408 



HYDRODYNAMICS. 



Hydrody- 



History. 



The general 

 principles of 

 Hydrody- 

 namics 

 known in 

 the earliest 

 ages. 



Invention 

 of Clepsy- 



ilra.'. 



PLATE 

 (JCCXIII. 

 Fig. 1. 



PLATE 

 CCCXIII. 



Fig. 2. 



from the Greek 'fluf, mater, and 

 &w*fi.if, power or force, is that branch of natural philo- 

 sophy whicli embraces the phenomena exhibited by 

 water and other fluids, whether they are at rest or in 

 motion. It treats of the pressure, the equilibrium, the 

 cohesion, the motion, and the resistance of fluids; and 

 of the construction of the machines by which water is 

 raised, and in which it is the first mover, or the pri- 

 mary agent. This science is generally divided into 

 Hydrostatics and Hydraulics, the former of which con- 

 siders the pressure, equilibrium, and cohesion of flu- 

 ids ; and the latter, their motion, the resistance which 

 they oppose to moving bodies, and the various machines 

 in which tjjey are the principal agent. 



HISTORY. 



ALTHOUGH Hydrodynamics is but a modern science, 

 and was studied by the ancients only in its most ge- 

 neral principles, yet many of the leading doctrines and 

 phenomena upon which it is founded are familiar to 

 the rudest nations, and must have been well known in 

 the very earliest ages of society. Even at that remote 

 period when man first trusted himself to the waves, 

 the pressure of fluids, and the phenomena of floating 

 bodies, were undoubtedly known to him ; and in the 

 more advanced stage of navigation, when the Pheni- 

 cians were able to colonise the most distant regions of 

 the globe, the directing power of the helm, the force 

 and management of the oars, the action of the wind 

 upon the sail, and the resistance opposed to the motion 

 of the vessel, were well known facts, which implied 

 practical acquaintance with some of the most important 

 doctrines of Hydrodynamics. 



The motion of fluids, as affected by the size of the aper- 

 ture from which they issued, and by the height of the 

 superincumbent column, formed the fundamental prin- 

 ciple of the Clepsydra (from K^IVIU, to steal, and v?g, 

 water) or water clocks, which were employed in die ear- 

 liest ages, before the invention of sun dials, to measure 

 time. The simplest, and probably the earliest form in 

 which the Clepsydrae appeared, is that of two inverted 

 cones, as represented in Plate CCCXIII. Fig. 1. This 

 species of Clepsydra consisted of a hollow cone A, perfo- 

 rated at its vertex, and of a solid cone B, which was made 

 to fill A with the greatest exactness. The aperture of A 

 was so adjusted to the size of the cone, that, when filled 

 with water, it emptied itself in the course of the short- 

 est day in winter. The length of the cone was divided 

 into 12 equal parts, which indicated the hours by 

 the descent of the fluid, or the same result was obtained 

 by divisions upon the vessel into which the water 

 flowed. When the days lengthened, and the hours be- 

 came longer, the solid cone B was introduced into the 

 hollow cone A, and, according to the depth of its pene- 

 tration, the water flowed from the aperture with less 

 facility. A graduated index BC enabled the observer 

 to accommodate the position of the solid cone to the 

 varying length of the day. 



Another Clepsydra, of a more ingenious construc- 

 tion, is represented in Plate CCCXIII. Fig. 2. The 

 water is first received into the reservoir A, which is al- 



History. 



ways kept full, and descends by the pipe B into a hole 

 in the great drum MN. This hole corresponds to one 

 of the openings in the groove round the circumference Clepsydra. 

 of the small drum LO, which is drawn out in the fi- 

 gure for the purpose of showing it, but when the machine 

 is in use, it is fitted into the drum MN. The apertures 

 of the groove in LO are of different sizes, so as to admit 

 different quantities of water, according to the length of 

 the day, and the proper aperture for the given day is 

 found by placing the index L opposite the sun's place in 

 the zodiac shewn at N, the index O being used for the 

 night hours. The water which descends through the 

 openings in the drum LO is conveyed by the pipe F, 

 and falls through the aperture at G into the reservoir-H. 

 As the water rises in the reservoir, the inverted vessel 

 I, suspended by a chain which passes over the axis R, 

 and balanced by a counterweight P, ascends, and conse- 

 quently the hour hand X, fixed upon the extremity of 

 the axis, is made to revolve, and indicate the hours 

 uptm the dial plate. 



Notwithstanding the ingenuity of these inventions, n ; SCOTer ; e 

 and the hydrodynamical knowledge which they indi- O f Archi- 

 cated, the doctrine of fluids may still be considered as medes, 

 deriving its origin from the discoveries of Archimedes. A.C. 250. 

 The history of these discoveries hp.s been rendered ri- 

 diculous by vulgar fables, which have long been dis- 

 credited ; but it appears unquestionable, that they ori- 

 ginated in the detection of a fraud committed by the 

 jeweller of Hiero, king of Syracuse. Archimedes was 

 applied to by the king to ascertain, without injuring 

 its workmanship, whether or not a new crown, which 

 had been made for him, consisted of pure gold : The 

 method of solving the problem is said to have occur- 

 red to him when in the bath, and he applied it suc- 

 cessfully in detecting the fraud. * The hydrostati- 

 cal doctrines to which Archimedes was thus conduct- 

 ed, were illustrated in a work consisting of two books, 

 and entitled, -STI^I ixvptvav, de insidentibus in Fluido. He 

 maintained, that every particle of a fluid mass in equi- 

 librio is pressed equally in every direction. He exa- 

 mined the conditions in consequence of which a float- 

 ing body assumes and preserves its position of equili- 

 brium, and he applied it to bodies that have a triangu- 

 lar, a conical, and a paraboloidal form. He shewed that 

 every body plunged in a fluid loses as much of its own 

 \veight as the weight of the quantity of water which it 

 displaces ; and upon this beautiful principle is founded 

 the process which he employed for ascertaining the 

 impurity of Hiero's crown. When the result was com- 

 municated to the king, he exclaimed, Kihil nan dicenti 

 Archlmede, credam ! The screw of Archimedes, which 

 is still used in modern times for raising water, is said 

 to have been invented by him when in Egypt, for the 

 purpose of enabling the inhabitants to free themselves 

 of the stagnant water which was left in the low grounds 

 after the inundations of the Nile ; and Atheneus in- 

 forms us, that navigators held the memory of Archi- 

 medes in the highest honour, for having furnished 

 them with the means of carrying off the water in the 

 holds of their vessels. 



Hydraulic machinery appears to have been first in- 

 vented in the Alexandrian school, which flourished un- 





