HEL 
disc are not so equally defined as the other 
parts ; on this account his image appears 
somewhat extended in the vertical direc- 
tion. This is owing to the decomposition 
of light, which is know'll to consist ot rays 
differently refrangible in their passage 
through our atmosphere. Thus the blue 
and violet rays, which proceed from the 
tipper part of the disc at the same time with 
those of other colours, are somewhat more 
refracted than the others, and therefore 
seem to us to have proceeded from a higher 
point ; whereas, on the contrary, the red 
rays proceeding from the lower edge of the 
disc, being less refracted than the others, 
seem to proceed from a lower point ; so 
that the vertical diameter is extended, or 
appears longer, than the horizontal diame- 
ter. 
HELIOCENTRIC latitude of a planet, 
the inclination of a line drawn between the 
centre of the sun and the centre of a planet, 
to the plane of the ecliptic. 
Heliocentric place of a planet, in astro- 
nomy, the place of the ecliptic wherein the 
planet would appear to a spectator placed 
at the centre of the sun. 
HELIOPHILA, in botany, a genus of 
the Tetradynamia Siliquosa class and order. 
Natural order of Siliquos®, Cruciformes. 
Crucifer®, Jussieu. Essential character: 
nectaries two, bowed back towards the 
bladder of the calyx. There are ten spe- 
cies. These plants are all natives of the 
Cape of Good Hope. 
HELIOSCOPE, in optics, a sort of te- 
lescope, peculiarly fitted for viewing the 
sun, without hurting the eyes. See Te- 
lescope. 
HELIOTROPE, in mineralogy, a spe- 
cies of the flint genus. It is of a green 
colour, and occurs massive, in angular 
and rolled pieces ; it is commonly trans- 
lucent on the edges; the specific gravity 
from 2.6 to 2.7. It is found in rocks, 
and is said to be the connecting link be- 
tween jasper and chalcedony. In Asia, it 
is found in Bucharia, Persia, and Siberia ; 
and in Europe, ip Iceland and in Upper 
Saxony. From the beauty of the colour, 
and its great hardness, it is reckoned of 
great value among lapidaries, and that 
which lias the greatest degree of translu- 
cency, and most numerous red spots, is of 
most value. 
HELIOTROPIUM, in botany, a genus 
of the Pentandria Monogynia class and or- 
der. Natural order of Asperifoli®. Bor- 
ragine®, Jussieu. Essential character : co- 
v / HEL 
rolla salver-shaped, five-cleft, with teeth in- 
terposed ; throat closed with arches. There 
are twenty-four species, of which H. peru- 
vianum, Peruvian turnsole, or Heliotrope, 
is a small shrubby plant, from two to three 
feet in height; the leaves are long, hairy, 
and much veined, of an ash-colour on their 
under side, on short foot-stalks ; the flowers 
are produced at the ends of the branches, 
in short reflex spikes, growing in clusters ; 
the peduncles divide into two or three, and 
these again into smaller ones, each sustain- 
ing a spikelet of pale blue flowers, which 
have a strong, sweet odour, somewhat re- 
sembling bitter almonds. It grows natu- 
rally in Peru ; it flowers with us great part 
of the year, and those flowers which come 
out early in the summer, are succeeded by 
ripe seeds in autumn. 
HELIX, in geometry, the same with Spi- 
ral, which see. 
Helix, in natural history, the snail,, a ge- 
nus of the Vermes Testacea class and or- 
der. Animal a limax; shell univalve, spi- 
ral, subdiaphanous, brittle; aperture con- 
tracted, semi-lunar or roundish. Of this 
genus more than three hundred species have 
been enumerated ; they are separated into 
divisions ; A. whorls, with a carinate acute 
margin; B. umbilicate, the whorls round- 
ed; C. rounded imperforate ; D. tapering ; 
E. ovate, imperforate. Of the species, we 
shall notice H. cornea, the shell of which 
above is umbilicate, flat, blackish, with 
four round whorls. It is found in Europe, 
and on the coast of Coromandel, freiii a sin- 
gle line to an inch in diameter ; shell ches- 
nut, brown, rufous, whitish, yellowish, or 
blueisb, polished and very fine striate trans- 
versely ; whorls four or five, rarely turned 
contrary; the inhabitant is black, with 
dirty-grey tentacula, and produces a scar- 
let, but not very durable dye. H. forma- 
tia, a snail with five spires remarkably ven- 
tricose, slightly umbilicated, fasciated with 
a lighter and deeper brown : this is found 
in the woods of ttye southern counties of 
England ; it is said to have been introduced 
into this country by Sir Kenelm Digby, for 
medical purposes. These are confined to the, 
southern counties, attempts having been 
made, but without success, to bring them 
into Northamptonshire. This snail is used 
in many parts of Europe as food, particu- 
larly at Rome during the weeks of Lent : 
here they are fattened, and grow to a very 
large size. It is ovipar*us, very tenacious 
of life, and, towards winter, covers its aper- 
ture with a calcareous lid. H. hortensis, 
