HEL 



HEL 



This genus has been distinguished into 

 four species; viz. 1. the rhomboidal or 

 witherite, which is a carbonate of baryta, 

 occurring in lead veins ; 2. the prismatic, 

 or heavy spar, or sulphate of baryta, 

 found in primitive and transition rocks, 

 in secondary limestone, and in lead 

 mines ; 3. the di-prismatic or sirontianite, 

 a carbonate of strontia occurring in 

 veins which traverse gneiss ; and 4. the 

 axifrangible or celestine, a sulphate of 

 strontia, occurring in trap-tuff and in red 

 sandstone. 



HEDENBER'GITE. A variety of 

 augite, of a dark-green colour, sometimes 

 nearly black: it resembles those horn- 

 blendes in which iron prevails, and is 

 found in Sweden. 



HE'DYPHAN. A phosphate of lime, 

 in which a portion of phosphoric acid is 

 replaced by arsenic acid. 



HE'GIRA. This mode of computing 

 time among the Mahomedans has been 

 noticed under the article Era. It may 

 here be added, that the years of the 

 Hegira are divided into cycles of thirty 

 years, nineteen of which are termed 

 common years, of 354 days each, and the 

 other eleven are called intercalary, or 

 abundant, from their consisting of one 

 day more. To reduce the Christian era 

 to the Mahomedan, subtract 622 from the 

 current year; multiply by 10307; cut 

 off four decimals, and add '46 : the sum 

 will be the year and decimal of the day, 

 Old Style. 



HEIGHT. This term, in Geometry, 

 is synonymous with altitude, and denotes 

 the perpendicular let fall from the ver- 

 tex, or top, of any rectilinear figure, upon 

 the base or side subtending it. It also 

 represents the position of any object, in a 

 vertical direction above the horizon. 



The meaxurement of heights is gene- 

 rally effected by observing the differences 

 of atmospheric pressure as indicated by 

 the barometer. In 1815, Capt. Smith 

 ascertained, trigonometrically, that the 

 height of Etna was 10,874 feet. In 

 1824, Sir J. Herschel determined, by 

 barometrical measurement, that the 

 height was 10,872^ feet. This singular 

 agreement of results, so differently ob- 

 tained, was spoken of by Herschel as "a 

 happy accident ; " but WoUaston re- 

 marked that " it was one of those acci- 

 dents which would not have happened to 

 two fools." 



HE'LIACAL (rjXtoc, the sun). A term 

 applied to the rising and setting of a 

 star, when it rises in the morning a little 

 161 



I before the sun, or sets in the evening a 

 I little after him. It is only in his heliacal 

 rising and setting that the planet Mer- 

 cury is ever visible to the naked eye. See 

 Cosmical. 



HELIANTHOI'DA {helianthus, the 

 sun-flower, eldov, likeness). An order of 

 the polypipherous Radiata, named from 

 their resemblance, when expanded, to 

 the sun-flower. They are also termed 

 actiniform polyps, from their general 

 resemblance to the sea-anemone. 



HELI'CIDiE {helix, a snail). Snails ; 

 a family of phytophagous Gasteropods ; 

 these are pulmonary animals, breathing 

 by a lateral opening ; the shell is light, 

 turbinated, or spiral ; the aperture al- 

 ways entire, rarely closed by an opercu- 

 lum, and sometimes only rudimentary. 



HELI'CIN^. Comfnon Land Snails ; 

 a sub-family of the Helicidcc, having 

 perfect turbinated shells more or less 

 depressed; the aperture entire, but 

 without teeth. 



HE'LICOID (fcXtf , a helix, eldoc, like- 

 ness). Parabolic spiral. In Geometry, 

 a curve generated by the bending of the 

 axis of the common parabola into the 

 circumference of a circle, the ordinates 

 still retaining their places and perpendi- 

 cular positions with respect to the circle, 

 all these lines still remaining in the same 

 plane. The equation of this curve re- 

 mains the same as when it was a para- 

 bola. 



HE'LIOCE'NTRIC (^\<of, the sun, 

 Kevrpov, a centre). Having the sun as a 

 centre ; a term applied to the place of a 

 planet, as seen from the centre of the 

 sun, and opposed to its geocentric place, as 

 seen from the centre of the earth. 



1. The heliocentric longitude oi a. planet 

 is the angle at the sun's centre formed 

 by the projection of its radius vector on 

 the ecliptic, and the straight line drawn 

 from the centre of the sun to the first 

 point of Aries. 



2. The heliocentric latitude of a planet 

 is the inclination of a line drawn between 

 the centres of the sun and the planet, to 

 the plane of the ecliptic. 



HELIO'GRAPHY {nXtoi, the sun, 

 '/pd(pb}, to write). Photography. A me- 

 thod of giving permanency to images ob- 

 tained by means of convex lenses, by the 

 chemical effects of light. 



HELIO'METER (riXiot, the sun, /ue- 

 rpov, a measure). A kind of micrometer 

 invented by M. Bouguer for measuring 

 the diameters of the heavenly bodies. 



HE'LIOSCOPE {riXioi, the sun, cko- 



