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It has been shown that it is entirely possible for a spiral to be formed and 
that it is probable that more spirals would be formed than any other kind. 
Here we approach the speculative a little closer and I would remind you that 
we have no record of any permanent form of nebula ever being formed. 
Of course the time over which we have any accurate record of the nebulae 
is very short, only the last few years in fact. Very few of these objects can 
be recognized in the telescope, and it is only since the invention of the 
rapid photographic dry plate, and the perfection of the large reflecting 
telescopes, that their true form and number have been found. Even with 
our present equipment and resources if one should be recorded on a plate 
tonight it might be impossible to say that it was there a year ago, or that 
it was not, unless it should be exceptionally bright. 
With this class of objects then we will not expect much observational 
confirmation. From mathematical investigation we know that it is possible 
for a spiral nebula to be formed from the close approach of two stars. We 
know of about two hundred million stars in the sky and there are probably 
many more that we can get no direct evidence of. We know that they are 
all in motion with velocities ranging up to 300 or even 400 miles per second. 
Under these conditions we will at times have collisions. These will be 
relatively rare because the average distance between stars is large, thickly 
as they seem to be sown in the heavens. <A close approach without actual 
contact will be much more frequent, and it is from such an encounter that 
a spiral nebula might easily arise. 
The moon with only a the mass of the earth and at a mean distance of 
over a quarter of a milhon miles has enough attraction for the earth to 
cause a distortion of figure, the liquid surface showing the effect of course 
more easily than the solid parts. Under the action of the moon there are 
twe tides raised in the earth, one of which tends to stay directly below the 
moon and the other at the opposite side. That is to say, the moon causes 
the earth to assume an ellipsoidal form, the long axis of which would point 
toward the moon if it were not for the rapid rotation of the earth. What 
would this effect be if the moon were as massive as the earth, or perhaps 
twenty times as massive? If, in addition to this increased mass, we should 
decrease the distance between the bodies to a few thousand miles, the tides 
would be many times as great as they are now. 
When we remember that the stars for the most part are gaseous, in many 
cases with an average density less than that of air at sea level, and at the 
