1881.] and its Effect on Astronomical Observations. 507 
With b as centre, and same radius, describe an arc of 30°, 
viz. N B o ; N o will be the arc described by the zenith at the 
pole during February. 
With c as centre, and same radius, describe an arc of 30°, 
viz. o c p ; op will be the apparent course of the zenith at 
the pole during March. 
Proceed in the same manner, and the Nautilus curve is 
completed by the arc x Y, which is the arc described by the 
zenith at the pole during December. 
The month indicated is written against each portion of the 
curve. 
As the arc A M is towards o hours right ascension, 12 hours 
right ascension will be in the opposite direction, viz., in the 
direction M A ; whilst 6 and 18 hours right ascension will be 
indicated by two lines drawn at right angles to AM and 
through M, 6 hours being on the left, 18 on the right. The 
scale at foot of diagram shows seconds of arc. The value 
of this curve can now be demonstrated, and its accuracy or 
inaccuracy tested by any reader acquainted with the mere 
elements of astronomy, for he will by aid of this curve be 
able to measure off the scale the adtual changes in seconds 
in the declination of stars near the pole for each month in 
the year ; also the changes in right ascension of all stars in 
the Northern Hemisphere. Any stars near the pole may 
have their direction plotted on the diagram, and their 
changes in right ascension and declination can be shown by 
this diagram. 
Four stars near the North Pole will be seledted, viz. the 
Pole Star, 51 Cephei (hev), 8 Ursae Minoris, and /3 Ursae 
Minoris, the direction of which stars is shown on the dia- 
gram ; but it must be remembered that the distance of these 
stars from the pole is so great, when compared with the 
breadth of the curve shown on diagram, that the arcs from 
the pole to the star are nearly parallel to one another, no 
matter where the pole may be situated on this curve. 
The effects on the polar distance of the Pole Star will 
first be examined, and the direction of this star is plotted on 
the diagram. 
During the month of January the pole moves from M to N, 
or nearly at right angles to the arc joining the pole and the 
Pole Star. Consequently, on February 1st and January 1st, 
the polar distance of the Pole Star ought to be nearly the 
same. The observations of many years show that the polar 
distance of the Pole Star at these dates varies only i-iothof 
a second. The curve, however, between M and N does bend 
slightly towards the Pole Star. So, if observation be correct, 
2 L 2 
