e heavens into five parallel circles. First, the
equinoctial, which lies in the middle, between the poles of the
earth, and obtains its name from the equality of days and nights
on the earth while the sun is in its plane. On each side are the
two tropics, at the distance of 23 deg. 30 min., and described by
the sun when in his greatest declination north and south, or at
the summer and winter solstices. That on the north side of the
equinoctial is called the tropic of Cancer, because the sun
describes it when in that sign of the ecliptic; and that on the
south side is, for a similar reason, called the tropic of
Capricorn. Again, at the distance of 231/2 degrees from the poles
are two other parallels called the polar circles, either because
they are near to the poles, or because, if we suppose the whole
frame of the heavens to turn round on the plane of the
equinoctial, these circles are marked out by the poles of the
ecliptic. By means of these parallels, astronomers have divided
the heavens into four zones or tracks. The whole space between the
two tropics is the middle or torrid zone, which the equinoctial
divides into two equal parts. On each side of this are the
temperate zones, which extend from the tropics to the two polar
circles. And lastly, the portions enclosed by the polar circles
make up the frigid zones. As the planes of these circles produced
till they reached the earth, would also impress similar parallels
upon it, and divide it in the same manner as they divide the
heavens, astronomers have conceived five zones upon the earth,
corresponding to those in the heavens, and bounded by the same
circles.]

[Footnote 15: _That which is the middle one._--Ver. 49. The
ecliptic in which the sun moves, cuts the equator in two opposite
points, at an angle of 231/2 degrees; and runs obliquely from one
tropic to another, and returns again in a corresponding direction.
Hence, the sun, which in the space of a year, performs the
revolution of this circle, must in that time be twice vertical to
every place in the torrid zone, except directly under the tropics,
and his greatest distance from their zenith at noon, cannot exceed
47 degrees. Thus his rays being often perpendicular, or nearly so,
and never very oblique, must strike more forcibly, and cause more
intense heat in that