H Y D 
*5. Hydrachna orbiculata : deprefied; orbicular, vio¬ 
let, with a white fpot and circle. Breaft fulvous, with a 
greenifh tip ; feelers and legs whitilli; eyes black. In¬ 
habits muddy ditches. 
26. Hydrachna lugubris: globular; brownifh, with 
black Breaks; legs green.' Eyes black; feelers green. 
Inhabits bogs. 
27. Hydrachna truncatella : grey; oblong, truncate be¬ 
hind, with dufky dots and lines. Eyes black; feelers 
white. Inhabits marfhes. 
III. Eyes two ; body glabrous. 28. Hydrachna defpi- 
ciens : rounded ; red, with numerous fpots ; eyes placed 
beneath. Body flightly deprefied, thick, and wrinkled ; 
eyes blackifh; legs pale yellow. Inhabits ditches. 
29. Hydrachna geographica : fo named from the fan¬ 
cied map-like diftribution of its variegations. It is one 
of the largeft of the genus, and occafionally feen in clear 
ponds and other ftagnant waters, but is one of the rarer 
kinds; its fhape is globular, and its colour a polilhed 
black, decorated with carmine-coloured fpots and patches, 
which, in a certain liglit, are accompanied by a kind of 
gilded luftre : four round fpots of this colour are fituated 
on the middle of the upper part of the body, and are 
bounded by four larger angular patches ; belides which, 
on the fides and lower part of the body, are fome others ; 
the legs are varied with black and red, and, as in the reft 
of the genus, beautifully ciliated with long hairs; eyes 
two. This infeft fwims with great celerity, and appears 
in almoft conftant motion. When feen in its ftate of per¬ 
fection, it may be numbered among the molt beautiful of 
the Britifh infeffs. 
30. Hydrachna abftergens: globular; carmine, imma¬ 
culate, with black legs. Body with black lines beneath ; 
eyes and feelers black. Inhabits ftagnant water. 
31. Hydrachna extendens: rounded; red, hind-legs 
con trailed. Inhabits ditches. 
32. Hydrachna cruenta: diftended ; red; legs equal. 
Eyes red ; feelers pubefcent. Inhabits wet meadows. 
33. Hydrachna impreffa : diftended ; red, with impreffed 
dots ; feelers lliort. There is a variety with the feelers 
long. Inhabits marfhes. 
34. Hydrachna lunaris : oval; rufous, with a black lu¬ 
nate fpot on the back. Eyes black ; legs grey. Inhabits 
fifh-ponds. 
35. Hydrachna liliacea: obovate; whitifh, the difk 
lily-whir#, with a black fpot each fide. Feelers, legs, 
and breaft, pale; the latter fpotted with black. Inhabits 
fens. 
36. Hydrachna ovata: obovate; grey-brown, with a 
triangular fulvous fpot; legs pale black. Back marked 
with a fulvous crofs; legs black ; eyes and feelers black. 
Inhabits ditches. 
37. Hydrachna torris: oval; yellowifh, with lateral 
black fpots, and red in the middle ; hind-fhanks lamel¬ 
late. Eyes black ; legs white. Inhabits ditches. 
38. Hydrachna ftrigata : oblong ; yellowifh-green, the 
fore-part deprefied, with two blackifh Breaks which unite 
behind. Eyes black ; feelers and legs white. Found near 
the banks of rivers. 
39. Hydrachna nodata: red; truncate before, and fpot¬ 
ted each fide; hind-legs knotty. Eyes black ; feelers an¬ 
nulate; legs yellowifh, and longer than the body. Inha¬ 
bits ditches. 
40. Hydrachna obfoleta : rounded; reddifli-brown, with 
a double obfolete Break behind the eyes. Eyes black ; 
feelers pellucid; legs pale brownifh. Inhabits moift 
woods. 
41. Hydrachna complanata: green; deprefied and emar- 
ginate before, with a white band in the middle. Body 
elliptic ; eyes black ; feelers and legs pellucid. Inhabits 
ditches. 
42. Hydrachna mufculus: green; oval, comprefied; 
above grooved, beneath carinate. Varies in colour. Eyes 
black; feelers white, pellucid. Inhabits marfhes. 
V«L. X. No. 691. 
H Y D (>25 
43. Hydrachna latipes: oval; yellowifh, and fpotted 
each fide ; fourth joint of the hind-legs dilated. Legs' 
pale, or bluifh, tipt with brown. Inhabits.marfhes. 
44. Hydrachna verficolor : nearly liquate ; with, whitifh 
blue and brown fpots. Body white; beneath brown, with 
a yellowifh margin ; feelers and legs white, pellucid. In¬ 
habits wet meadows. 
IV. Eyes four. 45. Hydrachna calcarea: rounded ; 
brown, with a clear white difk. Eyes black; feelers 
pointed; legs pale, peliucid. Inhabits marfhes. 
46. Hydrachna fulcata: ovate; reddiih-brown, with a 
darker difk, and pale rufous fork. Body convex; fpotted 
on the back; eyes black; legs and feelers white, pellucid. 
Inhabits muddy ditches. 
47. Hydrachna undulata : oval; pale yellow, with flexu- 
ous black Breaks. Body fubglobular; pellucid, with 
black eyes. Inhabits marfhes. 
48. Hydrachna maculata : oval; red, with black fpots 
on the back. Eyes black; legs pale yellow, pellucid ; 
feelers pale yellow, and fometimes long and fometimes 
fhort. Inhabits ditches. 
V. Eyes fix. 49. Hydrachna umbrata: rounded; red, 
with numerous darker fpots. Body glabrous ; fubde- 
preffed, with ten darker fpots above ; eyes black; feelers 
yellowifh at the tip; legs yellow. Inhabits marfhy woods. 
HYDRAGO'GA, oi-Hy'dragogues,^ [from v&vg, Gr. 
water, and a.yu , to lead.] Medicines to purge off watery 
humours. 
HYDRAGOG'ICAL, adj. Conveying water. 
HYDRAG'OGY, f. The art of conveying water; the 
conveyance of water from one place to another. 
HYDRAO'TES, a river of India, crofted by Alexander. 
See Hydaspes. 
HYDRAR'GYRAL, adj. Belonging to quickfilver, 
HYDRAR'GYRUM, oi-Hydrar'gyrus,/ Gr. 
.water, and apyvpo;, iilver; from its having the app.earance 
of fluid filver.] Quickfilver, or mercury. For its che¬ 
mical properties, fee Chemistry, vol. iv. alfo the article 
Mineralogy. 
HY'DRAS, a port on the Pirate Coaft in Weftern Hin- 
dooftan; fo called in allufion to the fabled ferpent which 
makes every thing its prey. Pennant. 
HYDRAS'TIS, f. [fo named by Ellis, a noblijjima. 
virgine Americana. Linn. Probably it was fent to Ellis 
by Mifs Colden. May it not be derived from v^pairw? 
Miller calls it Warneria, from Richard Warner, elq. of 
Woodford Row, Effex, a very curious botanift, and a 
great collector of growing plants.] In botany, the Yel¬ 
low Root; a genus of the clafs polyandria, order poly- 
gynia, natural order of ranunculacete, jjvjf. The geneiic 
charailers are—Calyx: none. Corolla: petals three, ovate, 
regular. Stamina: filaments numerous, linear, comprefied, 
a little fhorter than the corolla; antherae comprefied, 
blunt. Piftillum : germs numerous, ovate, collefted into 
an ovate head; ftyles very fhort; ftigmas broadifh, com- 
preffed. Pericarpium: berry compounded of oblong acini, 
or granulations. Seeds folitary, oblong.— Ejfential Cha¬ 
racter. Calyx none; petals three; neftary none; berry 
compofed of one-feeded acini, or granulations. 
Hydraftis Canadenfis, or Canadian yellow-root: a fingie 
fpecies ; the.Hydrophyilum verum Canadenfium of Lin¬ 
naeus. The root is compofed of thick flefliy tubers, of a 
deep yellow colour within, but covered by a brown flcin, 
fending out fibres from feyeral parts in the fpring. It 
fends up one or two Balks about nine inches high, at 
their firft appearance of a light green, but afterwards 
changing to a purplifh colour, and hairy towards the top. 
Each Balk has one or two leaves, the lower petioled, but* 
the upper embracing; they are fix or feven inches in dia¬ 
meter, and are deeply cut into three, four, or five, lobes, 
which are irregularly ferrate; they are of a light green in 
the fpring, but change afterwards to a deep green, with 
fome dark fpots or marks, and after the flower is decayed 
turn to a purplifh colour. The Balk is terminated by 
7 U ,._j one 
