7 
the plane of the equator, is but an extremely minute quan- 
tity, viz., 50" in longitude and 20" in latitude; yet its continual 
action from year to year makes itself very conspicuous, and that 
in a way highly inconvenient to practical astronomers. It 
destroys, in the lapse of a moderate number of years, the 
arrangement of their catalogues of stars, with reference to the 
stations on earth, and renders it necessary to reconstruct them 
from time to time. 
Since the earliest catalogue on record — that made by 
Hipparchus 2,140 years ago — the stations of reference have 
moved towards the north-west 30°, and have, in round num- 
bers shifted northward during the same time 12° in latitude. 
That is equal to the cosine of the angle of the spiral plane (of 
23° 30') the direction of the superficial movement. The effect 
of this change in the aspect of the heavens is to make the 
southerly stars appear to recede southward, and those 
situated in the north to approach at the rate of 20" per annum 
in the meridian. Hence it appears that the superficial film of 
our globe has been made free to move, like the ocean, from 
south to north, but in a spiral path : this movement has been 
determined to a fraction of a second of a degree, and is seven 
and a half furlongs in longitude W. and three furlongs in 
latitude N. per annum. 
As a further illustration of this terrestrial change, let us, by 
way of an example, take y Ursae Majoris as a convenient fixed 
star to determine annually our geographical position. The 
situation of this star is very favourable for making observations 
in this latitude, inasmuch as it passes within 3° of the zenith, 
and therefore is, when in that position, unaffected by refraction 
on its transit. 
In January, 1853, Greenwich was 3° 2' 5" to the south 
of the transit of y Ur see Majoris. In January, 1864, the 
Observatory was 2° 58 ' 24" S. of this star. In 435 years 
hence the Observatory will have arrived at the same parallel 
as y Ursae Majoris, when the star's transit will be seen in 
the zenith. It might be urged that such a small movement, 
which is only detected after the lapse of ages, would not be 
sufficient to account for the geological changes referred to; but 
I shall endeavour to show that, small as it is, it is quite suffi- 
cient to produce them, and in the exact order in which they are 
seen. I shall take the Isle of Portland as an example. In the 
deposits of this island is a petrified tropic'al forest, proving 
that that part of England has not only been upheaved, but 
also exposed to a tropical, or at least, a semi-tropical sun. Many 
of the fossil trees are still standing erect, with the roots in the 
very ground in which they grew. The plants are similar to 
