P UM 



PUR 



enjoyed an undisputed celebrity, for 

 1400 years. 



PUBE'SCENCE {puhesco, to begin to 

 have a beard). The down of plants, con- 

 sisting of soft short hairs, which partially 

 cover the cuticle, as in geranium moUe. 

 Also, the soft hairy down of insects. 



PUDDlisrG-STONE. Conglomerate. 

 Rounded water-worn fragments of rocks 

 or pebbles, cemented together by another 

 mineral substance, which may be of a 

 siliceous, calcareous, or argillaceous 

 nature. 



PULLEY. One of the mechanical 

 powers, consisting, in its simplest form, 

 of a wheel, capable of turning about its 

 axis, which may be either fixed or move- 

 able. A cord passes over a portion of 

 the circumference ; if the axis of the 

 pulley is fixed, its only eflfect is to change 

 the direction of the force exerted by the 

 cord ; but if it is moveable, a mechanical 

 advantage may be gained. Combinations 

 of pulleys may be made in endless 

 variety. 



PULMOBRANCHIA'TA {pulmo, the 

 lungs, branchice, gills). Pulmonia. An 

 order of terrestrial Gasteropods, named 

 from the resemblance between their 

 mode of breathing and that of animals 

 possessed of proper lungs. The snail, 

 for instance, breaths air, which is alter- 

 nately drawn into and expelled from a 

 cavity lined with a vascular network. 

 Many of these animals have no shell. 



PULMONA'TA (pulmo, the lungs). 

 An order of the Arachnida, comprising 

 those species which breathe by means of 

 pulmonary sacs. See Tracheata. 



PULMONI'GRADACpm/wo, the lungs, 

 gradior, to advance). An order of the 

 Acalephcs, so named from the contrac- 

 tions and dilatations of their umbrella- 

 shaped disc, by means of which they 

 move, having some resemblance to the 

 motion of the lungs in respiration. 



PUMICE ipumex). A light spongy 

 lava, chiefly felspathic, of a white ap- 

 pearance, produced by gases or watery 

 vapour gaining access to the particular 

 kind of glassy lava called obsidian, when 

 in a state of fusion. It may be called 

 the froth of melted volcanic glass. On 

 account of its fibrous and vesicular state, 

 it is so light as to float on water. 



PUMP. A machine formed on the 

 model of a syringe, for raising water from 

 various depths to the surface of the earth. 

 1. In the common suction pump, the water 

 18 pressed into the pump-barrel by the 

 atmosphere, and thrown out by lifting. 

 276 



2. In the lifting pump, the piston is 

 situated in the lower part of the barrel, 

 and raises the water through the whole 

 distance, by forcing it upward, without 

 the agency of atmospheric pressure. 



3. The forcing pump differs from both 

 these, in having its piston solid, or with- 

 out a valve, and also in having a side 

 pipe, through which the water is forced, 

 instead of rising in a perpendicular direc- 

 tion, as in the others. 



Chain pump. — This kind of pump is 

 used on board of ships, and consists of 

 an endless chain moving over a wheel on 

 the deck, which is turned round by 

 winches, and over a roller in the pump- 

 well, having saucers or flat circular pis- 

 tons at certain intervals. Chain pumps, 

 in large ships, throw out a ton of water 

 per minute. 



PULSE GLASS. A tube of glass, 

 having a bore of about two lines in dia- 

 meter, but at right angles at each end, 

 where it is expanded into two bulbs of 

 about 1^ inch in diameter. The instru- 

 ment contains water or spirits of wine, 

 the remaining space being a vacuum : if 

 one bulb be grasped in the hand, the 

 liquid will boil violently in the other, 

 owing to the atmospheric pressure being 

 removed from the surface of the liquid. 



PUPA. A term literally denoting a 

 baby wrapped up in swaddling bands, 

 and applied to the chrysalis, or nymph, 

 or second state of insect existence, in 

 which, the last skin of the larva being 

 thrown off, the animal appears in a dif- 

 ferent form, enveloped in an oblong case, 

 without external limbs, and almost in- 

 capable of the slightest motion. See 

 Imago. 



PUPI'PAROUS {pupa, and pario, to 

 produce). A designation of Dipterous 

 insects, which bring forth their young in 

 the pupa state. They comprise two 

 families, known by the names of forest- 

 flies and bat-lice. 



PURBECK LIMESTONE. Purbeck 

 Beds. Limestone strata, belonging to 

 the Wealden Group, which intervenes 

 between the Greensand and the Oolite. 

 It is composed of fresh-water shells 

 united by a calcareous cement. It is 

 slaty, argillaceous, of a brownish colour, 

 alternating with slaty marl, and some- 

 times containing beds of compact lime- 

 stone. 



PURPLE OF CASSIUS. A purple- 

 coloured powder, precipitated when pro- 

 tochloride of tin is added to a dilute 

 solution of gold. 



