HEL 



HEL 



two, vi:. H; annus and H. indicus. They 

 are all natives of America. 



HEL1COID parabola, or Parabolic Spi- 

 ral, is a curve arising- from the supposition 

 that the common parabola is bent or 

 twisted, till the axis comes into the cir- 

 cumference of a circle, the ordinates still 

 retaining their places and perpendicu- 

 lar positions with respect to the cir- 

 cle, ail these still remaining in the same 

 places. 



HEL1CONIA, in botany, a genus of the 

 Pentandria Monogynia class and order. 

 Natural order of Scitaminese. Musae, Jus- 

 sieu. Essential character: spathes; perianth 

 none; corolla, three-petalled; nectary two- 

 leaved ; pericarpium tricoccous ; seeds 

 solitary. There are three species, natives 

 of the West Indies and South America. 



HELICONII, in natural history, a divi- 

 sion of the genus Papilio, butterfly ; the 

 wings are narrow, entire, usually trans- 

 parent, the anterior ones are oblong, and 

 the interior short and rounded. See 

 PAPILIO. 



HEL1CTERES, in botany, a genus of 

 the Gynandria Decandria class and order. 

 Natural order of Columniferse. Malvaceae, 

 Jussieu. Essential character: pentagy- 

 nous ; calyx one-leafed, oblique ; petals 

 five ; nectary of five leaflets ; capsule 

 five-twisted. There are nine species, 

 shrubs or trees, natives of both Indies, 

 mostly tomentose; leaves alternate ; pe- 

 duncles axillary, few-flowered. 



HELIOCARPUS, in botany, a genus of 

 'the Dodecandria Digynia class and order. 

 Natural order of Columniferze. Tiliaceae, 

 Jussieu. Essential character : calyx four- 

 leaved ; corolla four-petalled ; styles sim- 

 ple; capsule two-celled, compressed, lon- 

 gitudinally radiated on both sides. There 

 is only one species, viz. H. americana, 

 American heliocarpus. It is found grow- 

 ing wild about La Vera Cruz, in New 

 Spain. 



HELIO METER, 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, 

 consisting 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 accura- 

 cy ; nor is it necessary 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 an- 

 other : for if the object be large, the im- 

 ages will approach, or perhaps lie even 

 over one another, and the object-glasses 

 being moveable, the two images may 

 always be brought exactly to touch one 

 another, and the diameter may be com- 

 puted from the known distance of the 

 centres of the two glasses. Besides, as 

 this instrument has a common microme- 

 ter 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 to both the im- 

 ages may be measured with great ex- 

 actness, as being viewed upon a ground 

 that is only one half less luminous than 

 itself; whereas, in general, the heaven- 

 ly bodies are viewed upon a dark ground, 

 and on that account are imagined to be 

 larger than they really are. By a small 

 addition to this instrument, provided it 

 be of a moderate length, Mr. 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 vertical diameter, though 

 somewhat diminished by the astronomi- 

 cal refraction, is longer than the hori- 

 zontal diameter; and, in ascertaining 

 this phenomenon, he also found, that the 

 upper and lower edges of the sun's 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 decomposi- 

 tion of light, which is known to consist 

 of rays differently refrangible in their 

 passage through our atmosphere. Thus 

 the blue and violet rays, which proceed 

 from the upper 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 ; 



