ON PHOTOGRAPHS OF METEOROLOGICAL PHENOMENA. 143 



Discharges may consist of a single f3ash, but they frequently consist 

 of a series of flashes following one another with considerable rapidity 

 along the same or related paths. The eye is often able to detect alterna- 

 tions of brilliancy during a discharge, and may resolve it, as a moving 

 camera will, into a series of flashes accompanied by a persistent luminosity, 

 which it has already been suggested is probably the flame of burnin» 

 nitrogen. 



Last year your Committee referred to a photograph taken by Mr. Glew 

 at Brixton. This was taken in a camera the lens of which was attached 

 to the hammer of an electinc bell and kept in oscillation during exposure. 

 The object was to deduce from the known rate of movement of the lens 

 the duration of the discharge. Unfortunately, however, there is nothino- 

 to show in which direction the lens was travelling at the moment of each 

 component flash. 



There is one very simple method by which it is quite possible to make 

 a rough measurement of the duration of a discharge. Let two observers 

 A and B, agree that A shall carefully notice the seconds hand of his watch 

 while B looks at the sky to be sure that A does not confuse two separate 

 discharges. If the night is otherwise dark, A will see the hand only when 

 the face is illuminated by the lightning. The secretary to your Com- 

 mittee has, with the aid of Mrs. Clayden, made many such observations, 

 and has found that a lightning discharge often lasts as much as two or 

 thi-ee seconds, and may extend further, the longest time hitherto observed 

 being no less than seven seconds. During these times, though the 

 brightness of the light varied considerably, it was quite possible to watch 

 the hand moving steadily, and not in a series of jerks, as must have been 

 the case if the continuity of illumination had been an illusion due to per- 

 sistence of vision. In a similar way it is quite possible to follow the 

 movements of swaying tree-tops and other objects. It was noted with 

 some surprise that the light, as far as the eye can see, is often perfectly 

 steady for as much as a couple of seconds. Since beginning these observa- 

 tions not a single discharge has been noted of suSicient brevity to prevent 

 any movement of the watch hand from being seen. 



Now, although such observations are rough, their bearing upon light- 

 ning photography is important. 



An argument commonly advanced to prove that all photographs of 

 reduplicated flashes are due to movement of the camera is that the track 

 to be followed by successive flashes in a given discharge is marked out 

 by the first, which creates a path of minimum resistance in the form of a 

 partial vacuum. 



But it seems to be forgotten how far this tube of rarefied air must 

 be moved, and how far the discharging point of the cloud (so to say) 

 may be displaced by the movement of the air. We know that the wind 

 is often quite strong during a thunderstorm. 



Now, a movement of one mile an hour corresponds to 176 inches a 

 second. 



Suppose, therefore, we take the first seven values of the Beaufort 

 scale and see how far such a tube of minimum resistance would be dis- 

 placed during the existence of a discharge. 



Hence it appears that if a dischai-ge lasts as long as three seconds, the 

 path of minimum resistance marked out by the first flash might be dis- 

 placed as much as fifty yards by a strong breeze. Moreover, since the 

 clouds would be moving at the same rate as the upper part of the vacuous 



