565 



PLUTUS. 



PNEUMATICS. 



580 



of Hesiod (455, 767), the abode of Hades is'said, as has been already 

 stated, to be beneath the earth ; but in the ' Odyssey ' it is placed in 

 the regions of darkness beyond the stream of ocean. (' Od.,' x. 508 ; 

 xii. 81.) 



The temples and statues of Pluto appear to have been very few. 

 Pausauias relates (i. 28, 3 6), that there was a statue of Pluto in the 

 temple of the Eumenides, on the Areiopagus of Athens ; and the same 

 writer also informs us (v. 20, 1), that there was a statue of this god 

 in the temple at Olympia. There were also temples sacred to Pluto in 

 Elis, Pylos, and other parts of Greece. Black sheep, a male and a female, 

 were the sacrifices offered to Pluto and Persephone ; and the person 

 offering them was obliged to avert his face. 



Few representations of Pluto by Greek artists have come down to 

 us. In the statues and busts which are known, he resembles his 

 brothers Zeus and Poseidon ; but the countenance has a gloomier cast, 

 the hair is lower on the forehead, and the drapery heavier. On vases, 

 urns, and sarcophagi, he is more frequently represented carrying off 

 Persephone, or enthroned with Persephone seated beside him. 



In later writers, the word Hades also signifies the abode of the dead, 

 as well as the deity who presided over it. 



PLUTUS (lUowos), the personification and the giver of wealth, is 

 said by Hesiod (' Theog.,' 969) to have been the son of Demeter and 

 the hero lasius. He is said to have been bliuded by Zeus, in order 

 that he might distribute his favours indifferently to the evil as well as 

 to the good. (Aristoph., ' Plutus,' 90.) Plutus appears as an actor in 

 the comedy of Aristophanes named after him ; but we have no par- 

 ticulars respecting his worship. In the temple of Fortune at Thebes, 

 Plutus was represented as a child in the arms of Fortune (Paus., ix. 16, 

 1 ), and at Thespeia in Bceotia there was a statue of Plutus placed by 

 the side of one of Athena Ergane. (Paus., is. 26, 5.) Pausanias also 

 mentions a statue of Plutus in the arms of Peace, which Cephisodotus 

 made for the Athenians. The attribute of Plutus was a cornucopeia. 



PLUVIOMETEU. [R.vix-G.vccE.] 



PNEUMA'TICI (ol TrvfviutTutol), a medical sect founded by 

 Athentcus about the middle or end of the 1st century A.D. [ATHKN.ECS 

 in Bloc;. Div.] Their principal doctrines have been noticed in the life 

 of their founder. The sect appears to have existed but a short time, 

 namely, from the reign of Vespasian to Antoninus Pius, and never to 

 have enjoyed any great celebrity. For further information concerning 

 it the reader may consult Wigan, ' Pnefat. in Arct. ;' Ackermann, 

 ' De Aretsco ;' and especially Osterhajisen, ' Dissert. Inaugur. de 

 Secte Pneumaticorum Medicorum Historia.' 



PNEUMATICS. This name is given to that part of physics in 

 which are contemplated the mechanical properties of elastic fluids, 

 principally atmospheric air. The word is derived from ryivfut, ' breath 

 or spirit,' and pneumatology is a term which was once applied to the 

 division of science now called metaphysics. 



The mechanical properties of air appear to have been, to a certain 

 extent, known to the ancients. Aristotle must have been aware of its 

 materiality, and of its tendency to descend towards the earth, like 

 other heavy bodies, for he observes that if a bladder be filled with air, 

 it weighs more than when empty : the observation proves at least that 

 he had the idea, but it may be doubted whether or not he ever made 

 the experiment, since it is not probable that he had the means 

 of weighing the full bladder in vacuo, or that he could keep it dis- 

 tended when the air was extracted from it. The invention of a species 

 of forcing-pump for raising water, and of instruments for producing 

 sounds by the passage of air through orifices in tubes, both of which 

 are ascribed by Vitruvius (lib. ix., x.) to Ctesibius, who lived about 

 ISO years B.C., are proofs that the elastic force of compressed air must 

 have been then apprehended and made subservient to purposes con- 

 nected with practical utility. 



The ascent of water in pumps, on raising the piston, must have 

 been for ages observed, without a suspicion being entertained of the 

 cause ; and the principle that the existence of a vacuum was impos- 

 sible, was held to afford a sufficient explanation of the phenomenon. 

 Even Galileo, when made aware, by the formation of a vacuum between 

 ton and the water when the latter was at its greatest height in 

 a pump, that the principle was unfounded, had recourse to another 

 which was equally remote from the truth ; for he is said to have 

 ascribed the ascent of the water to the attraction of the piston, and to 

 have accounted for the height of the column never exceeding a certain 

 quantity (about 34 feet) by a supposed equilibrium between the weight 

 of the column and the attractive power of the piston. The steps 

 by which Torricelli arrived at the discovery of the true cause of the 

 phenomenon, and at the same time, the determination of the pressure 

 of the atmosphere by the weight of the column of mercury supported 

 in a tube closed at the upper extremity, are noticed under BABO- 

 METEH. 



Soon after the commencement of the seventeenth century, the idea 

 of ascertaining with precision the state of bodies with respect to 

 temperature by the expansion of fluids when acted on by heat, is said 

 to have occurred to the members of the Academy del Cimento, at 

 Florence ; and this gave rise to the invention of an instrument of great 

 importance, as well for many occasions of ordinary life, ag for the 

 purposes of pneumatical science. [THEKMOMETEB.] 



The desire of perfecting the theory for determining the heights of 

 mountains by the barometer, led to numerous researches in order to 



ascertain the relations between the condensation of air and the forces 

 compressing it, and also to discover the effects produced by variations 

 of temperature. About the year 1650, Boyle in England, and about 

 1676, Mariotte in France determined from experiments, that when the 

 temperature was constant, the density of air was proportional to the 

 compressing force ; but it was not till a century afterwards that De 

 Luc determined the expansion consequent upon given increments of 

 temperature. 



In the beginning of the eighteenth century, steam began to be 

 employed as a moving power for pumps and other machinery. The 

 law of the resistance of the air to bodies moving in it was discovered 

 by Sir Isaac Newton; but the intensity of this resistance against 

 military projectiles was first determined, for the inferior velocities, by 

 Robins, in 1740 ; and Dr. Hutton subsequently obtained a formula 

 which, with perhaps sufficient correctness, may be employed with any 

 velocity whatever. [GUNNERY.] The expansion of fired gunpowder 

 has been determined from the researches of Robins, Euler, and 

 Hutton ; and that of steam has been investigated by Dalton in Eng- 

 land, and by Prony, Arago, and other distinguished physicists in 

 France. [STEAM.] 



If a fluid be conceived to be perfectly elastic, it is evident that it 

 can be made to occupy a given volume only by being confined within 

 a close vessel, or by the pressure of a circumambient fluid. In the 

 latter case it is easy to perceive that while the temperature of the 

 elastic fluid remains the same, the spaces occupied by it will be 

 diminished when the compressing force is increased, and increased 

 when the latter is diminished ; and that the density, or degree of close- 

 ness of the particles, will vary directly with the change of volume. 

 Now the compressing forces which are made to act on the surface of 

 an elastic fluid are usually estimated in terms of the pressure exerted 

 by a vertical column of the atmosphere, its base being equal to some 

 unit of superficies, as one square inch or foot. Whatever be the con- 

 stitution of such column, its weight or pressure is known, in a given 

 state of the atmosphere, by direct experiment, and this is usually 

 designated the pressure of one atmosphere : the same pressure is also 

 frequently expressed by the height of the column of mercury which is 

 supported in a barometer-tube by the counteracting weight of the 

 atmospheric column. 



A simple experiment suffices to show the relation between the 

 density and elasticity of a fluid and the compressing force. Let 

 mercury be poured into a bent tube open at one end and closed at the 

 other ; the sum of the weights of the column of* mercury and of the 

 atmosphere above it in the open tube, will reduce the air in the closed 

 tube to a volume less than that which it previously occupied. Let, 

 then, more mercury be added, and the volume of air will be still 

 further reduced at every addition of mercury : and it will be found 

 that the volumes of air vary inversely as the sums of the weights of 

 the mercury and atmosphere. Therefore, since the density of an elastic 

 fluid is inversely proportional to the space it occupies, it follows that 

 the densities are directly proportional to the compressing weights ; 

 and since the weights of the mercury and atmosphere together, in the 

 open tube, are balanced by the elastic force of the condensed air in the 

 close tube, that elastic force is also directly proportional to the com- 

 pressing force, that is, to the density. [AiB.] This is the law of 

 Boyle and Mariotte, and though it cannot be said to be absolutely 

 correct for very great pressures, it is sufficiently so for all practical 

 purposes. (See the elaborate experiments of Magnus, ' Annales de 

 Chimie et de Physique, 1 3 serie, iv. ; and of Regnault, iv. and v.) 



The following table, extracted from the results of the experiments 

 made by MM. Prony, Arago, and thers, exhibits the volumes and 

 elasticities of a given quantity of atmospheric air under different 

 pressures, the temperature being nearly constant, and expressed by 14 

 of the centigrade thermometer (57'2 Fahr.), and, as far as it extends, 

 it confirms the law above mentioned. The first of the columns 

 expresses the pressure or elasticity in multiples of the weight of an 

 atmospherical column ; the second expresses the same by the height, 

 in inches, of the column of mercury supported in a barometer tube ; 

 and the third column contains the corresponding volumes. 



Atmospheres. 



1 

 I 



11-3 



16-5 



21-7 



34 



17 



Inches, En;. 

 30 

 150 

 339 

 495 

 651 

 720 

 S10 



Volumes. 



1-0000 

 1989 

 0884 

 0601 

 0457 

 0411 

 0369 



The law being admitted, it may be proved that the particles of an 

 elastic fluid repel each other with a force which varies inversely as the 

 distance between them. For let the volume of fluid be in the form 

 of a cube, and let the compressing force act perpefldieularly on one of 

 iU faces ; then, if d represent the distance between any two adjacent 

 particles of fluid, the number of particles in the surface pressed will 



vary aa -L. Now assume that the repulsive force (perpendicularly to 



rf* 



that surface) between any two adjacent particles in the volume varies 

 as d* ; then the whole repulsive force on that surface, and, con- 



