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1'beVfe of the Azimuth Compass, is for finding the Sun s magnctical Azimuth, or Amplitude ; and thence the Va- riation of the Compafi.
If the Obfervation be for an Amplitude at Sun-rifing, or an Azimuth before Noon, apply the Centre of the Index lc on the Weft Point of the Card, within the Box ; fo that the four Lines on the Edge of the Card, and thofe on the infide of the Box may meet. If the Obfervation be for the Sun's Amplitude fetting, or an Azimuth in the Afternoon) turn the Centre of the Index right againfl the Eaft Point of the Card, and make the Lines within the Box concur with thofe on the Card : The Inftrument thus fitted for Obfer- vation, turn the Index be towards the Sun, till the Shadow of the Thread de fall directly on the Slit of the Sight, and on the Line that is along the middle of the Index - then will the inner Edge of the Index cut the Degree and Minute of the Sun's magnctical Azimuth from the North or South.
But note, that if, when the Centpafi is thus placed, the Azimuth is lefs than 4.5° from the South, and the Index be turn'd towards the Sun, it will pafs off the Divifions of the 'Limb : the Inftrument, therefore, in this Cafe, mull be turn'd juft a quarter of the Compafi ; i. e. the Centre of the Index mail be plac'd on the North or South Point of the Catd, according as the Sun is from you ; and then the Edge will cut the Degree of the magnetic Azimuth, or Sun's Azi- muth from the North, as before. See Amplitude.
The Sun's maanetical Amplitude thus found, the Varia- tion of the Needle is thus determin'd.
Being out at Sea the 1 jth of May 171 5, in 45 N. Lat. the Tables give me the Sun's Latitude 15° North, and his Eaft Amplitude 27 25' North : By the Azimuth Compafi, 1 find the Sun's magnctical Amplitude at his riling and fet- ting ; and find he riles, v.g. between the 6zA and iJjd Deg. reckoning from the North towards the Eaft Point of the Compafi, i.e. between the 27th and 28th Deg. reckoning from the Eaft.
The magnetical Amplitude, therefore, being here equal to the true one, the Needle has no Variation : But if the Sun at his riling ftlould have appear'd between the 52d and 53d Deg. from the North towards the Eaft; his magneti- cal Amplitude would then be between 37 and 38 Deg. i. e. about 10 Deg. greater than the true Amplitude : therefore, the Needle would vary about 10 Deg. North-Eafterly.
If the magnetical Eaft Amplitude found by the Inftru- ment, fliould be lefs than the true Amplitude, their Diffe- rence would /hew the Variation of the Needle Eafterly.
If the true Eaft-Amplitude be Southwardly, as alio the magnetical Amplitude and this laft be the greater ; the Va- riation of the Needle will be North- Weft, and vice verfa.
What has been faid of North-Eaft Amplitudes, holds al- fo of South- Weft Am plitudes. And what of South-Eaft Am- plitudes, holds of North-Weft Amplitudes. See Amplitude.
Laftly, if Amplitudes be found of different Denomina- tions, v. g. if the true Amplitude be 6 Deg. North, and the magnctical Amplitude 5 Deg. South ; the Variation, which in this Cafe is North- Weft, will be equal to the Sum of the magnetical and ttue Amplitudes : Lnderftand the fame for Weft Amplitudes.
The Variation may likewife be found from the Azimuth : but in that Cafe, the Sun's Declination, Latitude of the Place, and his Altitude muft be given, that his true Azi- muth may be found. See Azimuth.
Compass of 'Proportion. See Sector.
Com? &ss-Z)ials, are fmall Dials, fitted in Boxes, for the Pocket, to fhew the Hour of the Day by direction of the Needle ; which ftiews how to place them right by turning the Dial about, till the Cock or Style ftand directly over the Needle, and point up to the Northward ; but thefe can ne- ver be very exaa, becaufe of the Variation of the Needle it felf. See Dial.
COMPASSES, or "Pair of COMPASSES, a Mathemati- cal Inftrument, ufed for the defcribing of Circles, meafuring the Diftances of Points, Lines, (go. See Circle, Line, Sgc.
The common Compajfes confift of two Branches or Legs, of Iron, Brafs, or other Metal, pointed at bottom ; and a-top join'd by a Rivet, whereon they move, as on a Centre.
The Invention of Compajfes is afcrib'd to Calus, Nephew of plains by his Sifter, whom the Poets fay, T)<sdalus kill'd out of envy.
We have Compajfes now of various Kinds and Contrivan- ces, accommodated to the various Ufes they are intended for : As,
Hair-Corn? asses, fo contrived within-fide, as to take an Extent to a Hair's-brcadth.
Geman Compasses, whofe Legs are a little bent out- wards towards the top : fo that when Ihut, only the Points meet.
l ** % ? ' nt8 are ufually made to take off, and on ; and other Ioints for particular occafixns put in their Places ; as Drawing-pen Point, Dotting-whecl Point, Potte-crayon Point, i£r.
Spring-Corn- asses, or dividers, made of harden'd Steel,
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the Head arch'd • which, by its Spring, opens the Compaj- fes; the opening be.ng directed by a circular Screw, faften'd to one Leg and let thro' the other, work'd with a Nut
Clock-makers Compasses are very fubftantial.ferving'toctit Paftboard, Brafs, &p. jointed like the common Comiaflhs with a Quadrant, or Bow, as the Spring- Compares ■' only its ufe different; as ferving here, to, keep the Inftrument farm at any opening.
'turn-up Compasses, a late Contrivance to fave the trou- ble of changing the Points : The Body is like the common Compajfes ; towards the bottom of the Legs, without-fide; are added two other Points, befides the ul'ual ones ; the one carrying a drawing Pen-point, the othet a Porte Craion 5 both adjufted fo as to turn round, and fo be in the way of ufe, or out of it, as occafion requires.
The Points of fmall Cempafjes are tetnper'd by a Lamp and Blow-pipe, heating 'em red-hot; when cold they are hard : The larger are temper'd by a charcoal Fire and a Blow-pipe, heating them to a Cherry-colour, then plunging them in Water. See Tempering.
Compasses of three Branches. Their Struflure is like that oUhe common Compajfes, fetting afide the Excefs of a Leg : Their ufe is to take three Points at once ; and lb to form Triangles ; to lay down three Pofnions of a Map to be copied at once, He.
TriffcBing Compasses, the Invention of M. tarragon, for theTriffeaionof Angies, geometrically. See Trissection.
The Inftrument confifts of two central Rules, and an Arch of a Circle of 120 Deg. immoveable, with its Radius : The Radius is faften'd with one of the central Rules, like the two Legs 1 of 3. Seflor, that the central Rule may be carried thro' all the Points of the Circumference of the Arch. The Ra- dius and Rule to be as thin as poffible, and the Rule faften'd to the Radius hammet'd cold, to acquire an Elafticity : The breadth of the other central Rule, to be triple the breadth of the Radius. In this Rule to be a Groove, with a Dove-tail, to be faften'd on it, for its Motion : In the Cen- tre of each Rule muft likewife be a Hole. See the fount, de Scavans, Sept. 1688.
Beam Compasses, confift of a long Branch, or Beanij carrying two brafs Curfors ; the one fix'd at one end, the other Hiding along the Beam, with a Sctew to fallen it, on occafion. To the Curfors may be fcrew'd Points of any kind ; whether Steel, for Pencils, or the like. It is ufed to draw large Circles, take great Extents, t£c.
Elliptick Compasses. Thcit ufe is to draw Ellipfes, or Ovals of any kind : They confift of a Beam A B,' (Piaiei Geom. Fig. 2.) about a Foot long, bearing three Curfors ; to one of which may be fcrew'd Points of any kind : to the bottom of the other two are rivetted two Aiding Dove-tails, adjufted in Grooves made in the crofs Branches of the Beam; The Dove-tails having a Morion every way, by turning a- bout the long Branch, go backwards and forwards along'the Crofsi fo that when the Beam has gone half w.iy about, one of thefe will have mov'd the whole length of one of the Branches ; and when the Beam has got quite round, the fame Dove-tail has got back the whole length of the Branch. Underftand the fame of the other Dove-tail.
Note, the Diftance between the two Hiding Dove-tails, is the Diftance between the two Foci of the Eliipfis ; fo that by changing that diftance, the Eliipfis will be rounder or flenderer. Under the Ends of the Branches of the Cro.lj are placed four Steel Points to keep it fall.
The ufe of this CompaJ's is eafy; by turning round the long Branch, the Ink, Pencil, or other Point, wili draw the Eliip- fis requir'd. Its Figure Ihews both its Ufe and Conftruclion.
Cylindrick and Spherick Comp asses, ufed in taking the Diameter, Thicknefs, of Calliber of round, or cylindrick Bo- dies; as, Cannons, Pipes, He. They confift (of tour Branches join'din a Centre ; two of 'em circular, two flat.a little bent at the Ends.
To ufe 'em, one of the flat Points is put within the Can« non, the other without : the two oppofite Points fhew the) Thicknefs. See Calliper.
There are alfo Spherical Compajfes, differing in nothing from the common ones, but that their Legs are arch'd ■ fer- ving to take the Diameters of round Bodies, £ijc.
'Proportional Compasses, confift of two Branches, (Plate: Geom. Fig. 3.) each pointed at cither end with Steel : the length of the Branches is cut thro', for a Curfor to Aide up and down ; in the middle of which Curfor is a Screw, fer- ving to join the Branches, and to fix 'em at any Point re- quir'd.
On the one Leg are Divifions, ferving to divide Lines into any number of equal Parrs, for reducing of Figures, tSei On the other are Numbers, for the inferibing any regular Polygon in a Circle propos'd.
The Ufe of the firfi is eafy : Suppofe, n. g. a right Line* to be divided into three equal Parts ; pufti the Curfor till the Screw be juft on the Figure 3 ; where fixing it, take' the length of the given Line between the longeft Parts of the Legs 1 the diftance between the two fhorteft, will b6
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