Page:Cyclopaedia, Chambers - Volume 1.djvu/1012

 HOR

( 2 $2 )

HOR

Horizon, in Geography, is a Circle paffing over a given Part of rhe Earth, and dividing the viiible Fart of the Earth and Heavens, from that which is invilible. See Earth.

The Altitude or Elevation of any Point of the Sphere, is an Arch of a Vertical Circle, intercepted between it and the ienfible Horizon. See Altitude and Elevation,

This is peculiarly denominated fenflbk Horizon, to diftin- guifh it from the rational or true t which paffes thro' the Centre of the Earth ; as already obierv'd.

By fenfible Horizon is alfo frequently meant a Circle, which determines the Segment of the Surface of the Earth, over which the Eye can reach j call'd alfo the <Phyfical Horizon.

In this Senfe we fay, a fpacious Horizon, a narrow fcan- ty Horizon. To find the Extent of the Horizon, or how far a Man's Profpect reaches, by means of the Height of his Eye, fuppofing the Earth an uninterrupted Globe, is a common Cafe of right-angled plain Triangles, where two Sides and an oppofite Angle are given. — Thus, iuppofe AHB (Tab. Geography, Fig. 8.) a great Cir- cle of the terraqueous Globe, C the Centre, H C irs Sethi- diameter, and E the Height of the Eye; fince HE is a Tangent, the Angle at H is a right Angle ; fo that there are given HC, «t>$ t $%6 Miles, 0^21,054781 Ettglljb Feet, CE rhe fame Length at the Height of the Eye on the Maft of a Ship, or at only a Man's Height, &c. added to ir, and EHC the oppofite right Angle.

By thefe three Parts given, it is eafy to find all the other Parts of theTrianele. ---And firft, for the Angle at C, in order to find the Side HE; the proportion is, as the Side CE to the Angle at H, fb is the Side HC to the Angle at E ; which being fubilracted from 90 Degrees, the Remainder is the Angle at C. Then, as the Angle at E is to its oppo- fite Side HC; or elfe, as the Angle at H is to its oppofite Side CE; fo the Angle at C to its oppofite Side EH, the vifible Horizon.

Or rhe Labour may be fhorten'd by adding together the Logarithm of the Sum of two given Sides, and the Loga- rithm of their Difference ; the half of which two Loga- rithms, is the Logarithm of the Side requir'd, nearly. For an Example, we will take the two Sides in Yards, by rea- fon fcarce any Table of Logarithms will lerve us any far- ther ; the Semi-diameter of the Earth is 7,011594 Yards ■ the Height of the Eye is two Yards more, the Sum of both Sides is 14,023190.

Logar. of which Sum is - - - 7,14(18468 Logar. of two Yards, the Differ, is 0,5010^00

Sum of both Logar, The half Sum -

7,447876a 3,7239384

is the Logarithm of 5296 Yards = three Miles, which is the Length uf the Line KH, or Distance the Eye can reach at fix Feet Height.

This, at leaf}, would be the Diffance on a perfect Globe, did the vifual Rays come to the Eye in a {trait Line 5 but by means of the Refractions of the Atmofphere, diftant Objects on the Horizon appear higher than really rhey are, and may be feen at a greater Diffance, efpecially on the Sea, which is a Matter of great Uie, efpecially to diicover the Land, Rocks, ££fc.

Father Laval, Profeffor of Hydrography at MarfeilleS, found that the Horizon of his Ohfervatory to the Seaward was never more than 15 Minutes, nor left than 13 rj tnat is, the Arch of the Circumference of the Earth, intercepted between the Ohfervatory and the Horizon, fluctuated be- tween thofe two Quantities; whence M. C&ffmi deduces, that the Extent of the Horizon is (even Trench Leagues of three Miles each; and that the Obiervatory is 175 Foot high.

The Extent of the Horizon, at the fame Place, and the fame Height, is very fubject to vary, by means of Diffe- rences in the Atmofphere, which occafion others in the Re- fractions. See Refraction-

When the Sea was full, or the North-Weft or South-Eafl Wind blew, and the Air haxy about the Horizon, F. Laval always found his Horizon the lower; i. e. the Re- fraction which fhould raife it in that Cafe was lefs than ordinary. — And yet on the common Principles, the Air being now much charged with Vapours, the very contrary were rather to be expected. — This makes M. Cafjini fuf- peer, that there is fome other refractive Matter in the At- mtifphere, befide the Air itfelf.

The fame Author obferves, that at a Height much greater than that of F. Laval*s Ohfervatory, he found the Arch terminated by the Horhon to the Seaward, 42', without any fenfible Variation ; whence he concludes, that the Variations are the greater, as the Height is the lets; which may feem contrary to what he had afftrted in another

Place, viz. that the Variations in the apparent Altitudes of Bodies are greater, as thefe Objects are mere re- mote, by reafbn they are leen thro* the larger Quantity of Air, which is all liable to be vary'd. — But the Contra- diction may be folv'd.

Horizon of the Globe, See Globe.

HORIZONTAL, fomething that has a Regard to the Horizon^ is taken in the Horizon, or on a Level with the Horizon. See Horizon.

In this Senfe we lay, a Horizontal Plane, Horizontal Line, Horizontal Diffance, £$c.

Horizontal "Plane, is that which is parallel to the Horizon of the Place; or nothing inclin'd thereto. See Plane.

The Bufinefs of Levelling, is to find whether two Points be in the horizontal Plane ; or how much the Deviation is. See Levelling.

Horizontal "Plane, in Perspective, is a Plane parallel to the Horizon, paffing thro' the Eye, and cutting the Perfpective Plane at right Angles. See Perspective "Plane.

Horizontal Line, in Perspective, is a right Line drawn thro' the principal Point, parallel to the Horizon : Or, it is the Interferon of the horizontal and perfpective Planes,

Such is the LinePQ_(Tab. Perfpective, Fig. 12.) paffing thro' the principal Point F.

Horizontal "Dial, is that drawn on a Plane parallel to the Horizon ; having its Gnomon, ot Style elevated ac- cording to the Altitude of the Pole of the Place it is de- iign'd for.

Horizontal "Dials are, of all others, the molt fimple and eafy. —The Manner of defcribing them, fee under the Ar- ticle Dialling.

Horizontal Range, or Level Range of a Piece of Ordnance, is the Line it defcribes, when directed parallel to the Horizon, or horizontal Line. See Range.

Dr. Halley gives two very ready Theorems, the one to find the greateft horizontal Range at 45 Degrees Elevation, in any Shot made upon any inclin'd Plane, with any Eleva- tion of the Piece whatfoever ; and the other to find Eleva- tions proper to flrike a given Object with any Force, greater than what fufficeth to reach it with the middle Ele- vation.

i°. A Shot being made on an inclin'd Plane : having the horizontal Diffance of the Object it ftrikes, with the Ele- vation of the Piece, and the Angle at the Gun between the Object and the Perpendicular ; to find the greateft horizon- tal Range of thar Piece laden with the fame Charge $ that is half the Latus Rectum of all the Parabola made with the fame Impetus.— Take half the Diffance of the Object from the Nadir, and rhe Difference of the given Elevation from thar half j fubffract the verfed Sine of that Difference from the verfed Sine of the Diftance of the Object from the Zenith : The Difference of thofe verfed Sines, will be to the Sine of the Diftance of the Object from the Zenith, as the horizontal Diftance of the Object ftruck, to the greateft Range at 45 Degrees.

2 . Having the greateft horizontal Range of a Gun, the horizontal Diftance and Angle of Inclination of an Object to the Perpendicular ; to find the two Elevations neceffary toftrike thar Object. — Halve the Diftance of the Ohjcct from the Nadir ; this Half is equal to the half Sum of the two Elevations fought ; Then fay, As the greateft horizontal Range is to the horizontal Diftance of the Object, fo is the Sine of the Angle of Inclination, or Diftance of the Object from the Perpendicular, to a fourth Proportional 5 which Fourth being fubftractcd from the verfed Sine of the Di- ftance of the Object from the Zenith, leaves the verfed Sine of half the Difference of the Elevations fought ; which Elevations are therefore had, by fubftracting that half of the Difference to and from the aforefaid half Sum. See Pro- jectile.

Horizontal Shelters, among Gardeners, are Defences difps'd parallel to the Horizon for tender Plants, Bloffoms, and Fruits in the Spring, againft Blafts and pinching Nights.

The ufual Shelters that have obtain'd, are Bafs-mats, and other warm Coverings, which are roSl'd up in the Day-time, and let down in the Night. -- In lieu of thefe, the Revd. Mr. Lawrence firft pronos'd horizontal Shelters, chiefly on this Principle, that moft of our Frofts and Blafts fall per- pendicularly ; i.e. the condens'd Vapours falling from the upper Region, do, at Night, form themfelves toward the Surface of the Earth, into Drops of Dews, fubject to be frozen by the Coldnefs of the Air. See Dew, Frost, ?£c.

The horizontal Shelters are to be made by laying Rows of Tyles, at certain Diftances one above another, in the Structure of the Wall, fo as to project or hanp over the Plane of the Wall, to carry off the Dew, Wet, &c. — Tia an Inconve- nience objected to this Method, that it is difficult to lead a Tree rightly among the Tyles, or to keep its Figure duly

Horizontal