THE MOON S FACE. 
263 
tion. This simplified conception is embodied in the diagram, 
Fig. 10. The angle of incidence is the angle included be¬ 
tween the direction of the incident meteor and a line normal 
to the moon’s surface at the same point. It is 0° at the 
center of the hemisphere turned toward the rain, and is 90° 
at the margin of that hemisphere. At any intermediate 
point, A, it is measured by the arc connecting that point 
with the center of the hemispherical surface. Through the 
point A draw a small circle in the plane parallel to the base 
of the hemisphere. It is evident that the zone of spherical 
surface above this plane includes 
the downfall of all meteors whose 
incidence angle is less than that 
of the meteors reaching A, and 
that the zone below it includes 
the downfall of meteors making 
greater angles. The number of 
those falling on the upper zone 
is measured by the area of the 
small circle. The number of 
those falling on the whole hemi¬ 
sphere is measured by the base 
of the hemisphere. The ratio of 
the one to the other, or the pro¬ 
portionate number of meteors 
having an incidence angle less than any given angle, i is 
equal to $m 2 i. Substituting 30° and 60° successively for 
i, we learn that 25 per cent, of all the meteors have inci¬ 
dence angles less than 30°, and 75 per cent, have incidence 
angles less than 60°; so that 50 per cent, of the angles fall 
within the middle third of the quadrant. The law of dis¬ 
tribution is graphically shown by curve A of Fig. 12, where 
abscissas represent angles of incidence, and ordinates the 
corresponding proportionate numbers of meteors. It will be 
noted that the number of meteors having incidence angles 
of 0° or 90° is a vanishing quantity, and that the incidence 
angle shared by the greatest number of meteors is 45°. 
Fig. 10.—Diagram illustrating inci¬ 
dence angle of meteors. 
