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WHEN MOUNTED IN THE COMMON MANNER.
ALTHOUGH I have, in the first part of this Essay, laid before
readers the reasons which induce me to prefer my father's manner of mounting the globes to the old or Ptolemaic form, yet as many may be in possession of globes mounted in the old form, and others may have been taught by these globes, I thought it would render these Essays more complete, to give an account of so many of the leading problems, solved on the common globes, as would enable them to apply the remainder of those heretofore solved, to their own use. This is the more expedient, as, since the publication of my father's Trea. tise, there have been a few attempts to do away some of the inconveniences of the ancient form, particularly that of the old hour-circle, which is now generally placed under the meridian.
I cannot, however, refrain from again observing to the pupil, that the solution of the problems on the old globes depends upon appearances ; that there. fore, if he means to content himself with the mere
mechanical solution of them, the Ptolemaic globes will answer his purpose; but if he wishes to have clear ideas of the rationale of those problems, he must use those mounted in my father's manner.
The celestial globe is used in the same way in both mountings, excepting that in my father's mounting it has some additional circles; but the difference is so trifling, that it is presumed the pupil can find no difficulty in applying the directions there given to the old form.*
PROBLEM 1. To find the latitude and longitude of
any given place, on the globe. Bring the place to the east side of the brass meridian, then the degree marked on the meridian over it shews its latitude, and the degree of the equator. under the meridian shews its longitude.
Hence, if the longitude and latitude of any place be given the place is easily found, by bringing the given longitude to the meridian ; for then the place will lie under the given degree of latitude upon the meridian.
PROBLEM II. To find the difference of longitude
belween any two given places. Bring each of the given places successively to the brazen meridian, and see where the meridian cuts the
* A trifling difference, will certainly sanction, by tuition, the simplest construction. -Evit.
equator each time; the number of degrees contained between those two points, if it be less than 180 degrees, otherwise the remainder to 360 deg. will be the difference of longitude required.
PROBLEM 111. To rectify the globe for the latitude,
zenith, and sun's place.
Find the latitude of the place by prob. 1, and if the place be in the northern hemisphere, elevate the north pole above the horizon, according to the latitude of the place. If the place be in the southern hemisphere, elevate the south pole above the south point of the horizon, as many degrees as are equal to the latitude.
Having elevated the globe according to its latitude, count the degrees thereof upon the meridian from the equator towards the elevated pole, and that point will be the zenith, or the vertex of the place; to this point of the meridian, fasten the quadrant of altitude, so that the graduated edge thereof may be joined to the said point.
Having brought the sun's place in the ecliptic to the meridian, set the hour index to XII at noon, and the globe will be rectified to the sun's place.
PROBLEM IV. The hour of the day at any place being given, to find all those on the globe, where it is noon, midnight, or any given hour at that time.
On the globes, when mounted in the common
manner, it is now customary to place the hourcircle under the north pole; it is divided into twice twelve hours, and has two rows of figures, one running from east to west, the other from west to east; this circle is moveable, and the meridian answers the purpose of an index.
Bring the given place to the brazen meridian, and the given hour of the day on the hour circle ; turn the globe, till the meridian points at the hour desired; then, with all those under the meridian, it is noon, midnight, or any given hour at that time. *
PROBLEM v. The hour of the day at any place being given, to find the correspondent hour (or what o'clock it is at that time) in any other place.
* Another preferable method, which I generally adopt in the New British Globes, is to place the north horary circles and indexes externally on the meridians, and to make the north polar pia with index to unscrew, so that the circle may be slid to any part of the meridian, to admit the entire free passage of the brass meridian through the horizon, and the circle to be brought over any place for problems relative to bearings, &c. upon the terres. trial globe. This method will answer all the desired purposes ; and it may be added, that a learner need not be under a difficulty with any
constructions of the hour circles of any maker's globes he may not have learnt by, when he has only to consider, that the place, star, planet, &c. is simpày to be brought to the meri. dian, and the hour-index sct to the given time. For hour-circles placed under the meridians, the graduated face of the meridian serves as the index. This method has been approved, and used by the most eminent preceptors.--Edit.
Bring the given place to the brazen meridian, and set the hour circle to the given time; then turn the globe about, until the place where the hour is required comes to the meridian, and the meridian will point out the hour of the day at that place.
Thus, when it is noon at London, it is
PROBLEM VI. The day of the month being given, to find all those places on the globe where the sun will be vertical, or in the zenith, that day.
Having found the sun's place in the ecliptic for the given day, bring the same to the brazen meridian, observe what degree of the meridian is over it, then turn the globe round its axis, and all places that pass under that degree of the meridian, will have the sun vertical, or in the zenith, that day,
A place being given in the torrid zone, to find those two days of the year on which the sun will be vertical to that place.
Bring the given place to the brazen meridian, and mark the degree of latitude that is exactly over it on the meridian; then turn the globe about its