Page:Cyclopaedia, Chambers - Volume 2.djvu/633

 RHU

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RHU

■which began with Monofyllables, and were continued in Words growing gradually longer and longer to the laft, which was the longeft of all. See Verse.

They had their Name from the Greek £«r«Ai», a Club, which, like them begins with a {lender tip, and grows bigger and bigger to the Head— Such is that Verfe of Homer,

And this Latin one,

SpesDeus aterna eft flat ionis Conciliator.

RHOPOGRAPHI, in Antiquity, a Name given to certain Painters, who confined themlelvea to low Subjects; fuch as Ani- mals, Plants, Landskips, &c.

The Title was alfo given to fuch as cut Figures of Men, &c. in Box., Phileria, Yew, &c. in Gardens.

The Word isform'd from the Greek feTe$, Toys, or odd Ware, and ygeupwj 1 write, I paint.

RHUBARB, a medicinal Root, large, compact, and heavy; yellow without, of a Nutmeg Colour within ; an aftringent, bit- teriih Tafte; and an agreeable, aromatic Smell; Of great ufe in Medicine, as a Purgative. See Root and Purgative.

Confidering the mighty Ufe of this Drug, 'tis iurprizing we fhould know Co little of the Place where the Plant grows that produces it. — Some fry it comes from the Kingdom of Routan, in the Extremities of India', others, that 'tis found in the Pro- vinces of Xmji and Suchen in China. Others will have it only grow in Perfia; and others on the Confines of Mufcovy; deriving its Name from Rha, the River among us calTd Volga, and Bar- barum, q. d. Root found by the Barbarians on the River Rba.

Rhubarb, in Lathi, Rhabarbarum Officmarwu, was not known of the Antients; and their Rhaponticum, which indeed refembles it, is not the real Rhubarb. See Rhaponticum.

Good Rhubarb fteep'd in Water gives it a Saffron Colour; and when broke, looks bright, with fomewhat of a Vermillion Caft. — Some Druggifts have the Art of renewing their old Roots, by giving them a yeliow hue ; but the Cheat is eatily known by handling them; the Powder wherewith they colour them {ticking to the Fingers.

Rhubarb is efteemed an excellent Cathartic ; proper for a Diar- rhsea, to fortify the Stomach, and to create an Appetite.

M. Boulduc informs us, in the Memoirs of the Trench Academy, that he drew Extracts fromj Rhubarb both with Water and Spirit of Wine,* but the purgative Virtue, he fays, was much more vi- fible in the former than the latter; which fhews that it confifts more in a Salt than an Oil.— He observes, however, that Rhu- barb taken in Subftance, has a ftiil better effect ; and adds, that of all the Operations he could make with it, none gave any Credit to the common Opinion, which afcribes an aftringent Quality to it.

Monks Rhubarb. See Rhaponticum.

White Rhubarb. See Mechoacan.

RHUMB, or Rumb, or Rum, Rhombus, in Navigation, a vertical Circle, of any given Place; or the Interferon of a Part of fuch a Circle with the Horizon. See Vertical.

Rhumbs therefore coincide with Points of the World, or of the Horizon. See Point and Horizon.

And hence the Mariners diftinguifh the Rhumbs by the fame Names as the Points, and Winds. See Wind.

They ufually reckon 32 Rhumbs ; which are reprefented by the 32 Lines in the Rofe, or Card of the Compafs. See Com- pass.

Aubin defines a Rhumb by a Line on the terreftrial Globe, Sea-Compafs, or Sea-Chart, reprefenting one of the 32 Winds which ferve to conduct a VelTel.— So that the Rhumb a Vcfiel purfucs is conceiv'd as its Route or Courfe. See Course.

Rhumbs are divided, and fubdivided like Points. — Thus, the •whole Rhumb anfwers to the Cardinal Point.— The half Rhumb, to acollaferalPoinr, or makes an Angle of 45 ° Degrees with the for- mer. — The Quarter Rhumb makes an Angle of 22 30 7 there- with.— And the half-quarter Rhumb makes an Angle of n Q 15'. See Cardinal, Collateral, Quarter, &c.

A Table of the Rhumbs, or Points, and their Diflances from the Meridian, fee under the Article Wind.

Rhumb-Line, or Loxwdromia, in Navigation, is the Line which a Ship keeping in the fame collateral Point or Rhumb, de- fcribes, throughout its whole Courfe. See Loxodromia.

The great Property of the Rhumb Line, or Loxodromia, and that from which fome Authors define it, is, that it cuts all the Meridians under the fame Angle. See Meridian.

This Angle is call'd the Angle of the Rhumb, or Loxodromic Angle. See Angle.

The Angle which the Rhumb Line makes with any Parallel to the Equator is call'd the Complement of the Rhumb. See Com- plement.

Nature and Origin of the Rhumb-Lines.

An Idea of the Origin and Properties of the Rhumb-Line, the

great Foundation of Navigation, may be conceived thus,— A

Veffel beginning its Courfe, the Wind wherewith it is driven makes a certain Angle with the Meridian of the Place; and as 'tis fuppofed the VelTel runs exactly in the Direction of the Wind, it makes the fame Angle with the Meridian which the Wind makes.

Suppofing, then, the Wind to continue the fame, as each Point or lnilant of the Progref, may be efteem'd the Be- ginning,- the Veffel always makes the lam:: Angle with the Me- ridian of the Place where it is each Moment, or in each Point of its Courfe, which the Wind makes.

Now, a Wind, e. gr. that is North-Ea£t, and which of Con- fequence makes an Angle of 45 °, with the Meridian, is equally North-Eaft wherever.it blows; and makes the fame Angle of 45 with all the Meridians it meets.— A Veffel therefore driven by the fame Wind, always makes the fame Angle with all the Meridians it meets withal on the Surface of the Earth.

If the VelTel fail North and South, it makes an Angle infi- nitely acute with the Meridian, i. e. 'tis parallel to it, or rather fails in it.— If it run Eaft and Welt, it cuts all the Meridians at right Angles.

In the nrft Cafe it defcribes a great Circle; in the fecond, either a great Circle, sws. the Equator, or a Parallel to ic— If its Couric be between the two, it does no: then defcribe a Cir- cle ; fince a Circle drawn in fuch a manner would cut all the Meridians at unequal Angles, which the Veflel cannot do.

It defcribes, therefore, another Curve, the effential Property whereof is, that it cuts all the A'leridians under the fame Angle. —This Curve is what we call the Loxodromic Curve, Rhumb-Line, or Loxodromy. See Curve.

s Tis a kind of Spiral, which, likethe Logarithmic Spiral, makes

an Infinity of Circumvolutions without ever arriving at a certain Point to which it ftill tends, and towards which it approaches e- very ftep. See Spiral.

This afymptotic Point of the Rhumb-Line is the Pole ; to which were it poflibie for it to arrive, it would rind all the Meri- dians conjoin'd, and be loft in them. See Pole.

The- Courfe of a Velfel, then, except in the two firft Cafes, is always a Rhumb-Line; which Line is the Hypotheiuife of a Redhngle-Tri angle, whole two other Sides are the Ships Way or Diftance run in Longitude and Latitude. Now the Latitude is ufually had by Observation, (See Latitude,);; and the Angle of the Rhumb, with one or other of the two Sides, by the Com- pais. See Compass.

All therefore that is required by Calculation in Sailing, is the Value of the length of the Rhumb-Line, or Ships Courle. See Course.

But as fuch Curve Line would prove very perplexing in the Calculation; it was neceffary to have the Ships Way in a right Line; which right Line however muff, have the effential Proper- ty of the Curve Line, to, to cut all the Meridians at right An- gles — The Method of c feeling which fee under the Article Chart and Mercator.

. If PA, PF, PG, &c {Tab. Na-i-igation, Frg. 19.) be fuppofed Meridians, AI the Equator, and AE another great Circle of the Sphere ; AO will represent a Rhuwb-Lhie, the Angles whereof with the fevera! Meridians, being leis than thofe of rhe great Circle; it follows, that the Rhumb js not a great Circle of the Sphere—If a Ship therefore be at firft directed towards E, and conftantly perfift in the fame Rhumb, it will never arrive at the Place E, but at the Place O which is further from the Equator.

Hence, as on the Surface of a Sphere, the fhorteft Way be- tween A and O is an Arch of a great Circle between A and O; the Rhumb-Line is not the morteic Way, or Icaft Diftance from one Place to another. See Circle, Sphere, Distanxe,

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Application of Rhumb-Lines in Navigation.

i°. If the Meridians PA, PC, PD, &c. be not very far a- part, the Rhumb-Line AIHG is divided by the equidiftanr Paral- lels BI, KH, FG, &c. into equal Parts.

Hence, i°. the Pans of the Rhumb AI, and AG, are as the Latitudes AL and AN of the Places I and G.— 2. Since the Arches AB, IK, HF, are equal in Magnitude, and therefore un- equal in Number of Degrees; the Sum of the Arches, call'd the Latus Mecofynamhum, or Mihs of Longitude, is not equal to the Difference of Longitude AB of the Places A and G- See Mecodynamicum.

2 . The Length of the Rhumb-Line AG is to the Change or Difference of Latitude GD, in the fame Ratio, as the whole Sine to the Co-fine of the Angle of the Rhumb,

Hence, i°. the Rhumb fail'd on being given, together with the Difference or Change of Latitude, tura'd into Miles; the length of the Rhumb-Line, or the Diftance from the Place A to the Place G upon the fame Rhumb, is had by the Rule of Three. — 2 . The "Rhumb being given, together with the Quantity of the Ship's Way on the fame Rhumb, \. e. the length of the Rhumb AG; the Difference of Latitude DG, is had by the Rule of Three, in Miles to be converted into Degrees of a great Circle.— 3 °. The Difference of Latitude DG being given in Miles; as alfo the Length of the Rhumb-Line AG ; the Angle of the Rhumb, and

confe.