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to shine on all those places which are in the parallel that just touches the edge of the broad paper circle, and will fnr several days seem to skim all around, and but a little above their horizon, just as it appears to us at its setting; but with this observable difference, that whereas our setting-sun appears in one part of the horizon only, by them it is seen in every part thereof; from west to south, thence east to north, and so to west again.
Or if the latitude be given, elevate the globe to that latitude, and on the back of the strong brass meridian, opposite to the latitude, you obtain the day of the month ; then all the other requisites are answered as above.
As the two concentric spaces, which contain the days of the month on the back side of the strong brass meridian, are graduated to shew the opposite days of the year, at 180 degrees distance; when the given day is brought to coincide with the broad paper circle, it shews when the sun begins to shine on that parallel, which is the first day of its appearance above the horizon of that parallel.
And the plane of the broad paper circle cuts the day of the month on the opposite concentric space, when the sun begins to disappear to those inhabitants.
The length of the longest day is obtained by reckoning the number of days between the two opposite days found as above, and their difference from gives the length of their longest night.
Problem xxx. To make use of the globe as a tellurian, or that kind of orrery which is chiefly intended to illustrate the phenomena that arise from the annual and diurnal motions of the earth.
Describe a circle with chalk upon the floor, ai large as the room will admit of, so that the globe may be moved round upon it; divide this circle into twelve parts, and mark them with the characters of the twelve signs, as they are engraved upon the broad paper circle; placing 25 at the north, VJ> at the south, y in the east, and •£=. in the west: the mariner's compass under the globe will direct the situation of these points, if the variation of the magnetic needle be attended to.
Note. At London the variation is now 23 degrees from the north-westward.
Elevate the north pole of the globe, so that 66^ degrees on the strong brass meridian may coincide with the surface of the broad paper circle, and this circle will then represent the plane of the ecliptic, or a plane coinciding with the earth's orbit.
Set a small table, or a stool, over the centre of the chalk circle, to represent the sun, and place the terrestrial globe upon its circumference over the point marked VJ, with the north pole facing the imaginary sun, and the north end of the needlepointing to the variation; and the globe will be in the position of the earth with respect to the sun at the time of the summer solstice, about the 21st of June; and the earth's axis, by this rectification of the globe, is inclined to the plane of the large chalked circle, as well as to the plane of the broad paper circle, in an angle of 66f degrees; a line, or string, passing from the centre of the imaginary sun to that of the globe, will represent a central solar ray connecting the centres of the earth and sun: this ray will fall upon the first point of Cancer, and describe that circle, shewing it to be the sun's place upon the terrestrial ecliptic, which is the same as if the sun's place, by extending the string, was referred to the opposite side of the chalked circle, there representing the earth's path in the heavens,
If we conceive a plane to pass through the centre of the globe and the sun's centre, it will also pass through the points of Cancer and Capricorn, in the terrestrial and celestial ecliptic; the central solar ray, in this position of the earth, is also in that plane: this can never happen but at the times of the solstice.
If another plane be conceived to pass through the centre of the globe at right angles to the central solar ray, it will divide the globe into two hemispheres; that next the centre of the chalked circle will represent the earth's illuminated disc, the contrary side of the same plane will at the same time shew the obscure hemisphere.
The reader may realize this second plane by cut. ting away a semicircle from a sheet of card pasteboard, with a radius of about 1-1 tenth of an inch greater than that of the globe itself.*
If this plane be applied to 66^- degrees upon the strong brass meridian, it will be in the pole of the ecliptic; and in every situation of the globe round the circumference of the chalked circle, it will afford a lively and lasting idea of the various phenomena arising from the parallelism of the earth's axis, and in particular the daily change of the sun's declination, and the parallels thereby described.
Let the globe be removed from Vf to xz, and the needle pointing, to the variation as before, will preserve the parallelism of the earth's axis; then it will be plain that the string, or central solar ray, will fall upon the first point of Leo, six signs distant from, but opposite to the sign zz, upon which the globe stands; the central solar ray will now describe the 20th parallel of north declination, which will be about the 23<hof July.
If the globe be moved in this manner point to point round the circumference of the chalked circle, and care be taken at every removal that the north end of the magnetic needle, when settled, points to the degree of variation, the north pole of the globe will be observed to recede from the line connecting the centres of the earth and sun, until the globe is placed upon the point Cancer;
* Or he may have a plane made of wood for this purpose.
after which, it will at every removal tend more and more towards the said line, till it comes to Capricorn again.
Problem xxxi. To rectify either gl'ile to the latitude and liariznn of any place.
If the place be in north latitude, raise the north pole; if in the south latitude, raise the south pole, "until the degree of the given latitude, reckoned on the strong brass meridian under the elevated pole, cuts the plane of the broad paper circle; then this circle will represent the horizon of that place, while the place remains in the zenith, but no longer. This rectification is therefore unnatural, though it is the mode adopted in using the globes when mounted in the old manner.
Problem xxxn. To rectify for the sun's place.
After the former rectification, bring the degrees of the sun's place in the ecliptic line upon the globe to the strong brass meridian, and set the horary index to the Xllth hour upon the equator which is nost elevated.
Or if the sun's place is to be retained, to answer various conclusions, bring the graduated edge of the moveable meridian to the degree of the sun'* place in the ecliptic, and slide the wire which crosses the centre of the artificial horizon thereto;