262 
PRESTON. 
other end it would be found that the lines of sight would 
be separated at one point by nearly six feet (l m ,8). Calcula¬ 
tion shows that the area included between the two lines, 
which on a sphere would be identical, but which on account 
of the ellipsoidal form of the earth really diverge, is equal 
to a good-sized farm, being no less than 145 acres. 
The pursuit of scientific truth is often accompanied by 
unexpected discoveries and the revelation of new difficulties. 
The fact that the earth is traveling through space at the 
rate of about 18 miles per second necessitates a correction 
to our astronomical observations dependent on this motion; 
and because it is not only flying along its path at this enor¬ 
mous speed, but also on account of its revolving on its axis, 
we are obliged to introduce corrections to reduce observed 
facts to an uniform basis. We can no longer ignore the 
convergence of the equipotential surfaces of a rotating spher¬ 
ical body, and observations for latitude as well as those for 
leveling must be corrected accordingly. The latitude of a 
place above the sea level as determined by observation is 
always greater than that of a point vertically under it on 
the plane of mean sea level, and the relative height of 
any point determined by spirit-levels when the line is in the 
meridian is greater by actual determination than it would 
be were the earth not subject to the influence of attraction 
and rotation. The value obtained for the elevation of a 
point 400 feet above the gulf of Mexico, depending on a 
line of levels through a distance of 500 miles, may be nearly 
two inches in error—that is to say, that on account of the 
revolution of the earth a line of precise levels extended from 
south to north and increasing in elevation gives a height 
for the terminal point two inches greater than is actually 
the case. 
III. Aberration of Light. 
One of the greatest accomplishments in scientific work is 
the ability to extract from a great mass of observations all 
the information that they are capable of yielding. The 
