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Euphrof. I thank you, Cleonicus: But there is that ftrait, curious Rod or Wire in the Middle of the Sphere, about which it turns ; is it not what you call the Axis of the Sphere?
Cleon. Yes, Euphrosyne, it is ; and it represents the Axis of the World, and the two Extremities thereof, (N and S) are called the Poles of the World; the former (N) the North Pole, which is above our Horizon; and the latter (S) the South Pole, below the Horizon.
Euphrof. But this fmall Circle, fixed on the Meridian about the North Pole, I presume, you call the HourCircle; from the Hours I fee engraved thereon, and the Hand on the Axle, pointing to them.
Cleon. You judge aright, 'tis called the Hour-Circle ; and is rather an Appendage to the Sphere, than a Part of it: The Use of which, and of all the Circles of the Sphere, great and small, we will more fully consider, and apply, in the next ensuing Opportunities; the Daylight being too far gone to admit any Thing farther on this Subject at present.
and the Names of the Circles which compofe it, Cleonicus; I shall be glad to have a more distinct and particular Explanation of the Nature and Use of those Circles, that I may the more compleatly comprehend the Design of the Machine.
Cleon. I'll now begin to inform you of the Name, Nature, and Use of all the Circles ; how they divide the Sphere; and the Terms of feveral Points and Parts of the Machine derived therefrom.-And first of all, let us consider this broadeft Circle, the Horizon.
Euphrof. With Pleasure I attend you, Cleonicus ; pray then, whence has it the Name? And what does it import? Cleon. The Word is derived from the Greek, and the
Use of the Circle is contained in its Name; for it comes from a Word which fignifies to bound, limit, or terminate; as the Horizon is appointed to shew that Circle in the Heavens, which bounds or terminates the sight of the Spectator any where situated on the Earth.
Euphros. I suppose you mean, that distant Boundary of our Sight, where the Heavens and the Earth seem to join all around us, as it appears from the Top of an high Mountain or Tower.
Cleon. The very fame, Euphrosyne ; 'tis that imaginary Circle which intercepts from our View the Sun, Moon, and Stars, each Night; and when they descend below it, we say, they set; as on the contrary, each Morning, when they appear above it, we say, they rise.
Éuphros. How far is this Circle distant from us in the Heavens ?
Cleon. It is every Way distant from the Point over our Heads in the Heavens just 90 Degrees; and therefore divides the Heavens into two equal Parts, called the Upper and the Lower Hemispheres. Euphrof
. By the upper Hemisphere, I suppose, you mean all that Part of the Heavens which is open to our View; and by the under or lower Hemisphere, all that Part which is hid from our Sight.
Cleon. Very right, Sister, 1 do: And the two Points, (Z and D) which are equally distant froin the Horizon on every Part above and below it, are called the Poles of the Horizon; and further, this which is above it, (Z) and over the Head of the Spectator, is called the Zenith; as that (D) which is under his Feet, is called the Nadir. Of this great Circle Manilius thus fings :
To find the spacious Line, cast round thine Eyes,
This calld the Horizon, is the Round's Design,
Astron. Book I. Euphrof. The Poet intimates what I was just going to observe, viz. that this Circle must always be variable and new, according to the different Situation of the Spectator on the surface of the Globe, or Sphere.
Cleon. Very true, Euphrofyne ; every Person has an Horizon peculiar to himself." Thus the People of Paris have a different Horizon from that of the People at London, or Madrid; their visible Hemisphere extending more to the South than that of the former, and less than that of the latter. All which I shall more fully explain to you on the terrestrial Globe.
Euphros. But, pray, what is this other Circle on the outer Part of the Horizon, marked all round with Letters ?
Ciean. This Circle is called the Compass; it is of great Use at Sea; and also in the Sciences of Astronomy and Geography; for which Reason it is fixed on the Horizon of Spheres and Globes.
Euphrof. What are those various Divisions in it, by which the Letters stand ?
Cleon. They are called the Points of the Compass; and are 32 in Number, each cont ining 11 Degrees. Four of which are called the Cardinal Points, and are marked with the first Letter of their Names, viz. S, the South; W, the Weft; N, the North; E, the East. These four principal Points are sometimes called the four Winds of Heaven. They divide the Horizon, you see, into four Quarters, in each of which are eight Points.
Euphrof. What are the Names of these intermediate Points ?
Cleon. They receive their Names from their relative Distances from the four Cardinal Points, in each Quarter respectively; which Names are denoted by the initial Letters also.
Thus in the Quarter between the South and Weft, the Points are denominated as follows:
5. S. W.
7. W. S. W.
Euphros. I understand you perfectly well, as to that; but what is the Use of these Points ?
Cleon. By them we usually distinguish the Course of the Winds and Clouds, and give them their proper Names ; thus if the Wind blows from the North-Eaj Point, we fay it is a North-East Wind; and the Course of the Clouds we say is South-West, which is opposite to it.
Again, in Geography, we thereby distinguish and relate the Situation and Bearing of Places, Countries, or Cities, one from another : Thus, we fay, Bourdeaux in France lies South of London ; and Bagdat bears South-East from London.
Lastly; in Astronomy, we say, the Sun, Moon, or Planets, rise on such or such a Point; or so many Points from the East or West toward the South or North, which Diftance from the East or West Point is called the Amplitude of their Rising or Setting. I shall not mention the Use of the Compass at Sea, as yet. --These are the chief Uses that it ferves for in common, and are fummarily included in the following Verfos :
The Compass-Points by Art have been design'd,
Euphros. Well, I never knew so much of the Compass, and its Ule, before ; but I fee now it is very considerable: If there remain any other Uses of the Horizon, or Compass, which we have not enquired into, pray, let me know them, Cleonicus.
Cleon. What remains will be better explained when we come to the Globes ; at present we will proceed to the General Meridian, and consider its Properties.
DIALOGUE III. Of the GENERAL MERIDIAN, and the Degrees of
of their Nature and Use; pray, what is implied by the Name of this Circle, the Meridian?
Cleon. It is derived of the Latin Word Meridies, which signifies Mid-day, or Noon; because, when the Sun comes upon the Meridian of any Place, it is then Noon, or Mid-day, at that Place.
Euphros. Very good; but why do you call it the General Meridian?
Cleon. To distinguish it from particular Meridians : Now you must know, that a particular, or special Meridian, is an imaginary Circle in the Heavens, which passes through the Poles of the World, and the Zenith Point of every Person wheresoever situated on the Earth’s Sure face : Consequently, there will be as many such Meridians as you can imagine Points from Eajt to Weft, all round the Earth; for those which lie North and South of each other, have the same Meridian; because these Circles lie North and South themselves. Now, in Spheres and Globes, this general Meridian suffices for all the reft; for since it is fixed, and the Sphere or Globe is moveable about its Axis under it, you may by turning the same, bring any different Part of their Surface under this Meridian, which will then represent the Meridian of that Part or Place, VOL. I.