HYD 



HYD 



negations. It is one of the largest of the 

 genus, and is occasionally seen in the 

 clear ponds, and other stagnant waters. 

 This is reckoned one of the most beauti- 

 ful of the British insects. 



HYDRANGEA, in botany, a genus of 

 the Decandria Digynia class and order. 

 Natural order of Succulent*. Saxifrage, 

 Jussieu. Essential character: capsule 

 two-celled, two-beaked, containing ma- 

 ny seeds ; corolla five petalled ; calyx 

 five-cleft, superior. There are three 

 species. 



HYDRARGYRUM, an old name given 

 to mercury. 



HYDRA STIS, in botany, a genus of 

 the Polyandria Poly gynia class and order. 

 Natural order of Ranunculacese, Jussieu. 

 Essential character ; calyx none ; petals 

 three; nectary none ; berry composeH of 

 one seeded acini, or granulations. There 

 is but one species, viz. H. canadensis, 

 Canadian yellow root. 



HYDRATE, in chemistry, lately intro- 

 duced by Proust to express the chemical 

 union of water with any substance, and 

 especially with certain metallic oxides. 

 The hydrate of copper is a blue-green 

 oxide of this metal, which differs from 

 the brown oxide only in containing a 

 large quantity of water, which a low red 

 heat will expel. 



HYDRAULICS teach us to ascertain 

 the velocity and impetus of fluids when 

 in motion, and serves as the basis for com- 

 puting the powers of various machinery 

 acted upon by running water. 



The first principle we shall inculcate 

 in this service is, that water, being an in- 

 elastic fluid, (though many have thrown 

 away much time in the attempt to prove 

 the contrary,) can only be set in motion 

 by two causes, viz. the increased pressure 

 of the air, as in the air-vessels of fire-en- 

 gines, and by gravitation ; that is, where 

 it is liberated from confinement, and al- 

 lowed to descend to an inferior level. In 

 the former case, water may be made to 

 rise by machinery suited to the purpose ; 

 in the latter, it will inviolably seek a low- 

 er situation. 



The velocity of water, proceeding 

 through a hole in the side of a vessel, is 

 ever proportioned to the distance of the 

 aperture from the level of the fluid, the 

 square root of the intermediate space be- 

 ing the guide. It must, however, be re- 

 collected, that in consequence of the de- 

 crease of that space, as the water is let 

 out, the pressure becomes gradually less ; 

 therefore the medium, or mean distance, 

 between the surface and the vent whence 



the water issues, will be found, in gene- 

 ral, a correct standard. Hencr we see, 

 that, in order to force double the qaan- 

 tity of water through the lowest of two 

 apertures, the distance must be quadru- 

 pled. For if a hole made at C in the 

 pipe A B, fig. 1, will supply one gallon 

 of water in a minute'; to draw double 

 that quantity in the same time, the low- 

 er hole, D. must measure from the sur- 

 face, B, four times as much as from C to 

 the surface. 



This establishes the above position, 

 and proves, besides, that the force is 

 equal to the velocity, as indeed we know 

 to result in every branch of mechanism. 

 To shew this, let the pipe, A B, be per- 

 forated in several parts, as at C D E ; the 

 first, . e. C, being one foot ; that at D be- 

 ing four feet ; and that at E being seven 

 feet below the surface, B ; between E 

 and A we will suppose only one foot in- 

 terval, so that D may be in the centre of 

 the height A B. Draw the horizontal 

 line, A F, and from D describe the semi- 

 circle, EGA, having D G equal to D A, 

 or D B, for its radius. Now the water 

 will, as it flows from D, describe a para- 

 bola, and will fall upon the line, A F, at 

 such a distance from A, as will be equal 

 to double the radius, D G. In like man- 

 ner the water flowing from the aperture, 

 C, will re ach that point, viz. K, on the hori- 

 zontal A F, which may measure double 

 the sine, C H, on the same semi-circle : 

 and the sine of the arc taken opposite to 

 E, i. e. E L, is equal to the sine, C H, the 

 water rushing from E will intersect, or 

 meet, the water falling from C, at the 

 point K. It is to be observed, that the 

 parabolic curve of the water proceeding 

 from C to K has a greater tendency to 

 gravitation than that issuing from E, 

 which rushes with far more force, and 

 consequently has a greater tendency to 

 an horizontal direction. For the aper- 

 ture at C is only acted upon by a column 

 of one foot deep, i. e from B to C, but the 

 column of water from B to E measures 

 seven feet. We have already stated, 

 that the velocity is equal to the square 

 root of the column's height above the 

 aperture. 



It is the peculiar property of fluids to 

 preserve their level, notwithstanding any 

 varieties of course, or inequality of eleva- 

 tion. Thus, supposing the pipe, A B C D, 

 fig. 2, to be bent into the form required 

 for passing over declivities, as shown: the 

 water will rise to the height, A D ; but 

 where the channel exceeds the level of 

 that line, there will be a break in the 



