HOR 



HOR 



racier: calyx lateral, two-valved, one-flow- 

 ered, by threes, at each toothlet of the 

 rachis. There are nine species. 



HOftlZON, in astronomy and geogra- 

 phy, that great circle which divides the 

 heavens and the earth into two equal 

 parts, or hemispheres, distinguishing 1 the 

 upper from the lower. The horizon is 

 either sensible or rational : the sensible 

 horizon is that circle, which, being dis- 

 covered by our senses, limits our pros- 

 pect. 



When we are on terra firma, this cir- 

 cle commonly seems rugged and irregu- 

 lar, occasioned by the unevenness of the 

 ground; but at sea, there are no such 

 irregularities. The semi-diameter of this 

 circle varieth according to the height of 

 the eye of the observer. If a man, six 

 feet high stood upon a large plain, or the 

 surface of the sea, he could not see quite 

 three miles round. To find the distance 

 to which a person can see, at any given 

 height of the eye, or the extent of the vi- 

 sible horizon, is a problem of some utili- 

 ty, particularly to manners : the rule is, 

 "multiply the square root of the height 

 of the eye in feet, by 1.225, and the pro- 

 duct is the distance in miles to which we 

 can see from that height ;" thus a sailor, 

 standing at the top-mast of a ship 120 

 feet high, can see a distance in miles = 

 \fflO X 1.225=13 45= to thirteen miles 

 and a half nearly. 



The rational, or true horizon, is a great 

 circle of the apparent celestial sphere, di- 

 viding it into two equal hemispheres, and 

 serving as the limits of elevation or de- 

 pression of celestial objects. This hori- 

 zon being parallel to the sensible horizon, 

 is distant from it by the semi-diameter of 

 the earth, through whose centre it passes: 

 for the astronomers reduce the appear- 

 ance of the heavens to a spherical sur- 

 face, which is not concentrical to the eye, 

 but to the earth. It divides the heaven 

 and earth into two parts, the one light, 

 and the other dark, which are greater or 

 lesser, according to the condition of the 

 place, &c. It determines the rising and 

 setting of the sun, moon, or stars, in any 

 particular latitude; for, when any of these 

 appear just at the eastern part of the ho- 

 rizon, we say it rises ; and when it does 

 so at the western part, we say it sets. 

 And from hence also the altitude of the 

 sun or stars is accounted, which is their 

 height above the horizon. This circle is 

 divided by astronomers into four quad- 

 rants, or cardinal points The poles of 

 this horizon are the zenith and the nadir : 



and the innumerable circlesdrawn through 

 these poles to the horizon are called the 

 vertical circles, or azimuths. These two 

 horizons produced to the fixed stars will 

 appear to coincide into one, since the 

 earth compared to the sphere in which, 

 the fixed stars appear is but a point; 

 therefore the two circles, which are but a 

 point distant from each other, may be 

 well considered as coinciding into one. 



Homzoyrofafflobe. See GLOBE. 



HORTZONTALj-something relating to 

 the horizon r or that is taken in, or on a 

 level with the horizon : thus we say, an 

 horizontal plane, &c.- 



it frequently happens at sea, that the 

 atmosphere is so hazy as to prevent a 

 distinct view of the horizon, which is a 

 great hindrance to accurate observations*. 

 This inconvenience is remedied by an 



HORIZONTAX speculum^ which consists 

 in a well polished metal speculum, about 

 three or four inches in diameter, inclosed 

 within a rim of brass; so fitted, that the 

 centre of gravity of the whole shall fall 

 pear the point on which it turns. This is 

 the end of a steel axis running through 

 the centre of the spepulum, above which 

 it finishes in a square, for the convenience 

 of fitting a roller on it, by which it is set 

 in motion by means of a piece of tape 

 wound round the roller. The cup in 

 which it turns is made of agate, flint, or 

 other hard substance, and a cover to the 

 whole may be made of glass; by this 

 means an observation may be taken with ' 

 it as Weil covered as open, which will 

 prevent injury from the spray of the sea. 

 These specula are as useful by night as 

 by day ; for as* the images of the stars 

 may be seen in the speculum, conse- 

 quently any object that can be seen re- 

 flected upon the glasses of the quadrants, 

 maybe observed by the speculum, and 

 these are all the stars of the first magni-, 

 tude, the planets Venus, Mars, Jupiter, 

 Saturn, and the Moon; so that by having 

 the declinations of these bodies in the 

 Nautical Almanack, or indeed in any 

 ephemeris, they may be used in observa- 

 tions as well as the sun. 



HORIZONTAL dial, that drawn on a 

 plane parallel to the horizon, having its 

 style elevated accordingto the altitude of 

 the poll, in the place it is designed for. 

 See DIAL. 



HORIZONTAL line,] in perspective, a 

 right line drawn through the principal 

 point parallel to the horizon ; or it is 

 in the intersection of the; horizontal and 

 perspective planes. See PERSPECTIVE. 



