i88i.] and its Effect on Astronomical Observations , 443 
an axis, joining the two poles. A meridian of 15 0 E. longi- 
tude passes through this centre of gravity. 
Before entering into a description of the results which 
follow, it must be pointed out that if the North and South 
Poles of the Earth be equidistant from all parts of the 
Equator, it must be a most marvellous coincidence if the 
axis of the Earth passes through the centre of gravity of 
the Earth, because the land and water on the Earth’s surface 
are most unequally distributed. If the axis of the Earth 
does pass through the centre of gravity of the Earth, then 
the Poles cannot be equidistant from all parts of the 
Equator. 
It may appear, at first sight, to those persons unacquainted 
with practical astronomy and geometry, that so small a 
divergence as 1115 feet for the position of the centre of 
gravity of the Earth from the Earth’s centre would not 
affedt, in a perceptible manner, any astronomical observa- 
tions. The effedts, however, would be most singular and 
most marked. 
The zenith of any locality on Earth (from which zenith 
measurements are made to determine the zenith distance of 
stars, and hence their declinations) is obtained by means of 
a trough of mercury, or a plumb-line ; whichever method 
was adopted the results would be the same, and for simplicity 
of explanation I will seledt the plumb-line. 
The plumb-line at any locality would be diredted to the 
centre of gravity of the Earth , and not to the true centre of the 
sphere , and this law would hold good for every locality on 
Earth. The zenith of each locality, therefore, would be 
assigned a position in the Heavens found by drawing a line 
from the centre of gravity of the Earth through each locality, 
and producing this to the sphere of the Heavens. The 
results of this fadt are most singular, and a few of these will 
now be demonstrated. 
I will first suppose two observers to be located one at each 
Pole, and using a transit instrument. The Poles being sup- 
posed equidistant from all parts of the Equator, each 
observer ascertains the position of his zenith by means of a 
plumb-line or trough of mercury, and then determines the 
zenith distance of certain stars. Each observer being at a 
Pole of the Earth, zenith distances and polar distances 
would be supposed identical, and the effedts on the supposed 
polar distances of stars can now be demonstrated. 
epqw represents a sedtion of the Earth through the 
Poles c w ; c the centre of the Earth ; g the centre of 
gravity of the Earth. 
2 G 2 
