HEL 
HEL 
lieliacally in the evening, after the new 
moon; and sets heliacally in the morning, 
when old and approaching to a conjunction 
with the sun. 
The inferior planets, Venus and Mercury, 
which sometimes seem to go westward 
from the sun, and sometimes again have a 
quicker motion eastward, rise heliacally 
in the morning, when they are retrograde ; 
but when direct in their motions they rise 
heliacally in the evening. The heliacal 
rising or setting of the moon, happens when 
she is seventeen degrees distant from the 
sun; but for the other planets, twenty de- 
grees ate required ; and for the fixed stars, 
more or less according to their magnitude. 
HELIANTHUS, in botany, sun-flower, 
a genus of the Syngenesia Polygamia Frus- 
tranea class and order. Natural order of 
Composite Oppositifoliae. Corymbiferas, 
Jussieu. Essential character : calyx imbri- 
cate, somewhat squarrose; down two- 
leaved ; receptacle chaffy, flat. There are 
twelve species. These are hardy herbace- 
ous plants, most of them tall and large, all 
perennial excepting two, viz. H. annus and 
H. indicus. They are all natives of Ame- 
rica. 
HELICOID parabola, or Parabolic Spiral, 
is a curve arising from the supposition that 
the common parabola is bent or twisted, till 
the axis comes into the circumference of a 
circle, the ordinates still retaining their 
places and perpendicular positions with 
respect to the circle, all these still remaining 
in the same place. 
HELICONIA, in botany, a genus of the 
Pentandria Mohogynia class and order. Na- 
tural order of Scitamineae. Mus®, Jussieu. 
Essential character : spathes ; perianth none; 
corolla tkree-petalled ; nectary two-leav- 
ed ; pericarpium tricoccous ; seeds solitary. 
There are three species, natives of the West 
Indies and South America. 
HELICTERES, in botany, a genus of 
the Gynandria Decandria class and order. 
Natural order of Colunmiferse. Malvaceae, 
Jussieu. Essential character: pentagynous ; 
calyx one-leafed, oblique ; petals five ; 
nectary of five leaflets ; capsule five-twist- 
ed. There are nine species, shrubs or trees, 
natives of both Indies, mostly tomentose ; 
leaves alternate ; peduncles axillary, few- 
flowered. 
HELIOCARPUS, in botany, a genus of 
the Dodecandria Digynia class and order. 
Natural order of Columniferas. Tiliacere, 
Jussieu. Essential character : calyx four- 
leaved ; corolla four-petalled ; styles sim- 
ple ; eapsule two-celled, compressed, lon- 
gitudinally radiated on both sides. There 
is only one species, viz. H. americana, 
American heliocarpus. It is found growing 
wild about La Vera Cruz, in New Spain. 
HELIOMETER, the name of an instru- 
ment for measuring with particular exact- 
ness the diameters of the heavenly bodies, 
and especially those of the sun and moon. 
This instrument is a kind of telescope, con- 
sisting of two object-glasses of equal focal 
distance, placed one of them by the side of 
the other, so that the same eye-glass serves 
for both. The tube of this instrument is of 
a conical form, larger at the upper end, which 
receives the two object-glasses, than at the 
lower, which is furnished with an eye-glass 
and micrometer. By the construction of 
this instrument two distinct images of an 
object are formed in the focus of the eye- 
glass, whose distance, depending on that of 
the two object-glasses from one another, may 
be measured with accuracy ; nor is it neces- 
sary that the whole disc of the sun or moon 
should come within the field of view, since, 
if the images of only a small part of the disc 
be formed by each object-glass, the whole 
diameter may be easily computed by their 
position with respect to one another : for if 
the object be large, the images will ap- 
proach, or perhaps lie even over one an- 
other, and the object-glasses being moveable, 
the two images may always be brought 
exactly to touch one another, and the dia- 
meter may be computed from the known 
distance of the centres of the two glasses. 
Besides, as this instrument has a common 
micrometer in the focus of the eye-glass, 
when the two images of the sun or moon 
are made in part to cover one another, that 
part which is common 1-j both the images may- 
be measured with great exactness, as being 
viewed upon a ground that is only one half 
less luminous than itself ; whereas, in gene- 
ral, the heavenly bodies are viewed upon a 
dark ground, and on that account are ima- 
gined to be larger than they really are. By 
a small addition to this instrument, provided 
it be of a moderate length, M. Bouguer, 
the inventor, thought it very possible to 
measure angles of three or four degrees, 
which is of particular consequence in taking 
the distance of stars from the moon. With 
this instrument he found that the sun’s ver- 
tical diameter, though somewhat diminished 
by the astronomical refraction, is longer 
than the horizontal diameter ; and, in as- 
certaining this phenomenon, he also found, 
that the upper and lower edges of the sunk 
