 the storm Avas about 24° C, and the barometer varied betAveen 29-89 

 iiud 29*92. The flash Avas obtained Avhen the storm Avas most seA^ere 

 and Avhile it Avas raining A'ery hard. 



This flash is composed of forty separate discharges, made up of 

 one band, Avhich in all probability is composed of a number of sepa- 

 SM 1905 12 



