defcend in the Glafs, till at Length it touches, with its lowest Limb, the horizontal Edge of the Sea: And when I have got it there it will appear at reft, notwithstanding the Quadrant, or Glaffes are moveable by the Motion of the Ship: The Navigator, therefore, has it in his Power to make this Contact as nicely as he pleases; and from what I fhewed you before, the angular Motion of the Image of the Sun is twice as great as that of the Index of the Quadrant: And therefore I need only observe, that that Angle is expreffed in the proper Degrees on the Limb, by reckoning every half Degree a whole one; for you observe, the Limb of the Quadrant is divided into 90 Degrees, whereas it is in Reality but 45, or the 8th Part of a Circle (for which Reafon it is properly an Olant, and not a Quadrant :) And thus the Arch of the Sun's Altitude is, at any Time, very easily taken on the unfteady Bafis of a Ship.

Euphrof. I make no doubt but you can exemplify the Ufe of this Inftrument by Moon-light, as well as that of the Sun: If so, there is now as delightful a clear Moon as you could wish,

Cleon. There is fo: And I fhall command the Queen of Night to defcend, in Complaifance to you, from her lofty ætherial Seat, and be perched upon the Top of yonder Chimney, without any Power to move: or, to be ferious, I will inftantly measure her Height above that Chimney, which the feems to be perpendicular over.→→→→ Here, take the Quadrant, and hold it just in the Pofition as you fee me do, and, looking through the finall Hole in the Sight-Vane, you will perceive the Moon depreffed to the Edge of the Chimney; and moving the Quadrant any how, you will observe the Image of the Moon to be as immoveable as the Chimney itself.

Euphrof. I do, indeed; and fo curious a Sight I never before beheld: For, notwithstanding I move the Quadrant, or Glaffes, the Moon appears as abfolutely fteady as if nailed on to the Top of the Chimney. -Now let me fee the Number of Degrees on the Limb,the Vernier cuts 27° 30'; that, I prefume, is what you call the Height of the Moon above the Top of the Chimney?

Cleon. It is: And after the fame Manner you will find the Degrees contained in an Arch of the Heavens between the Moon and any particular Star, which is called the Distance of the Moon from the Star. Thus you obferve yonder bright Star, called Regulus, or the Lion's Heart; the Distance of that Star from the Moon I easily find, by putting the Quadrant into a proper Position for bringing the Reflection of the Star upon the quickfilvered Part of the fmall Glafs, and then I move the Index till I' bring the Image of the Star to touch the Limb of the Moon next to it, and then you will obferve the curious Appearance they make both together:Here, take the Quadrant, and put it into the fame Pofition for your; Qwn Eye.

Euphrof. This I will do as foon as I can:-I find it requires a little Practice to be ready at this Matter:-L have juft now brought it to bear, and see the Star nicely touch the Limb of the Moon next to it.-This is a very pretty Phænomenon, indeed.

Cleon. On the Limb, you obferve, the Index has moved over 23 Divifions, which is therefore the Number of Degrees contained in the Arch of a great Circle, between the Moon and the Star: And to measure this Distance of a Star or the Sun from the Moon, is a principal, and moft ufeful Problem in Navigation, and was the original grand Defign of this Invention, by Sir Ifaac Newton.

Euphrof. You have thoroughly convinced me of the important Ufes of this valuable optical Inftrument :But I imagine, fince you can fo readily measure the Arch of Distance between any two Objects, it must be of Ule in many Cafes on Land, as well as at Sea: Is it not, Cleonicus?

Gleon. You may think it very strange, that a Perfon hould be able, by any Inftrument whatever, to take the Height of the Sun, or any other Object, in a Ship rolling on the Waves of the Sea, more certainly, than on the firmeft footing by Land: But you will foon recollect, that at Sea there is fometimes a clear and evident HORIZON, from which we can measure the Height of the Sun, Moon, or Star; but, on Land, fuch an Horizon is wanting, and therefore this excellent Inftrument is but

of

of little ufe in the common Conftruction: But it is yet capable of an artificial Horizon being adapted to it, and thereby the Altitude of the Sun may be obferved, not only by Land, but likewife at Sea in foggy Weather, when the Sun, but no Horizon, can be feen: By this Means alfo, it may be rendered of very great Ufe in Surveying, and in all Cafes where a Theodolite, Circumferentor, or common Quadrants are applicable.

This new Conftruction of a Quadrant I fhall take fome other Opportunity of giving you a farther Infight into, having faid, at prefent, as much as I thought neceffary, to give you a general Idea of this noble Invention, as it pertains to the Clafs of optical Inftruments.

DIALOGUE XVI.

The Defcription and Ufe of the HELIOSTATA, or the folftitial and planetary CLOCK, on a new Conftruction, for fixing the SUN, MooŃ, and PLANETS in aftronomical OBSERVATIONS.

I

Cleonicus.

Have referved this Opportunity as the most convenient, for inftructing you in the Knowledge of the Nature and Ufe of an Inftrument, whofe Theory is the moft fublime of all in Philofophy. This grand Invention, like all others, has arrived at its present Perfection by feveral Gradations. Its firft Principle is founded in Optics, and its general Nature is derived from optical, mechanical, and aftronomical Principles; and this you will eafily conceive when I tell you, that it is a Clock of such a peculiar Nature, as to fix the Rays of Light from any radiant Object, and thereby to render the Sun Beams of Ufe in the Camera Obfcura more perfectly than any Inftrument

*It was not judged neceffary to exhibit, in a Plate, any Thing relating to the Structure or Ufes of the Quadrant, as that is fully done in a fmall Treatife, which is given with every Inftrument of this Kind that is fold.

Inftrument has hitherto done; and alfo, what is still of much greater Confequence, the Aftronomer can hereby detain the Sun, Moon, or Planet, in his Telescope,-in a quiefcent State; by which Means they become the proper Subjects of his Obfervation, and that for any Time propofed. This hath been hitherto the Defideratum of Aftronomy; for as often as the Aftronomer has been intent upon determining the Eclipfes of the Sun and Moon, upon delineating their Macula, or Spots, or meafuring their apparent Diameters with a Micrometer, fo often he could wish to fay, Sun, ftand thou ftill; and thou, Moon, proceed no farther in thy Path: Since the conftant Motion of thofe Luminaries, in the Telescopes, does every Moment elude his utmost Efforts to accomplish the abovementioned Defigns.

Euphrof. Well, and are thofe great Points effected now by this new Clock, which I here obferve in fo elegant a Conftruction?

Cleon. They are, in a Degree fuperior to any Thing that could be expected in fo complicated a Machine, and fully fufficient to answer all the Intentions of the most accurate Aftronomer.It may feem very ftrange, that though it be wholly impoffible to ftop the Sun in its Courfe, yet that it fhould be fo eafy to fix, as it were, his Rays, and thereby render him apparently motionless : But nothing has been fo arduous and paradoxical, which Philofophers have not, at one Time or other, endeavoured to effect, and often have found, to their great Satisfaction and to the Perfection of the Sciences, that they are far lefs fo in Fact than in Appearance.-The first Step towards the Difcovery of this univerfal, optical, and aftronomical Clock, was the Invention of Sir Ifaac Newton, in regard to fixing the Image of the Sun, by two Reflections from two Speculums in Hadley's Quadrant, as I explained to you in the laft Conversation: and fince it appeared, from the Experiment in the fmall. Machine you then faw, that the Beam of Light from moveable Object was rendered stable by one Reflection, and of Confequence the Image was thereby rendered quiefcent, or at reft, by a Reflection of that fixed Ray, it was easy to conclude, that if Rays, thus fixed by al firft Reflection, could be thrown upon the Object-glafs VOL. II.

X

a

of

of a Telescope, by Reflection from a fecond Speculum, then might the Image of the Object be made to appear at Reft through fuch a Telescope, and this was foon found to be practicable, by Experiment: But then they found a conftant manual Operation was neceffary, to keep the first Speculum moving, fo as always to keep the reflected Rays in one and the fame Pofition in the Telescope.- -Thus far went the illuftrious Inventor himfelf here in England, and the celebrated Farrenheit and Boffat, abroad.

Euphrof. This seems to be a large Step taken at once; but, pray, what was the next Enterprise, to render this Machine more perfect?

Cleon. The next Improvement was an Addition of Clock-work, by the late learned S' Gravefande, who, in a moft fuperb Manner, conftructed one of these Machines, by which the folar Beams, reflected from a fingle Speculum, were rendered permanent, and confequently fit for all Experiments of the Camera Obfcura, and all the Purposes of telescopic and aftronomical Obfervation.

Euphrof. No doubt but fuch an Atchievement in the Sciences was received with great Eagernefs and Applaufe: I fuppofe, those who had Occafion for trying Experiments on Prifms, the folar Microfcope, &c. took the firft Opportunity of availing themfelves of this Invention, and that no public or private Obfervatory would be without fuch a telescopic Clock.

Cleon. Alas! my Euphrofyne, it is much to be wished, the Fate of the Sciences were not fo much like that of Virtue itself, viz. to languifh under univerfal Applause: For notwithstanding the Philofopher had, by this Means, rendered the Sun obedient to all their Wills, and fixed him for their View for every Kind of Obfervation, yet have I never heard of a Purchafer of one of thofe noble Machines, nor have I convertel with any Gentlemen who had ever so much as feen them. I thought it would reflect great Difhonour for fuch an Invention not to be found in any English Author, and therefore procured a Copper-plate of it to be engraved and published, to gratity fuch English Artifts who might be willing to enquire into its Nature and Ufes; and this, as far as I