iQ/y Dr. Herschel's Observations of a Comet, 
prove it to be spherical ; but in addition to this, I have already 
shown that it is of an elastic nature, for which reason alone, 
had we no other, its globular figure could not be doubted. 
A most singular circumstance, which however must cer- 
tainly be admitted, is, that the constant appearance of the bright 
envelope, with its two opposite diverging branches, can arise 
from no other figure than that of an inverted hollow cone, 
terminating at its vertex in an equally hollow cap, of nearly 
a hemispherical construction ; nor can the sides or caps of this 
hollow cone be of any considerable thickness. 
The proof of this assigned construction is, that the bright 
envelope has constantly been seen in my observation as being 
every where nearly equidistant from the transparent atmo- 
sphere ; now if that part of it which in a semi- circular form 
surrounds the comet, on the side exposed to the sun, were not 
hemispherical, but had the shape of a certain portion of a ring, 
like that which we see about the planet Saturn, it must have 
been gradually transformed from the appearance of a semi- 
circle into that of a straight line, during the time that we have 
seen it in all the various aspects presented to us by a geocen- 
tric motion of the comet, amounting to 90 degrees. 
That this hemispherical cap is comparatively thin, is proved 
from the darkness and transparency of that part of the atmo- 
sphere which it covers ; for had the curtain of light., which 
was drawn over it, been of any great thickness, the scattered 
rays of its lustre would have taken away the appearance of 
this darkness ; nor would the atmosphere have remained 
sufficiently transparent for us to see extremely small stars 
through it. 
It remains now only to account for the semi-circular 
