4 
ASTRONOMY: A. VAN MAANEN 
Proc. N. A. S. 
The results found by subtracting these values from the motions of the 
nebular points are assumed to represent internal motions. In plate I those 
for the 30 points of the 25-foot focus plates are indicated by arrows with 
full lines and for the 22 very probable nebular points measured on the 
80-foot focus plates by arrows with broken lines; the comparison stars 
of the first pair are enclosed in small squares, those of the second pair in 
circles. 
A first inspection of the plate leaves us again in doubt as to whether 
we are dealing with a rotation of the nebula as a whole or with a motion 
along the arms of the spiral; upon further examination the latter motion 
seems to be more clearly indicated. 
Resolving the motions into components perpendicular to and along the 
radius, we find the following results : 
25-FOOT FOCUS 
80-FOOT FOCUS 
Right-handed 
Left-handed 
Outward 
Inward 
Mean tangential motion 
Mean radial motion 
23 or 77% 
7 or 23% 
18 or 60% 
12 or 40% 
+0".020 
+0".006 
17 or 81% 
4 or 19% 
15 or 71% 
6 or 29% 
+0".014 
4-0". 012 
+ means right-handed and outward 
Classifying the points according as they have a component of motion 
outward or inward along the arms of the spiral, we have the following 
results : 
25-FOOT FOCUS 
80-FOOT FOCUS 
Motion outward 
Motion inward 
Mean outward motion 
28 or 93% 
2 or 7% 
0".024 - 
18 or 86% 
3 or 14% 
0".021 
Before discussing the possibility of an increase or decrease of motion 
with distance from the centre it will be necessary to measure a large num- 
ber of additional points in the nebula, which is deferred for the present. 
The principal conclusion to be drawn from the present material is that 
here again we find motions analogous to those occurring in Messier 101, 
81 and 51. In general they seem to be outward along the arms of the 
spiral. 
In 1916 Pease published values for the radial velocity of the centre of 
Messier 33 and for the bright knot 10 ' nf the nucleus. The two values 
differ by about 200 km. /sec. Taking into account the probable inclina- 
tion of the nebula with respect to the tangential plane, we can gain some 
