1874.] Curved Appearance of Comets’ Tails. 509 
to the trace of the equino¢ctial on the sphere of the heavens, 
and pointed out how to an observer in middle latitudes 
the equinoctial would appear like an arch, cutting the east 
and west points of the horizon, and rising on the meridian 
to an altitude equal to the colatitude of the station on which 
the observer is located, whilst to the individual at the 
equator the trace of the equinoctial would appear as a 
straight line, rising from the east and west points straight 
to his zenith. Again we showed that to an observer at 
either pole the equinoétial traced in the heavens would 
coincide with the horizon, and would again appear as a 
straight line. It was also shown that a great circle passing 
from the east and west points of the horizon and through 
the pole would, for that part above the horizon and visible, 
appear as an arch to an observer in middle latitudes. 
We will now suppose that three stars or any three 
celestial bodies (distant from the earth many million miles) 
R 
H 0 Z 
were located on the equinoctial; that to an observer at the 
earth’s equator these three stars, which we will call P, R, s, 
were so situated that the star R was at the zenith, the star 
p east, and the star s west of the observer, and both 45° 
above the horizon. These three stars being coincident 
with the equinoctial would appear in a straight line to the 
observer at the equator, and the line joining P R s would 
also appear as a Straight line. 
To an observer in, say, 51 north latitude, the equino¢tial ap- 
pears as an archor curve in the heavens and not asa straight 
line, and all portions of the equinoctial appear as portions 
of an arch or curve, and therefore the line joining the three 
stars, P RS, would appear as an arch orcurve to the observer 
in lat. 51°, though it would appear a straight line to an 
observer at the equator. The three stars on the equino¢ctial 
would appear curved to an observer in lat. 51°, as shown in 
the diagram, where H is the east, z the west points on the 
VOL. IV. (N.S.) 3T 
