HEX HEL HEL <505 
have no latitude; after which it will increase 
again, till it come to B, or its utmost latitude ; 
and, lastly, decrease again, till the planet 
come to be in n, whence it set out. 
Heliocentric place qfa planet, in astro- 
nomy, the place of the ecliptic which in the 
planet woidd appear to a spectator placed at 
the centre of the sun. 
The ingenious Dr. Halley gives the follow- 
ing method to find the heiiocentrical places 
of a planet, and its distances from the sun, 
which supposes only that the periodical time 
of the planet is known. Let KLB (fig. 120.) 
be the orbit of the earth, S the sun, P the 
planet, or rather the point in the plane of the 
ecliptic, in which the perpendicular let fail 
from the planet meets that plane. And first 
when the earth is in K, observe the geocen- 
tric longitude of the planet, and having the 
theory of the earth, we have the apparent 
longitude of the sun, and, consequently, the 
angle PKS. The planet, after it has com- 
pleted an entire revolution, returns again to 
the point P, at which time suppose the earth 
in L ; and there again, let the planet be ob- 
served, and find tine angle PLS, the planet’s 
elongation from the sun. Having the times 
of observations, we have the places of the 
earth in the ecliptic, or the points K and L; 
and, consequently, the angle LSK, and the 
sides LS and SK: wherefore we shall have 
the angles SKL and SLK, and the side LK. 
From the known angles SKP and SLP, take 
away the known angles SKL and SLK, and 
we shall have the angles PKL and PLK 
known; therefore in the triangle PLK, hav- 
ing all the three angles, and the side LK, 
we shall find the side PL ; and in the triangle 
PLS, having the sides PL and LS, and the 
intercepted angle PLS, we shall have the 
angle LSP, which determines the heliocen- 
tric place, and its distance from the node ac- 
cording to the ecliptic, as also the side SP. 
But as the tangent of the geocentric latitude 
is to the tangent of the heliocentric, so is the 
curtate distance of the planet from the sun to 
its curtate distance from the earth. But as 
the geocentric latitude may be found by ob- 
servation, the heliocentric latitude will also 
be found ; by which, and the curtate distance 
of the planet from the sun, we can find the 
true distance. 
HELIOCOMETES, a phenomenon some- 
times observed about sun-setting ; being a 
large luminous tail or column of light pro- 
ceeding from the body of the sun, and drag- 
ging after it, not unlike the tail of a comet ; 
whence the name. 
HELlOPIilLA, a genus of the siliquosa 
order, in the tetradynamia class of plants, 
and in the natural method ranking under the 
39th order, siliquosa;. There are two necta- 
ria recurvated towards the vesicular base of 
the calyx. There are ten species, all herba- 
ceous plants of the Cape. 
HELIOSCOPE, in optics, a sort of tele- 
scope, peculiarly fitted for viewing the sun, 
without hurting the eyes. See Optics. 
'As the sun maybe viewed through colour- 
ed glasses, without hurt to the eyes, if the 
object and eye-glasses of a telescope are made 
of coloured glass, as red or green, such a te- 
lescope will become an helioscope. But Mr. 
Huygens only used a plain glass, blacked at 
the flame of a candle on one side, and placed 
between the eye-glass and the eye, which an- 
swers the design of an helioscope very well. 
VOL. I. 
HELIOSTATA, in optics, an instrument 
invented by the late learned Dr. S. Graves- 
ande, who gave it this name, from its fixing 
apparently the rays of the sun in an horizon- 
tal direction across the dark chamber, all the 
while it is in use. 
HELIOTROPE, a precious stone of a 
green colour, streaked with red veins. Pliny 
says it is thus called, because, when cast into 
a vessel of water, the sun’s rays falling on it 
seem to be of a blood-colour ; and that, when 
out of the water, it gives a faint reflection of 
the figure of the sun, and is proper to observe 
eclipses of the sun as a helioscope. The he- 
liotrope is also called oriental jasper, on ac- 
count of its ruddy spots. It is found in the 
East Indies, as also in Ethiopia, Germany, 
&c. According to Dr. Thompson, the co- 
lour of this stone is different shades of green, 
often stained or striped with olive, •yellow, 
and red. Found in mass, and in angular 
pieces. Specific gravity 2.6. Brittle, infu- 
sible before the blow-pipe. 
HELIOTROPIUM, turnsole, a genus of 
the polygynia order, in the pentandria class 
of plants, and in the natural method ranking 
under the 4 1st order, asperifolise. The co- 
rolla is salver-shaped and quinquefid, with 
lesser dents interjected alternately ; the throat 
closed up by small arches formed in the co- 
rolla itself. There are 24 species, all of them 
natives of warm countries. Only one, called 
the tricoccum, grows in Europe, and is a na- 
tive of France, Spain, and Italy. It is only 
remarkable for the property of its berries. 
See Colour-making. 
HELIX, in geometry, the same with spi- 
ral. In architecture some authors make a 
difference between the helix and the spiral. 
A stair-case, according to Daviler, is an he- 
lix, or is helical, when the stairs or steps wind 
round a cylindrical newel ; whereas the spiral 
winds round a cone, and is continually ap- 
proaching nearer and nearer its axis. 
Helix, the snail, in zoology, a genus be- 
longing to the order of vermes testacea. The 
shell consists of one spiral, brittle, and almost 
diaphanous valve ; and the aperture is nar- 
row. There are 60 species, principally dis- 
tinguished by the figure of their shells. They 
are of various sizes, from that of a small apple 
to less than half a pea. Some of them live 
on land, frequenting woods and gardens, or 
inhabiting clefts of rocks and dry sand-banks. 
Others of them are aquatic, inhabiting ponds, 
deep rivers, and the ocean. The principal 
species are. 
The janljiina, with a violet-coloured shell, 
is remarkable for the extreme thinness of its 
texture, which breaks with the least pressure, 
and seems therefore entirely calculated to 
keep the open sea, or at least to shun rocky 
shores. It inhabits the seas of Europe, espe- 
cially the Mediterranean, those of Asia and 
Africa, and also the ocean. The living ani- 
mal, when touched, exudes a juice which 
stains the hands of a violet-colour. Dr. 
Hawkesworth, in his Account of Cook’s 
Voyage, mistakes this shell for that which 
yielded the purpura of the antients. But 
whoever looks into Pliny, can never have the 
least idea that the thin shell aforementioned 
could be the same with it. They had seve- 
ral shells which yielded the purple dye; but 
these were all rock-shells, and very different 
both in figure and hardness from the little 
5Y 
helix jantliina, which is not calculated fop the 
neighbourhood of rocks. 
2. The pomatia, or exotic snail, with five 
spires, most remarkably ventricose, and fas- 
ciatecl with a lighter and deeper brown, is a 
native of France, where it inhabits the woods, 
but has been naturalized in England, where 
it inhabits (lie woods in the southern coun- 
ties. It was introduced, it is said, by sir 
Kenclm Digby, whether for medical pur- 
poses, .or as lood, is uncertain: tradition says, 
t hat to cure his beloved wife of a decay was 
the object. They are quite confined ie our 
southern counties. An attempt was made to 
bring them into Northamptonshire, but they 
would not live there. These are used as a 
food in several parts of Europe during Lent, 
and are preserved in an escargatoire, or a 
large place boarded in, with a floor covered 
half a foot deep with herbs, in which the 
snails nestle and fatten. They were also a 
favourite dish with the Romans, who had 
their cochlearia, a nursery similar to the 
above. Fulvius Hirpinus was the first in- 
ventor of this luxury, a little before the civil 
wars between Ca'sar and Pompey. The 
snails were fed with bran and sodden wine. 
If w ! e could credit Varro, they grew so large, 
that the shells of seme would hold ten quarts ! 
People ueed not admire the temperance of 
the supper of the younger Pliny, which con- 
sisted of only a lettuce, three snails, two eggs, 
a barley-cake, sweet wine and snow, in case 
his snails bore any proportion in size to those 
of Hirpinus. Its name is derived not from 
any thing relating to an orchard, but from 
iru^ee, au operculum, it having a very strong 
one. This seems to be the species described 
by Pliny, lib. viii. c. 39. which he says was 
scarce ; that it covered itself with the opercle, 
and lodged under ground; and that they were 
at first found only about the maritime Alps, 
and more lately near Velitrae. 
3. The hortensis, or garden-snail, is in form 
like the last, but less, and not umbilicated and 
clouded, or mottled with browns. It abounds 
with a viscid slimy juice, which it readily 
gives out by boiling in milk or water, so as 
to render them thick and glutinous. The 
decoctions in milk are apparently very nu- 
tritious and demulcent, and have been recom- 
mended in a thin acrimonious state of the 
humours, in consumptive cases and emacia- 
tions. 
The eyes of snails are lodged in their 
horns, one at the end of each horn, which 
they can retract at pleasure. The manner of 
examining these eyes/which are four in num- 
ber, is this : When the horns are out, cut off 
nimbly the extremity of one of them; and 
placing it before the microscope, yon may 
discover the black spot at the end to be really 
a semiglobular eye. The dissection of this 
animal is very curious; for by this means the 
microscope not only discovery the heart beat- 
ing just against the round hole near the neck, 
which seems the place of respiration, but also 
the liver, spleen, stomach, and intestines, 
with the veins, arteries, mouth and teeth, arc 
plainly observable. The intestines of this 
creature are green, from its eating herbs, 
and are branched all over with fine capillary- 
white veins; the mouth is like a hare’s oe 
rabbit’s, with four or six needle-teeth, resem- 
bling those of leeches, and of a substance like 
horn. Snails are all hermaphrodites, having 
both sexes united in each individual. They 
