Hi. M. Parkhurst—Tails of Comets. 41 
when we undertake, for the 30th day after the perihelion pas- 
sage, to trace the entire tail which has left the head during t 
preceding 60 days, we must pursue a curve commencing at the 
position of the head of the comet, passing to the right and 
ownward, crossing the perihelion line at a distance from the 
sun one-third greater than that of the planet Neptune, and re- 
entering the earth’s orbit above. 
ow much of this will be visible? ‘As we follow the tail 
from the head it becomes fainter, first, because it is more dis- 
tant from the sun, and therefore less illuminated. Another 
occupies less than one-fourth the space traveled over by the 
comet while it was issuing. 
While the dark line in the center of a comet’s tail seems to 
disprove the theory naturally suggested by the fact of the front 
edge being the line of computation, that the resistance of the 
ether is the cause of the expansion of the tail, the ether may 
have a perceptible effect upon its form and position. Had the 
sun an ether of its own, carried with it through that still more 
rare ether which brings to us the pulsations of light, the effect 
of the ether upon the tail would be only that resulting from 
the motion of the comet itself; but leaving the ether behind it 
m its course toward the constellation Hercules, it will tend to 
Sweep the tail back from that point: and here we may find a 
possible explanation of the apparent wafting of the tail out of 
the plane of its orbit, leaving the front edge still on the right, 
and causing a thin, flat tail to have a visible width tenfold 
—" than it would have had if strictly in the plane of the 
rbit. 
Note-—Observations in England on July 9, 14, 17, 18, 19 and 21, confirm the 
Positions above given, but are not sufficiently definite for exact comparison. The 
theory of Prof. Norton (Am. Jour. Sci., 1861), that the width of the tail arises 
ftom the variation of the repulsive force exercised upon different particles of the 
etary matter, and its thickness from the repulsion of the nucleus itself at the 
time of emission, I had not seen when the above was written. I have com 
i nes re with the former compute pat eater 
: 8® of the tail, apparently indicating that the tail was, fro ’ 
chiefly South of the lane of the orbit. The theory of Prof. Norton affords a 
Plausible explanation of that cause. 
New York, Oct., 1874. 
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