1874.] Curved Appearance of Comets’ Tazis. 511 
sextant, which we had with us, we measured the length of 
the tail by measuring from the disc of the comet to those 
stars which could be seen within the tail. We found that 
110 was the length of the tail, and we noted those stars 
which appeared to be in the centre line of the tail. 
At about four o’clock in the morning our friend called 
us to come and look at the comet, which he announced 
had undergone a marvellous change. 
When we first saw the comet and up to nearly midnight 
the tail appeared straight, and to point past the pole and 
up to the zenith; but at 4 A.M. the comet’s tail had quite 
altered its shape, and, as our friend remarked, was more 
curved than a Turkish scimitar. Being aware at the time 
of those laws of curves and straight lines demonstrated in 
the preceding papers, we saw how good an example this 
comet afforded of the laws, for it was a practical illustration 
of a straight line becoming in appearance a curved line in 
consequence of six hours of diurnal rotation. We remarked 
that the comet’s tail had not changed its position in the 
heavens as regards the fixed stars in the slightest degree, 
the very stars that were in the centre of the tail at Io P.M. 
were also there at 4 AM.; and, in fact, we had the same 
problem exhibited in the heavens as we have endeavoured 
to demonstrate by diagrams in the preceding papers. The 
particular position occupied by the comet of 1861 was such 
that in six hours it changed the apparent form from a straight 
line to a curve, as seen by an observer in England, the 
comet’s tail being directed from the northern horizon past 
the pole and zenith. If this comet instead of being so 
situated had its tail coincident with the equino@tial, it would 
never have appeared to us in England as a straight line, 
but always as a curve in the heavens during the whole time 
it was abovethe horizon. If, however, it had coincided with 
the equinoctial it would have appeared as a straight line to 
an observer at the equator of the earth, and never as a curve 
orarch. Thus, whether a comet’s tail appears as a straight 
line or as a curve depends mainly on the position of the 
comet in the heavens and on the position of the observer 
on the earth. If a comet’s tail were coincident with the 
equino¢tial, and of, say, 80° length, it would appear to an 
observer in 45 N. lat. as a curve or arch. If this observer 
could be instantly transported to the equator, the comet 
would then appear as a straight line without any sign of 
curvature. If,again, the person were transported to 45’ S. lat., 
the comet would again appear as a curve. A _ comet, 
therefore, that coincides with the equino¢tial appears as a 
