one-third of the way from
the equator to the pole, the pole of the heavens is raised one-third of
the way from the horizon to the point vertically overhead; and when this
is the case the observer knows that he is in latitude 30 deg. The builders
of the great pyramid, with the almost constantly clear skies of Egypt,
may reasonably be supposed to have adopted this means of determining the
true position of that thirtieth parallel on which they appear to have
designed to place the great building they were about to erect.

It so happens that we have the means of forming an opinion on the
question whether they used one method or the other; whether they
employed the sun or the stars to guide them to the geographical position
they required. In fact, were it not for this circumstance, I should not
have thought it worth while to discuss the qualities of either method.
It will presently be seen that the discussion bears importantly on the
opinion we are to form of the skill and attainments of the pyramid
architects. Every celestial object is apparently raised somewhat above
its true position by the refractive power of our atmosphere, being most
raised when nearest the horizon and least when nearest the point
vertically overhead. This effect is, indeed, so marked on bodies close
to the horizon that if the astronomers of the pyramid times had observed
the sun, moon, and stars attentively when so placed, they could not have
failed to discover the peculiarity. Probably, however, though they noted
the time of rising and setting of the celestial bodies, they only made
instrumental observations upon them when these bodies were high in the
heavens. Thus they remained ignorant of the refractive powers of the
air.[18] Now, if they had determined the position of the thirtieth
parallel of latitude by observations of the noonday sun (in spring or
autumn), then since, owing to refraction, they would have judged the sun
to be higher than he really was, it follows that they would have
supposed the latitude of any station from which they observed to be
lower than it really was. For the lower the latitude the higher is the
noonday sun at any given season. Thus, when really in latitude 30 deg.
they would have supposed themselves in a latitude lower than 30 deg., and
would have travelled a little further north to find the proper place, as
they would have supposed, for erecting the great pyramid. On the other
hand, if they determined the place from obs