An essay on the use of celestial and terrestrial globes; exemplified in a greater variety of problems, than are to be found in any other work; exhibiting the general principles of dialing & navigation. / By the late George Adams, mathematical instrument maker to His Majesty, and optician to the Prince of Wales.

PART I. A TREATISE ON THE USE OF THE TERRESTRIAL & CELESTIAL GLOBES.

OF THE ADVANTAGES OF GLOBES IN GENERAL, FOR ILLUSTRATING THE PRIMARY PRINCIPLES OF ASTRO∣NOMY AND GEOGRAPHY; AND PARTICULARLY OF THE ADVANTAGES OF THE GLOBES, WHEN MOUNTED IN MY FATHER'S MANNER.

UNIVERSAL approbation, the opinion of those that excel in science, and the ex∣perience of those that are learning, all concur to prove that the artificial representations of the earth and heavens, on the terrestrial and celestial globes, are the instruments the best adapted to convey natural and genuine ideas of astronomy and geography to young minds.

This superiority they derive principally from their form and figure, which communi∣cates a more just idea, and gives a more ade∣quate

representation of the earth and heavens, than can be formed from any other figure.

To understand the nature of the projection of either sphere in plano, requires more know∣ledge of geometry than is generally possessed by beginners, it's principles are more recluse, and the solution of problems more obscure.

The motion of the earth upon it's axis is one of the most important principles both in geography and astronomy; on it the greater part of the phenomena of the visible world de∣pend: but there is no invention that can com∣municate so natural a representation of this motion, as that of a terrestrial globe about it's axis. By a celestial globe the apparent mo∣tion of the heavens is also represented in a na∣tural and satisfactory manner.

In order to convey a clear idea of the va∣rious divisions of the earth, of the situation of different places, and to obtain an easy solution of the various problems in geography, it is necessary to conceive many imaginary circles delineated on it's surface, and to understand their relation to each other. Now on a globe these circles have their true form; their inter∣sections and relative positions are visible upon the most cursory inspection. But in objec∣tions of the sphere in plano, the form of these circles is varied, and their nature changed; they are consequently but ill adapted to convey

to young minds the elementary principles of geography.

On a globe, the appearance of the land and water is perfectly natural and continuous, fitted to convey accurate ideas, and leave per∣manent impressions on the most tender minds; whereas in planispheres one-half of the globe is separated and disjoined from the other; and those parts, which are contiguous on a globe, are here separated and thrown at a distance from each other. The celestial globe has the same superiority over projections of the hea∣vens in plano.

The globe exhibits every thing in true pro∣portion, both of figure and size; while on a planisphere the reverse may often be observed.

Presuming that these reasons sufficiently evince the great advantage of globes over either planispheres or maps, for obtaining the first principles of astronomical and geographi∣cal knowledge, I proceed to point out the pre∣eminence of globes mounted in my father's manner, over the common, or rather the old and Ptolemaic mode of fitting them up.

The great and increasing sale of his globes mounted in the best manner, may be looked upon at least as a proof of approbation from numbers; to this I might also add, the en∣couragement they have received from the principal tutors of both our universities, the

public sanction of the university of Leyden, the many editions of my father's treatise on their use, and its translation into Dutch, &c. The recommendation of Mess. Arden, Walker, Burton, &c. public lecturers in natural philoso∣phy, might also be adduced: but leaving these considerations, I shall proceed to enumerate the reasons which give them, in my opinion, a decided preference over every other kind of mounting.* 1.1

The earth, by it's diurnal revolution on it's axis, is carried round from west to east. To represent this real motion of the earth, and to solve problems agreeable thereto, it is neces∣sary that the globe, in the solution of every problem, should be moved from west to east; and for this purpose, that the divisions on the large brass circle should be on that side which looks westward.* 1.2Now this is the case in my father's mode of mounting the globes, and the tutor can thereby explain with ease the ra∣tionale of any problem to his pupil. But in the common mode of mounting, the globe must be moved from east to west, according to the Ptolemaic system; and consequently, if the tutor endeavours to shew how things obtain in nature, he must make his pupil unlearn in a degree what he has taught him, and by ab∣straction reverse the method he has instructed him to use; a practice that we hope will not be adopted by many.

The celestial globe being intended to re∣present the apparent motion of the heavens, should be moved, when used, from east to west.

Of the phenomena to be explained by the terrestrial globe, the most material are those which relate to the changes in the seasons; all the problems connected with, or depending upon these phenomena, are explained in a clear, familiar, and natural manner, by the globe, when mounted in my father's mode; for on rectifying it for any particular day of the month, it immediately exhibits to the pupil the exact situation of the globe of the earth for that day; and while he is solving his problem, the reason and foundation of it presents itself to the eye and understanding.

The globe may also be placed with ease in the position of a right sphere; a circumstance exceedingly useful, and which the old con∣struction of the globes did not admit of.

By the application of a moveable meridian, and an artificial horizon connected with it, it is easy to explain why the sun, although he be always in one and the same place, appears to the inhabitants of the earth at different alti∣tudes, and in different azimuths, which cannot be so readily done with the common globes.

On the celestial globe there is a moveable circle of declination, with an artificial sun.

The brass wires placed under the globes, serve to distinguish, in a natural and satisfac∣tory manner, twilight from total darkness, and the reason of the length of it's duration.

The next point, wherein they materially differ from other globes, is in the hour circle. Now it must be confessed, that to every contriv∣ance that has been used for this purpose there is some objection, and probably no mode can be hit upon that will be perfectly free from them. The method adopted by my father ap∣pears to me the least exceptionable, and to possess some advantages over every other me∣thod I am acquainted with. Agreeable to the opinion of the first astronomers, among others of M. de la Lande, he uses the equator for the hour circle, not only as the largest, but also as the most natural circle that could be employed for that purpose, and by which alone the solu∣tion of problems could be obtained with the greatest accuracy. As on the terrestrial globe, the longitude of different places is reckoned on this circle; and on the celestial, the right ascension of the stars, &c. it familiarizes the young pupil with them, and their reduction to time. This method does not in the least im∣pede the motion of the globe; but while it affords an equal facility of elevating either the north or south pole, it prevents the pupil from placing them in a wrong position; while the

horary wire secures the globe from falling out of the frame.

Another circumstance peculiar to these globes, is the mode of fixing the compass. It is self-evident, that the tutor, who is willing to give correct ideas to his pupil, should al∣ways make him keep the globes with the north pole directed towards the north pole of the heavens, and that, both in the solution of pro∣blems, and the explanation of phenomena. By means of the compass, the terrestrial globe is made to supply the purpose of a tellurian, when such an instrument is not at hand. I cannot terminate this paragraph, without testifying my disapprobation of a mode adopted by some, of making the globe turn round upon a pin in the pillar on which it is supported; a mode, that, while it can give little but relief to indo∣lence, is less firm in it's construction, and tends to introduce much confusion in the mind of the pupil.

In order to prevent that confusion and per∣plexity which necessarily arises in a young mind, when names are made use of which do not properly characterize the subject, my fa∣ther found it necessary, with Mr. Hutchins, to term that broad wooden circle which sup∣ports the globe, and on which the signs of the ecliptic and the days of the month are engraved, the broad paper circle, instead of horizon, by

which it had been heretofore denominated. The propriety of this change will be evident to all those who consider, that this circle in some cases represents that which divides light from darkness, in others the horizon, and some∣times the ecliptic. For similar reasons, he was induced to call the brazen circle, in which the globes are suspended, the strong brass cir∣cle.

In a word, many operations may be perform∣ed by these globes, which cannot be solved by those mounted in the common manner; while all that they can solve may be performed by these, and that with a greater degree of perspicuity; and many problems may be performed by these at one view, which on the other globes require successive operations.

But as, notwithstanding their superiority, the difference in price may make some persons prefer the old construction, it may be proper to inform them, that they may have my father's globes mounted in the old manner, at the usual prices.

Notes

• * 1.1

  The following note from Mr. Walker's Easy Introduc∣tion to Geography, in favour of my father's globes, will not, I hope, be deemed improper.

  "Simplicity and perspicuity should ever be studied by those who cultivate the young mind; and jarring, opposing, or equivocal ideas should be avoided almost as much as error or falsehood. Our globes, till of late years, were equipt with an hour circle, which prevented the poles from sliding through the horizon; hence their rectification was generally for the place on the earth, instead of the sun's place in the ecliptic; which put the globe into so unnatural and absurd a position re∣specting the sun, that young people were confounded when they compared it with the earth's positions during it's annual rotation round that luminary, and considering the horizon as the boundary of day and night. Being, therefore, sometimes obliged to rectify for the place on the earth, and sometimes for the sun's place in the ecliptic, the two rules clash so un∣happily in the pupil's mind, that few remember a single pro∣blem a twelvemonth after the end of their tuition. Globes, therefore, with a horary circle, are but partially described in this treatise; the great intention of which is, to make the elevations and depressions of the poles of a terrestrial globe to represent all the situations the earth is into the sun, for every day or hour through the year. The globes of Mr. Adams are the most favourable for the above mode of rectification of any plates we have at present; and to make a quiescent globe to represent all the positions of one revolving round the sun, turning on an inclined axis, and keeping that axis altogether parallel to itself, his globes are better adapted than any, I believe, in being."
• * 1.2

  See the Rev. Mr. Hutchin's New Treatise on the Globes.