22 Professor Leslie on Electrical Theories, 
point A ; since a flat surface exerts hardly any action on the air 
immediately contiguous. 
Hitherto we have considered the motion of the distant par- 
ticles as affected only by the repulsion and attraction of the bo- 
dies A and B But it was formerly shewn that the external 
re-action of the air surrounding a point, such as B, is diminished 
in consequence of its rapid influx, and therefore that the more 
remote portions of air are impelled towards B by the excess of 
the pressure. Thus, a particle M, besides being attracted by 
the opposite surface, will be pushed more powerfully in the di- 
rection M B ; and since both this force and the attraction of the 
point B will increase as it advances, while the action M Q is 
hardly affected, the course of the particle will bend continually 
nearer and nearer to B : and, in the ordinary state of the atmo- 
sphere, all the particles emitted from A will terminate their mo- 
tion in B, and those which pass beyond that point will be im- 
perceptible. If, however, the air be exceedingly rare, the force 
arising from any change of its weakened electricity must be ra- 
pidly impaired ; at the same time, the particle, suffering little 
obstruction, will recede to a greater distance from A B. In this 
case, therefore, the motions of the particles will be determined 
almost solely by the actions of the bodies A and B, and a large 
portion of the stream will flow beyond the point, and spread from 
B to Q. 
Hence, we may determine the direction of the luminous stream : 
For, in rare air, the body from which the discharge is made will 
seem dark, except at its points ; whereas, that which receives 
it will not only appear bright at the extremity, but beset with 
a bur of light, for a small part of its length. The experiment 
was accordingly made: Two brass-wires, about a quarter of an 
inch thick, and tapered, were fixed with their points opposite, 
and about 2 inches from each other, in a small receiver, and the 
air considerably exhausted ; and it w r as constantly observed, that 
the wire connected to the cushion of the electrical machine was 
decked with luminous streaks for the space of more than an inch, 
but the one connected to the prime conductor was bright at the 
point only. The curves described by the different portions of 
the stream could also be partly traced ; though, beyond the mid- 
dle of the two points the diffusion was so great, that the light 
