HYDlJODYNAMICa 



HVHUOOEN. 



preauras before and behind, and by the friction of the water against 

 the skin. Atiditional causes of retardation are the heaping up of the 

 water in front when the Telocity us considerable, and a diminution of 

 the preeture on the hinder face on account of the surface of the water 

 there being depressed below the general level ; a circumstance arising 

 from the lateral communication of motion in lluid.-, tiy which the 

 water, driven off from the front, and. proceeding in a diverging direc- 

 tion on each ride toward* the rear, carries away with it from thence 

 some of the water which should counteract in part the pressure on 

 the front. 



Mr. Scott Russell in the' Report of the British Association,' for 1835, 

 and Mr. Macneili in the ' Transactions of the Institution of Civil 

 Engineers,' have given the following laws for the rtmttanct to boats 

 moving on canals ; but for the practical reading of these laws we must 

 refer to STEAM-BOAT. 



1. The rise or emergence depends on the velocity of the vend. 



-. The resistance depends on the velocity or magnitude of the wave 

 which U generated. 



3. The resistance increases rapidly as the velocity of the body ap- 

 proaches the velocity of the wave, and is a moriwum when they are 

 equal 



4. Jf the velocity of the body be greater than the velocity of the 

 wave, the resistance diminishes ; for the body is poised on the summit 

 of the wave in stable equilibrium. 



5. The velocity of the wave is independent of the breadth of the 

 fluid, but varies as the square-root of the depth. 



6. In every navigable river there is a velocity with which it is easier 

 to ascend against the current, than to descend with it. 



7. Vessels on the summit of the wave may move about 20 or 80 

 miles per hour. 



With regard to the form of vessel beat adapted to diminish this 

 resistance, they add : 



1. A cylinder will meet with leu resistance in a fluid than a plate of 

 the same dimensions as the end of the cylinder ; and a cone with its 

 Lull-end foremost is better than either. 



2. There is no fixed ratio of breadth to length which is beat. The 

 longer the better, but the breadth varies with the burden. 



, lie section of greatest breadth should be always abaft the middle, 

 about 3-5th of the length from the bow. 



4. Lastly, the water-lines should be hollow, of the form called the 

 " mite-form" 6rst concave, then convex. 



We may conclude this article with a notice of one of the many hydro- 

 dynamic engines, referring to separate heads, as indicated under 

 HYDRAULICS, for other contrivances, in which the water U to be 

 raised, or the force of water to be employed. 



The HYDRAULIC RAH, which was invented by Montgolfier at the 

 close of the last century, and improved by his son, consists, indepen- 

 dently of the feeding cistern, of a pipe which carries the water to the 

 head of operations. This part consists of a short tube, at the upper 

 part of which, as well as at the end, are two valves, the ttop-ralve, 

 and the (ittauwx-raJi-e / the extremity is in a bell filled in its upper 

 part with air, and its lower with water. The ascension-valve being 

 closed, the water will come from the reservoir with increasing velocity ; 

 and leaving it by the stop-valve, will shut it : then, by the rit riva 

 which it has acquired, it will strike the ascension valve, and open it, 

 and so penetrating into the reservoir of air, will compress it, and make 

 the water in the ascension-tube rise ; then the elasticity of the air, and 

 the weight of water 1 in the ascension-pipe will, of course, absorb partly 

 the m rwa acquired with the water and will give it a powerful motion ; 

 hence, by reason of the retrograde motion of the water, the ascension- 

 valve will shut, and there will be formed a partial vacuum under the 

 (top- valve, which will open, and so on continually. Hence this machine, 

 when once set in motion with a continual supply of water, will v. 

 the momentum generated and destroyed for any length of time, if 

 kept in repair. 



The accompanying figure represents a vertical section of the im- 

 proved construction. The water arrives from a cistern at a higher 

 level, by the horizontal pipe A, over which is a circular opening con- 

 Uining a valve v, which acU as a Oonpagt valve, and is suspended by a 

 stem. Further on the pipe ascends into a small reservoir c, called the 

 air malrau ; the air contained in it is compressed by the ascending 

 water, while the lateral pressure of the water opens the valves v v', and 

 enters the larger reservoir r, which it partly fills, and compresses the 

 ah- confined in the other part of it. The reaction of this air on the 

 surface of the water cause* the water to ascend the force-pipe o. When 

 the stoppage valve is down, an in the figure, the water overflows the 

 < above it, and pnnes into a waste reservoir, thereby producing 

 a rapid increase in the velocity of the current in A, which, acting on the 

 under nirface of the valve, force* it up, and closes the opening by 

 which the water crapes. This momentary confinement of the water 

 causes it t-< force ita way into the cylinder o, where it compresses the 

 air and produces a reaction, which opens the valve* v v', and a portion 

 of the water eaters the vessel p, and further compresses the air 

 there. These resistances retard the current A, and relieve the stoppage 

 valve from the impuUe which taiaud it, so that that valve again falls, 

 and the valves v * are closed. The water again escape* from the 

 opening over the stoppage valve a* shown in the figure, the current 

 m A is again accelerated; the stoppage valve is once more closed, and 



the Mine erics of effect* is repeated. In this way by a aerie* of 

 pulsation* water is continually elevated in the pipe o. As air is 



gradually absorbed by water, the compressed air of the vessels c and F 

 is liable to be carried away by the water up the force-pipe o, the effect 

 of which would be to subject the machine to shocks which would 

 destroy the uniformity of its action and injure its working parts. To 

 prevent this, an air-valve is provided at s, which, opening inwards, 

 admits air during the intervals when the stoppage valve is closed. Tim 

 air in rushing through the valve s makes the sound like the sniffling of 

 a person's nose, and is hence called a snifting valve. It may consist of 

 a tube of capillary bore, and be left entirely open. 



HYDRO-ELECTRIC MACHINE. [ELECTRICAL MACHINE.] 



HYDROFERRIDCYANIC ACID. [FERRIDCYANIC ACID.] 



HYDROFERROCYANIC ACID. [FERROCYANIC ACID.] 



HYDROFLUOBORIC ACID. [FLUORINE.] 



HYDROFLUORIC ACID. [FLUORINE.] 



HYDROFLUOSILICIC ACID. [FLUORINE.] 



HYDROGEN (H), an elementary body, which, as it is known only 

 in the aeriform state, is usually termed hydrogen gat. From tin- 

 earliest dawn of chemical science, elastic fluids have been known which 

 had the property of burning on the approach of flame, and were con- 

 founded under the general name of injlammable air. As it wan after- 

 wards found that there was a difference in their densities, they were 

 distinguished as liijht and heavy inflammable air ; it is the former of 

 these which is now called hydrogen. Hydrogen gas was first minutely 

 examined, and the mode of preparing it in various ways stated, l>v Mr. 

 Cavendish. [CAVENDISH, HENRY, in NAT. HIST. Div.J 



In nature, hydrogen is found in the free or uncouibined state, as a 

 constituent of volcanic gases. In comparatively email quantity it 

 occurs associated with phosphorus, sulphur, carbon, and nitrogen, 

 forming gaseous compounds; it is a constituent of nearly all the 

 proximate principles contained in animals and vegetables ; but united 

 with oxygen, it constitutes Jth of the total weight of that familiar com- 

 pound water. 



Hydrogen is always prepared from water. If the gas is required 

 absolutely pure, the water is decomposed by a current of electricity, 

 the hydrogen being collected from the negative polo of the decompo- 

 sing cell, and, if necessary, dried by being passed through a tube lille<I 

 with pieces of chloride of calcium. Another process, valuable because 

 instructive, consists in passing a fragment of potassium or sodium up 

 into n small quantity of water in a test tube, the remaining portion 

 of w.iieh is filled with mercury, and inverted over the mercury trough; 

 the sodium rapidly attacks the water, combining with its oxyr 

 form oxide of sodium (soda), and displacing its hydrogen, wlm-h 

 latter element, being gaseous, expels the greater part or all of the 

 v from tin- tube. The following is the simple change that takes 

 place: 



Ka + HO = NiO + II 



: 



Water. 



Soda. 



Hydrogen. 



For ordinary purposes, hydrogen is prepared by acting upon granulated 

 /in.- with diluted sulphuric acid. The zinc, covered with water, 

 is placed in a bottle or other convenient vessel to which a cork can 

 be fitted, and strong sulphuric acid is then poured in by means of 

 a funnel, to which is attached a tube passing through the cork to the 

 bottom of the bottle. The gas in rapidly disengaged, and paining 

 through an exit-tube in the <-<>rk may be conveyed by flexiblo tubing 

 into a gasholder, Ac. [GAM or.] In thin process, 



sulphate of zinc is formed, which remains in the generating vessel 

 dissolved in the excess of water used. 



7.n + HO, 80, 



ZnO, SO, 



II. 



