35 
same effect as he usually does in the summer, when he passes through the 
middle of the firmament [that is, the zenith of the historian] ; for he attracts 
the water [the Mediterranean] to himself, and, having attracted it, throws it 
back again [? as rain] upon the higher regions of Libya.” Anciently, when 
the Delta was within the boundary of the tropics, the hippopotamus was 
found in the Lower Nile, where he was hunted. Now, this animal is rarely 
seen, even in Lower Nubia, within the northern limits of the tropics. Large 
crocodiles were common in the Nile, but now we only see occasionally a few 
small ones even in Cairo. We have to ascend to the cataracts within the 
tropics before we meet with large crocodiles. 
Hipparchus was the most eminent of the ancient Greek astronomers. 
After studying at Alexandria, he continued his astronomical observations at 
Rhodes 34 years. He first discovered the phenomenon called the “ preces- 
sion of the equinoxes ; ” he catalogued the fixed stars, and laid the founda- 
tion for a correct system of astronomical computations. At that period, the 
northern limit of the tropic extended to Rhodes : hence “ the sun in June 
passed through the middle of the firmament.” Ptolemy’s physical system of 
astronomy was introduced by Hipparchus. To show how little the theories 
of astronomy have to do with the astronomical computations, on which alone 
the science rests, he was able to calculate the period of the eclipses, the 
mean period of the planets’ revolutions, and, in fact, all the observed celestial 
phenomena. The same was subsequently done by Tycho Brahe, who also 
entertained a geocentric theory, as accurately as the instruments then 
provided could admit. We can have no assistance from the modern theory 
of physical astronomy to guide us in our discussions, as its very foundation 
has been completely destroyed, not only by restoring the plenum, or a 
resisting medium, but also by other new ideas regarding the sun, meteors, 
&c., &c. It must be borne in mind that the only science of astronomy we 
can depend on, is that founded upon a system of computation, and nothing 
else. I was desirous to restrict my paper on this occasion to giving a 
brief description of the observed facts connected with the movements of 
the surface of the earth from south to north. I thought that, as the 
operations going on at the poles could not be examined, and therefore 
would have to be determined from analogy, they might be left for future 
consideration. But whatever may be the opinions as to the character 
of the actions going on at the poles, they cannot affect the fact of the 
great superficial movement of the earth from south to north. If. we look 
at the maps of the southern and northern hemispheres, with their re- 
spective poles in the centre, it will be seen that the dry lands radiating 
from the Antarctic Circle are comparatively very limited, and they are 
composed principally of gravel and sand ; whereas, in the northern hemi- 
sphere, they are crowded, jammed, contorted, rising in ridges, and they 
contract the passage of the currents of the ocean to the north polar 
basin. A superficial glance will show that the dry lands, after passing 
the parallel of 40° N. lat., become so crowded as to allow but a 
small space for the ocean, as compared with the other parts of the globe. 
D 2 
