ASTRONOMY: W. W. CAMPBELL 
9 
for all helium stars is about 6 km. per second. Of the observed nebular 
velocities only one-fifth are under 10 km. per second, and the average 
for the 54 observed objects is 42 km. per second, or 7 times that of the 
helium stars. The velocities of the helium stars follow pretty closely 
the so-called 'probability curve' — a great number of small velocities 
and very few large velocities. The nebular velocities do not follow the 
probability curve at all. 
If we omit velocities greater than 60 km. per second, as abnormal, we 
find the average radial velocity of 45 nebulae to be 26.4 km. per second^ 
which is still 4 times the average for all the helium stars. 
Omitting 12 extended and ring nebulae we find that the average 
radial velocity of 42 planetary nebulae is 46.1 km. per second. 
The velocities of the extended and ring nebulae seem to be habitually 
low, but the number of these included in the program thus far is too small 
to serve as a safe basis for drawing conclusions. The nebulae N. G. C. 
6644 {a = 18h 26m, 5 = -25.2°) and N. G. C.2 4732 (a = 18h 28m, 
8 = — 22.7°), which are less than 3° apart, have velocities 202 km. per sec- 
ond recession, and 141 km. per second approach, respectively — a relative 
radial motion of 343 km. per second. 
In general, assuming that the motions of celestial bodies are sub- 
stantially at random in direction and speed, the average radial velocity 
of a considerable number of objects is exactly one-half of the average 
space velocity of the objects. We shall probably not be far wrong in 
assuming that the average radial velocity of 42 km. per second for the 
54 nebulae means that the average space velocity of the same objects 
is approximately 84 km. per second. 
All of the above results have been freed from the effects of the sim's 
motion as determined from spectrographic observations of the stars. 
The algebraic mean of the 54 observed velocities, after correcting for 
the effects of the sun's motion, is —2 km. per second. There is thus no 
indication of a systematic tendency of the planetary nebulae for motions 
of approach or of recession. 
These observations refer exclusively to nebulae whose spectra are 
composed chiefly of bright lines, and there are no spiral nebulae on this 
list. 
With reference to stellar motions, the planetary nebulae are rapid 
travellers. Does this fact throw any light upon their origin or their 
possible relation to stars? A few of the stars are known to be travelKng 
rapidly. These rapid travellers are found mostly amongst the yellow 
and red stars, which are usually held to be furthest removed from the 
nebular condition. Perhaps the best explanation of the so-called novae, 
